Physics of fundamental Symmetries and Interactions - PSI2025

7 Sept 2025, 17:00 → 12 Sept 2025, 17:00 Europe/Zurich

WHGA/001 - Auditorium (PSI)

Adrian Signer (PSI - Paul Scherrer Institut), Anita Anita Van Loon-Govaerts/Tanja Mülhaupt (PSI - Paul Scherrer Institut), Bernhard Lauss (PSI - Paul Scherrer Institut), Klaus Stefan Kirch (PSI - Paul Scherrer Institut), Stefan Ritt (PSI - Paul Scherrer Institut)

 

The workshop focuses on the physics at the low energy, high precision frontier without neglecting complementary approaches. It aims at highlighting present activities and future developments. The Paul Scherrer Institut (PSI) itself offers unique opportunities for experiments in this realm: it houses the world's most powerful proton cyclotron and the highest intensity low momentum pion and muon beams and the ultracold neutron source.

 

 

 

Information for PSI2025-Participants.pdf

T.Rostomyan_MUSE_2025.09.08.pdf

Sunday 7 September

18:00 → 20:00 Welcome Reception

Monday 8 September

09:00 → 10:35 Session: Mo - 1

Convener: Klaus Stefan Kirch (PSI - Paul Scherrer Institut)

09:00 Welcome address

Speaker: Marc Janoschek (PSI - Paul Scherrer Institut)

09:05 The Muon Anomalous Magnetic Moment: Final Results from the Muon g-2 Experiment at Fermilab

The Muon g-2 Experiment at Fermi National Accelerator Laboratory set out to measure the muon anomalous magnetic moment, $a_{\mu}$, with a target precision of 140 parts-per-billion (ppb), representing a four-fold improvement over the predecessor measurement at Brookhaven National Laboratory in the 2000s. The Muon g-2 collaboration recently published the analysis of the final three of a total of six years of data taking, achieving a combined precision of 127 ppb, surpassing its design goal. This new result will remain a benchmark test for any future extension of the Standard Model for years to come. This talk will provide a brief history and overview of the experiment, details of the latest analysis, and the final result on $a_{\mu}$.

Speaker: Simon Corrodi (Argonne National Laboratory)

MuonG2_PSI_PDF_for_backup_only.pdf

MuonG2_PSI_preferred.pdf

MuonG2_PSI_preferred.pptx

09:35 Current status of theory for medium and heavy muonic atoms

In my talk, I will present the latest advances in the theoretical study of medium and heavy muonic atoms. I will discuss the four largest QED contributions, current uncertainties, and the limitations in determining nuclear radii, illustrated by the case of the doubly magic nucleus ²⁰⁸Pb. Finally, I will address the feasibility of obtaining model-independent values of nuclear radii.

Speaker: Dr Natalia S. Oreshkina (MPIK Heidelberg)

2025_09_PSI.pdf

10:05 Peeping at the Universe through the keyhole: the neutron electric dipole moment

The Universe and its history are at the same time very well understood and a big mystery. We have amazing tools from satellites to observatories to weight the universe as it is today and also as it was in the past. But the content of the Universe can simply not be explained by physicists. To get the full picture, we need to identify and understand the interactions at play throughout the life of the Universe. This is the meeting point between particle physics and cosmology. At this meeting point stands the neutron, a very common particle that we can uniquely use in high precision experiments.
I will present how experiments searching for a permanent electric dipole moment of the neutron (nEDM) aim at discovering new sources of CP violation beyond the Standard Model of particle physics and understanding the origin of the matter-antimatter asymmetry of the Universe. The quest for the neutron electric dipole moment started more than sixty years ago. In recent experiments, polarized ultra-cold neutrons are stored in material bottles. I will present the ongoing efforts worldwide but also the latest measurement to date and the new project at the Paul Scherrer Institute in Switzerland.

Speaker: Stéphanie Roccia (UGA)

PSI2025.pdf

10:35 → 11:05 Coffee

11:05 → 12:35 Session: Mo - 2

Convener: Georg Bison (PSI - Paul Scherrer Institut)

11:05 Highly Charged Ion Clocks to Test Fundamental Physics

The extreme electronic properties of highly charged ions (HCI) render them highly sensitive probes for testing fundamental physical theories. The same properties reduce systematic frequency shifts, making HCI excellent optical clock candidates. The technical challenges that hindered the development of such clocks have now all been overcome, starting with their extraction from a hot plasma and sympathetic cooling in a linear Paul trap, readout of their internal state via quantum logic spectroscopy, and finally the preparation of the HCI in the ground state of motion of the trap. Here, we present the first operation of an atomic clock based on an HCI (Ar13+ in our case) and a full evaluation of systematic frequency shifts of the employed 2P1/2-2P3/2 fine-structure transition at 442 nm. The achieved uncertainty is almost eight orders of magnitude lower than any previous frequency measurements using HCI and comparable to other optical clocks. One of the main features of quantum logic spectroscopy is the flexibility of the investigated species. This allowed us to perform isotope shift spectroscopy of the 2P0-2P1 fine-structure transition at 569 nm in Ca14+ ions. In a large theory-experiment collaboration, we combined this data with improved measurements of the Ca+ 2S1/2-2D5/2 clock transition at 729 nm, new isotope mass measurements, and highly accurate calculations of the 2nd order mass shift. The resulting King plot allows us to put the currently most stringent bound from isotope shifts on a hypothetical 5th force coupling neutrons and electrons [1], despite a large (~900σ) residual nonlinearity, suspected to be dominated by nuclear polarizability. This demonstrates the suitability of HCI as references for high-accuracy optical clocks and to probe for physics beyond the standard model.

A next-generation HCI optical clock may be based on Ni12+, which offers a long excited state lifetime of >10 s. By developing efficient search strategies with quantum logic techniques [2] and precise atomic structure calculations [3], we identified the logic and clock transitions in this species within just a few hours of searching.

[1] A. Wilzewski et al., arXiv:2412.10277, Phys. Rev. Lett. in print
[2] S. Chen et al., Phy. Rev. Appl. 22, 054059 (2024)
[3] C. Cheung et al., arXiv:2502.05386

Speaker: Piet O. Schmidt (PTB and LUH)

20250908-PSI2025.pdf

20250908-PSI2025.pptx

11:35 On the horizon: the $^{229m}$Th nuclear clock

The quest for an optical nuclear frequency standard, the ‘nuclear clock’ based on the elusive and uniquely low-energetic ‘thorium isomer’ $^{229m}$Th, has increasingly triggered experimental and theoretical research activities in numerous groups worldwide in the last decade. Today’s most precise timekeeping is based on optical atomic clocks. How-ever, those could potentially be outperformed by a nuclear clock, based on a nuclear transition instead of an atomic shell transition. Only one nuclear state is known so far that could drive a nuclear clock: the ‘Thorium Isomer $^{229m}$Th’, i.e. the isomeric first excited state of $^{229}$Th, representing the lowest nuclear excitation so far reported. Such a nu-clear clock promises intriguing applications in applied as well as fundamental physics, ranging from geodesy and seismology to the investigation of possible time variations of fundamental constants and the search for Dark Matter [1].
After years of nuclear-spectroscopy driven identification and characterization activities of $^{229m}$Th, the year 2024 witnessed seminal breakthroughs with first laser-driven excitations of the isomeric nuclear resonance in $^{229}$Th, both using intense broad-band [2,3] and VUV frequency-comb based narrow-band lasers [4], respectively. Hardly any physical observable experienced an improvement by 11 orders of magnitude within only 5 years, as it was reached for the excitation energy of the thorium isomer. Hence, the question is no longer ‘Will there be a nuclear clock?’, but rather ‘Which types of nuclear clocks with which properties will be realized in the coming years?’, driven by the requirements of a variety of fundamental and applied physics applications. While recent progress with optical excitation of $^{229m}$Th was achieved via fluorescence detection in a solid-state approach using doped large-bandgap crystals and thin films [5], recently also laser-driven conversion-electron Mössbauer spectroscopy of the thorium isomer was demonstrated [6], while the complementary approach using individual laser-cooled trapped ions in vacuum is still under study.
The talk will review the status and perspectives of ongoing activities towards realizing a nuclear frequency standard based on the thorium isomer both in the solid-state and trapped $^{229m}$Th$^{3+}$ ion approach.

[1] E. Peik et al., Quantum Sci. Technol. 6, 034002 (2021)
[2] J.Tiedau et al., Phys. Rev. Lett. 132, 182501 (2024)
[3] R. Elwell et al., Phys. Rev. Lett. 133, 013201 (2024)
[4] Ch. Zhang et al., Nature 633, 63-70 (2024)
[5] Ch. Zhang et al., Nature 636, 603 (2024)
[6] R. Elwell et al., arXiv:2506.03018 (submitted to Nature)

Speaker: Peter G. Thirolf (LMU Munich)

2025-Thirolf-PSI-229mTh.pptx

12:05 Status of the MUSE Experiment

Speaker: Dr Tigran Rostomyan (PSI - Paul Scherrer Institut)

T.Rostomyan_MUSE_2025.09.08.pdf

12:35 → 14:00 OASE Restaurant

14:00 → 15:30 Session: Mo - 3

Convener: Prof. Martin Fertl (Johannes Gutenberg University Mainz)

14:00 Fundamental physics with ultracold neutrons at LANL

Ultracold neutrons (UCN) are neutrons with kinetic energies below approximately 340 neV—low enough to be confined using material or magnetic bottles. These unique properties make UCN an essential tool for precision experiments in fundamental physics, such as probing the Standard Model and searching for new physics beyond it.
At Los Alamos National Laboratory, we operate a UCN source that utilizes a solid deuterium converter driven by a spallation neutron source. This facility has supported a diverse set of experiments—including UCNA, UCNτ, UCNA+, UCNτ+, UCNProBe, and the LANL nEDM project—each contributing to a deeper understanding of neutron properties and symmetries.
In this talk, I will present an overview of the physics program at the LANL UCN source,
highlighting recent achievements, ongoing eUorts, and future directions in the study of
fundamental neutron physics at LANL.

Speaker: Takeyasu Ito (Los Alamos National Laboratory)

PSI2025_Ito.pdf

14:30 SuperSUN: A high-density Source of Ultracold Neutrons at ILL

SuperSUN: A high-density Source of Ultracold Neutrons at ILL

SuperSUN is a newly commissioned superthermal high-density ultracold neutron (UCN) source at the Institut Laue-Langevin (ILL). It employs isotopically pure superfluid helium-4, cooled below 0.6 K, to convert a broad-spectrum cold neutron beam into UCN via inelastic scattering. The source has demonstrated a continuous, reliable operation for over 60 days. During this time, we achieved continuous UCN extraction rates of 21,000 neutrons per second and an in-situ saturated UCN density of 273 cm⁻³. The combination of high UCN density, extended storage times, and the low-energy spectrum offers new opportunities for fundamental research. This talk will present the source, our experimental results, discuss the technical challenges encountered, and outline the next steps in developing SuperSUN.

Speaker: Estelle Chanel (Institut Laue Langevin)

PSI2025.pdf

14:50 Measurement of the Neutron Electric Charge using Grating Interferometry

We report on a precision measurement of the neutron electric charge using the QNeutron apparatus. It consists of a Talbot-Lau interferometer for cold neutrons in the ballistic regime. The setup employs three identical absorption gratings and a differential two-beam geometry to detect potential beam deflections induced by a strong transverse electric field. During an 84-hour data-taking campaign in 2024 at the PF1B beamline at ILL, we successfully demonstrated the feasibility of this experiment with a first measurement yielding $Q_n = (0.11 \pm 1.06) \times 10^{-19}~e$, where $e$ is the elementary charge. The experiment establishes a new strategy for probing the neutron charge neutrality. It sets the stage for a future improved apparatus aiming to surpass the current experimental limit at the European Spallation Source.

Speaker: Marc Persoz (Universität Bern)

QNeutron - PSI2025.pptx

15:10 Sensitive search for neutron to mirror-neutron oscillations at the PSI UCN source

Parity-conjugated copies of standard model particles, so-called mirror particles, could provide answers for several standing issues in physics.
Since they would at first only interact with ordinary matter gravitationally, they can be viable candidates for dark matter. If mixing between standard model and mirror particles was possible, they could contribute to baryon number violation.
The mirror-neutron experiment at the Paul Scherrer Institute (PSI) was designed to search for anomalous disappearances of ultracold neutrons that could be hinting at neutron-to-mirror-neutron oscillations. Allowing for the presence of hypothetical mirror magnetic fields, the experiment was conducted in a controlled magnetic field, scanning from 5 $\mu$T to 109 $\mu$T. No evidence for anomalous neutron losses was found. Furthermore we are examining neutron losses for mass differences of up to $\Delta m = 0.02$ neV. We provide an overview of the experiment, its data analysis based on Monte Carlo simulations and precise magnetic field maps, and conclude with the presentation of new limits on the oscillation time.

Speaker: Nathalie Ziehl (ETH Zurich)

ziehln_psi25.pdf

15:30 → 16:00 Coffee

16:00 → 17:40 Session: Mo - 4

Convener: Michael Jentschel (Institut Laue Langevin)

16:00 Searching for Neutrinoless Double-Beta Decay

The search for neutrinoless double-beta decay is currently one of the
most compelling challenges in physics, with the potential to reveal the origin of the neutrino’s mass, demonstrate lepton number violation, and provide hints of the mechanism behind the matter anti-matter asymmetry we observe in our universe. Detecting this ultra-rare process, however, requires us to build very large detectors with very low background rates. Currently operating hundred-kilogram-scale experiments like CUORE, LEGEND-200 and the soon-to-come KamLAND2-Zen are beginning to probe the parameter space expected by minimal models of high-scale lepton number violation, and progress on ton-scale experiments is advancing quickly, with several collaborations aiming to begin
construction in the coming years. These experiments aim to achieve 3σ discovery sensitivity at 10^28 year half lives. I’ll give an overview of the current state of the field and the status of next-generation experiments.

Speaker: Julieta Gruszko (University of North Carolina at Chapel Hill)

Gruszko_PSI2025_DBD_overview.pdf

16:30 Precision measurements in superallowed 0+ → 0+ β decays at GANIL and upcoming opportunities

Corrected transition rates ($\mathcal{F}t^{0^+→0^+}$ values) of superallowed $0^+ \rightarrow 0^+$ beta decays have served as a benchmark for validating the conserved vector current (CVC) hypothesis in weak interactions. They now provide the most precise value of $V_{ud}$, the dominant top-row element of the Cabibbo-Kobayashi-Maskawa (CKM) quark mixing matrix. By imposing stringent constrains on the CKM unitarity, these decays enable probing physics beyond the Standard Model in the electroweak sector. Recent reevaluation of the superallowed $Ƒt^{0+→0+}$ values have resulted in a value of $V_{ud}$ that challenges the unitarity of the CKM matrix.

In this presentation, I will briefly discuss this current situation and the experimental program at GANIL, which aims to constrain isospin symmetry-breaking (ISB) corrections. Together with radiative corrections, this allows to extract nuclear medium-independent $\mathcal{F}t^{0^+→0^+}$ values from the experimentally measured transition rates $(ft^{0^+→0^+})$. In this context, I will also present preliminary results for the SA emitters $^{18}$Ne and $^{30}$S. Finally, I will highlight the opportunities available for high-precision measurements of these SA observables at DESIR and S$^3$-LEB, the upcoming low-energy radioactive ion beam facilities at GANIL.

Speaker: Bernadette Rebeiro (GANIL)

Rebeiro_PSI_Sept2025.pdf

16:50 Measurement of the $2\,^3$S$_1$ → $2\,^1$P$_1$ transition in positronium

In this talk I will discuss measurements of the $2\,^3$S$_1 \rightarrow 2\,^1$P$_1$ ($\nu_F$) transition in positronium (Ps). Although this transition is strictly forbidden by charge conjugation symmetry (C), it becomes observable in the presence of a magnetic field. Ps atoms were produced using a pulsed positron beam and optically excited to the radiatively metastable $2\,^3$S$_1$ state. These atoms then traversed a rectangular waveguide where microwaves were propagated to drive the $\nu_F$ transition. Line shapes of the transition were measured for a range of applied magnetic fields and the transition frequencies and Rabi frequencies were extracted.

Speaker: Rebecca Daly (University College London)

RDaly_PSI2025.pptx

17:20 New results of positronium 1S-2S transition and Muonium Fine structure

Positronium and muonium, as purely leptonic atoms without internal structure, provide ideal systems for high-precision tests of quantum electrodynamics (QED) [1] and measurements of fundamental constants. Here, we present the recent results we obtained at ETH on the $\text{1}^\text{3}\text{S}_\text{1} \to \text{2}^\text{3}\text{S}_\text{1}$ transition in positronium, measured via two-photon optical spectroscopy with a continuous-wave laser. The preliminary analysis estimates that the total uncertainty of this measurement at 5 ppb, comparable to the most precise measurement to date (2.6 ppb) [2]. We also outline the ongoing efforts by the MuMASS collaboration to improve the precision on the state-of-the-art measurements of the 1S-2S in muonium via CW laser [3] spectroscopy. The future prospects of positronium and muonium 1S-2S spectroscopy employing a novel Ramsey-Doppler scheme [4] will also be presented.

In addition, we present a recent measurement at PSI by MuMASS of the fine structure of muonium, which follows from the experiment that determined the muonium Lamb shift [5, 6]. A preliminary analysis of the experimental data indicates that the observed transition frequency is consistent with theoretical predictions, with a total uncertainty of around about 7 parts in 10,000, making it the most precise determination to date. The upcoming High-Intensity Muon Beam (HiMB) at the Paul Scherrer Institute (PSI) in Switzerland will allow to increase the statistics on such a measurement to enable precise tests of bound state QED, while also providing tests of new physics [7].
[1] G. S. Adkins et al., Phys. Rep. 975, (2022).
[2] M. S. Fee et al., Phys. Rev. Lett. 70 (1993).
[3] N. Zhadnov et al., Optics Express 31 (2023).
[4] E. Javary et al. Eur. Phys. J. D 79 (2025).
[5] B. Ohayon, et al., Phys. Rev. Lett 128 (2022).
[6] G. Janka, et al. Nat Commun 13 (2022)
[7] P. Blumer, et al. Eur. Phys. J. D 79 (2025)

Speaker: Edward Thorpe-Woods (ETH Zurich)

PSI.pptx

Tuesday 9 September

09:00 → 10:20 Session: Tu - 1

Convener: Eberhard Widmann (Stefan Meyer Institute)

09:00 New Era in Fundamental Physics with Antihydrogen

Antihydrogen—a bound state of an antiproton and a positron—offers a platform for precision tests of fundamental symmetries in nature. Over the past two decades, experimental progress has transformed antihydrogen studies from the demonstration phase into the precision measurement phase.

In this talk, I will review recent advances in antihydrogen research, with a focus on results from the ALPHA experiment. I will also share my personal perspectives on future directions, including efforts toward the simultaneous confinement of antihydrogen and hydrogen ("ALPHA Next Generation"), and the development of antihydrogen fountains and interferometers through the HAICU project at TRIUMF.

Speaker: Makoto Fujiwara (TRIUMF)

PSI2025_Fujiwara_v2.pptx

09:30 Antihydrogen beam formation in the ASACUSA-Cusp experiment

ASACUSA plans to measure the ground-state hyperfine structure of antihydrogen at low magnetic field using the Rabi method, for which a slow atomic beam ($v < 1500\,\mathrm{m/s}$) is needed. We make antihydrogen by slowly combining large amounts of positrons and antiprotons in a Penning-Malmberg trap. The antiprotons are "mixed" with the positrons for $60\,\mathrm{s}$, during which time about $100$ antihydrogen atoms leave the trap as a beam. The atoms are mostly in Rydberg states (principal quantum number $n > 20$) and can be ionized by a strong electric field. We study the binding energy of the atoms by varying the strength of the electric field, and we measure time-of-flight by pulsing the field to chop the beam. This presentation will cover our recent measurements and progress toward making a slower beam with more atoms in the ground state.

Speaker: Eric Hunter (CERN)

2025-09-09 -- PSI.pdf

ASACUSA

10:00 Antihydrogen production in the GBAR experiment at CERN

The GBAR experiment at the Extra Low Energy Antiproton ring ELENA at CERN aims to produce positive antihydrogen ions (Hbar+), the pure antimatter bound state of two positrons together with one antiproton, and the charge conjugate to the (fragile) H- ion. The production of such ions requires an experimentally challenging two step formation process, but it will open the door for next generation precision experiments with antimatter, since the experimental expense for manipulation and cooling of those states is vastly reduced. Here we present the efforts of the GBAR collaboration over the last years towards such a goal, in particular the order-of-magnitude increase in the production of neutral antihydrogen atoms during the beam time of 2024. This yields the first formation step for Hbar+ ions and brings the project into the starting blocks for an exploration of the 2S-2P Lamb shift on beam like antihydrogen atoms. The experimental installations needed for such a first antimatter physics measurement in GBAR will also be detailed in this talk.

Speaker: Dr Christian Regenfus (ETH Zurich)

PSI 2025CR.pdf

10:20 → 10:50 Coffee

10:50 → 12:40 Session: Tu - 2

Convener: Dieter Achim Ries (PSI - Paul Scherrer Institut)

10:50 From MAMI to MESA: Precision Tools to Probe Fundamental Interactions and Search for New Physics

The Mainz Energy-recovering Superconducting Accelerator (MESA) is a cutting-edge facility designed to push the frontiers of particle, hadron, and nuclear physics. It will enable high-precision measurements, including the weak mixing angle at low energies, and contribute to the search for physics beyond the Standard Model. MESA’s two flagship experiments, P2 and MAGIX, will offer crucial insights into nucleon form factors, weak radii of nuclei, and dark matter.
Together with its predecessor MAMI, MESA offers a complementary approach to the high-energy frontier — enabling low-energy precision tests of the Standard Model and advancing our understanding of fundamental interactions. This talk will trace that journey: a chronicle of precision, perplexities, and uncertain tales.

Speaker: Concettina Sfienti (Johannes Gutenberg University)

CSfienti_PSI25.pdf

11:20 Fundamental Symmetry Studies with Radioactive Molecules

Rapid progress in the experimental control and interrogation of molecules is enabling new opportunities to investigate the violation of fundamental symmetries. In particular, molecules containing heavy, octupole-deformed nuclei, such as radium, can offer enhanced sensitivity for measuring parity- and time-reversal violating nuclear properties. In this talk, I will present recent results and perspectives on precision experiments with these exotic species.

Speaker: Prof. Ronald F Garcia Ruiz (MIT)

Ronald_PSI25_4.pdf

11:50 Beta Decay Correlation Program at Spallation Neutron Source

Recent progress in both theory and experiment has left the unitarity of the quark mixing matrix (CKM) somewhat of an open question. The Nab experiment at the Spallation Neutron Source is designed to improve precision of the extraction of the first matrix element and shed light on experimental tensions within the neutron beta decay dataset. Nab’s asymmetric spectrometer allows coincident reconstruction of the decay proton and electron energies, which are used to determine the electron-neutrino correlation coefficient. Additionally, its design lends itself to future use for a program of polarized decay correlation coefficient measurements in the same apparatus.
This talk will present preliminary results from Nab’s first data collection runs, plans for the upcoming campaign and proposed measurements for the future. Finally, we will present an outlook for the program’s sensitivity in tests of CKM unitarity and to new physics beyond the Standard Model.

Speaker: Nadia Fomin (University of Tennessee)

PSI_Fomin_2025.pptx

12:20 The future science program at the Fundamental Neutron Physics Beamline

The Fundamental Neutron Physics Beamline at the Spallation Neutron Source provides intense, pulsed, cold-neutron beams dedicated to fundamental physics. With the termination of the nEDM@SNS experiment, we are proposing a new strategy to develop the beamline into a multi-user facility. This would include upgrades to allow measurements of correlations in the decay of polarized neutrons with the Nab spectrometer at the primary beamline (‘pNAB’), in tandem with new instruments at the secondary monochromatic beamline. Since decay experiments like Nab transmit the majority of the neutron beam, there is potential for an extension of the primary beamline to establish another instrument station, allowing up to three simultaneous experiments.
This presentation will detail these plans and present the outcome of a recent review of the most promising future instruments that could take advantage of this expanded facility.

Speaker: Wolfgang Schreyer (Oak Ridge National Laboratory)

The future science program at the Fundamental Neutron.pptx

12:40 → 14:00 Lunch

14:00 → 15:00 Session: Tu - 3

Convener: Susan Seestrom (Los Alamos National Laboratory, Retired Senior Fellow and Guest Scientist)

14:00 Fundamental Physics with Very-cold and Ultracold Neutrons at the Institut Laue-Langevin

Very slow neutrons are excellent probes for fundamental physics at the precision frontier.
In recent years, the Institut Laue-Langevin (ILL) has been strengthening its corresponding infrastructure: The superthermal UCN source SuperSUN has been commissioned and successfully put into user operation. The work-horse of UCN physics, the instrument PF2, has been modernized, and its VCN-port entirely refurbished.
As a result, new exciting experiments have been performed or are planned for the close future,ranging from neutron interferometry using VCN, advanced searches for mirror neutrons, precision tests of gravity at short distances within the qBounce experiments, as well as the commissioning of the new-generation nEDM-spectrometer PanEDM.

In my talk, I will review news about the instruments as well as some experiments.

Speaker: Tobias JENKE (Institut Laue-Langevin)

2025-09-09_PSI25_UCN.pdf

2025-09-09_PSI25_UCN.pptx

14:30 The TRIUMF Ultra Cold Advanced Neutron Source and EDM Experiment: First UCN detected and EDM spectrometer status

The TUCAN collaboration is building a world-leading ultracold neutron (UCN) source at TRIUMF (Canada Particle Accelerator Center) to conduct a measurement of the neutron’s electric dipole moment (nEDM) with a sensitivity of 10^(-27) ecm. If discovered, it would be a new source of CP violation and contribute to our understanding of the Baryon Asymmetry of the universe as well as theories beyond the Standard Model of Physics.
The UCN source is now being commissioned, which is capable of production rates up to 10^7 UCN/s. Here spallation neutrons are cooled in room temperature heavy water and 20 K liquid deuterium, followed by UCN production in a spherical volume of superfluid liquid helium He-II at 1 K. UCN are extracted from the production volume to the nEDM apparatus using several meters of coated vacuum guides.
In June of 2025 several milestones were achieved where UCN were produced for the first time with the new source, without the liquid deuterium moderator. At a proton irradiation current of 36 uA and 60 sec, ~900k UCN were detected at the exit of the source in 120 sec. This presentation will discuss the recent UCN production and the status and plans of the TUCAN nEDM experiment.

Speaker: Russell Mammei (The University of Winnipeg)

PSI2025_TUCAN_slidesRMammei_final.pdf

15:00 → 15:30 Coffee

15:30 → 16:30 Session: Tu - 4

Convener: Kenji Mishima (RCNP, Osaka university)

15:30 Advances in Ultra-Cold Neutron Science at TRIGA Mainz: Detector development, spin control, simulations, and the τSPECT Experiment

This presentation details recent developments from the Ultra-Cold Neutron (UCN) Physics group at the TRIGA research reactor in Mainz, Germany. We report on the operational performance of the UCN sources, the advancement of various detector systems, and on the implementation and improvement of a dedicated UCN simulation framework supporting the τSPECT neutron lifetime experiment. The novel detectors include a segmented detector enabling spatially-resolved UCN detection, a specialized trigger detector synchronized with the reactor pulse to tag incident UCNs, and a beamline monitoring detector providing relative UCN counts per pulse for normalization. Complementing these, we present developments in neutron spin manipulation, including UCN spin-flipper systems, and precision magnetic field control via custom NMR probes for monitoring field amplitudes and gradients. Finally, we report the successful commissioning of the τSPECT experiment at TRIGA Mainz and its subsequent relocation to PSI in 2023.

Speaker: Sylvain Vanneste (Johannes Gutenberg University Mainz)

SylvainVanneste_PSI2025_v4.odp

SylvainVanneste_PSI2025_v4.pdf

15:50 Recoil-order and radiative corrections to the aCORN experiment

Recoil-order and radiative corrections to neutron decay correlations enter at the 10$^{-3}$ level, important at the precision of recent and future experiments, especially when comparing results for $\lambda = G_A / G_V$. The aCORN experiment obtains the neutron electron-antineutrino correlation ($a$-coefficient) from an asymmetry in proton-electron coincidence events, in contrast to previous experiments that obtained it from the shape of the proton energy spectrum. We show that at recoil order the interpretation of these two methods are quite different. We update the recent aCORN results to include recoil-order and radiative corrections and compare to previous neutron decay experiments.

Speaker: Fred Wietfeldt (Tulane University)

PSI aCORN few.pdf

16:10 Discovery prospects for eEDM and nEDM in the general two Higgs doublet model

Baryon asymmetry of the Universe offers one of the strongest hints for physics Beyond the Standard Model (BSM). Remarkably, in the general two Higgs Doublet Model (g2HDM) that possesses a second set of Yukawa matrices, one can have electroweak baryogenesis (EWBG), while the electron EDM is evaded by a natural flavor tuning that echoes SM. We show that eEDM may first emerge around 10^−30  𝑒 cm or so, followed by neutron EDM (nEDM) down to 10^−27  𝑒 cm. We illustrate a cancellation mechanism for nEDM itself, which in turn can be probed when a facility capable of pushing down to 10−28  𝑒 cm becomes available, followed by neutron EDM (nEDM) which in turn can be probed when a facility capable of pushing down to 10^−28  𝑒 cm becomes available. We illustrate a cancellation mechanism for nEDM itself, which in turn can be probed when a facility capable of pushing down to 10^−28  𝑒 cm becomes available.

Speaker: George W.S. Hou (National Taiwan University)

eEDM and nEDM Discovery Prospects in General 2HDM.pdf

eEDM and nEDM Discovery Prospects in General 2HDM.pptx

16:30 → 19:30 Poster Session and BBQ

16:30 Non-linearity correction of SiPMs in the MEG~II liquid xenon detector

The MEG II experiment is being carried out in the PSI, piE5 area in the experimental hall west. The MEG II aims to search for the charged lepton flavor violation process, $\mu^{+}\rightarrow e^{+}\gamma$. The physics run started in 2021 and will be planned by the end of 2026 with the target sensitivity of branching ratio of $6\times10^{-14}$). In 2025, we published the result with the data collected in 2021and 2022. No signal excess was observed and we set the most strict limit, Br $< 1.5 \times 10^{-13}$ ($90 \%$ C.L.) [1].
Gamma-rays are detected by the liquid xenon detector to reconstruct their energy, timing, and position. In the latest result [1], the energy resolution was $2.4\%$/$1.9\%$ ($w<2$ cm/$2$ cm$<w$) in the 2022 data. The resolution in the shallow region ($w<2$ cm) is worse than that in the 2021 data ($2.1\%$). A possible cause is non-linear response of the MPPC used as a photo-sensor. In the shutdown period between 2021 and 2022 an annealing campaign was conducted to recover the detection efficiency [2]. The detected number of photon in an MPPC in the 2022 run was much larger than the 2021 data and this may cause the non-linear response of the MPPCs.
In this presentation, I will report ideas to improve the energy resolution by correcting the non-linear response and its result.

[1] K. Afanaciev, et. al, arXiv:2504.15711 (2025)
[2] S. Ban, PSI2022, https://indico.psi.ch/event/11742/contributions/38752/ (2022)

Speaker: Sei Ban (ICEPP, The university of Tokyo)

16:31 Determination of Resonance Spin via Low-Energy Gamma-Ray Spectroscopy in Neutron Capture Reactions

A large enhancement of parity-violation via the weak interaction has been observed in nuclear reactions for several nuclei [1]. The enhancement is explained by the mixing of parity-unfavored partial amplitudes in the entrance channel of the compound nuclear states, s-p mixing [2]. The s-p mixing occurs between resonances with the same spin. Therefore, the spin of resonances is an important parameter for understanding the enhancement mechanism.
It has been proposed that the spin of resonances can be determined by measuring the low-energy gamma-ray emitted from compound states formed via neutron capture [3]. This approach utilizes the effect that the intensity of gamma rays emitted from low-lying excited states after multiple cascade transitions varies depending on the resonance spin. Although this concept can be understood qualitatively, no studies have clearly verified whether the observed differences are truly due to spin.
In this study, we aim to verify this by comparing two methods: measuring the intensity ratios of low-energy gamma rays and measuring the polarized neutron transmission through a polarized nuclear target. In this presentation, we report primarily on the measurement and analysis results of low-energy gamma rays and discuss future prospects for measurements using polarized nuclei.

[1] G. E. Mitchell et al., Physics Reports 354, 157 (2001).
[2] V. V. Flambaum and O. P. Sushkov, Nucl. Phys. A 435, 352, (1985).
[3] J. R. Huizenga and R. Vandenbosch, Phys. Rev. 120, 1305 (1960).

Speaker: Dr Shunsuke Endo (Japan Atomic Energy Agency)

PSI2025ポスター_v1.pdf

16:32 Ultracold neutron energy spectrum from magnetic depolarization

The nEDM Collaboration at PSI presents a novel method for extracting the energy spectrum of ultracold neutrons from magnetically induced spin depolarization measurements using the n2EDM apparatus. This method is also sensitive to the storage properties of the materials used to trap ultracold neutrons, specifically, how specular or diffuse is the surface. We highlight the sensitivity of this new technique by comparing the two different storage chambers of the n2EDM experiment, which, due to the geometry, result in different energy spectra. We validate our extraction by comparing to an independent measurement for how this energy spectrum is polarized through a magnetic-filter, and finally, we calculate the neutron center-of-mass offset, an important systematic effect of nEDM measurements.

Speaker: Efrain Patrick Segarra (PSI - Paul Scherrer Institut)

16:33 Chiral EFT Approach to Bremsstrahlung Corrections and Charge Asymmetry in Elastic Lepton-Proton Scattering

We present a detailed analytic evaluation of the soft-photon bremsstrahlung radiative corrections to the unpolarized elastic lepton-proton scattering cross-section within the framework of low-energy chiral effective field theory. Our study is motivated by the precision goals of the MUSE experiment, which aims to resolve the proton radius puzzle via simultaneous measurements of $e^\pm p$ and $\mu^\pm p$ scattering at low energies. In particular, we employ SU(2) heavy baryon chiral perturbation theory (HBChPT), a model-independent approach to the pion-nuclear sector, incorporating gauge-invariant couplings to photons. All possible next-to-leading order (NLO) contributions to the ${\mathcal O}(\alpha^3/M)$ cross-section are systematically included. The infrared divergences from soft bremsstrahlung emission are isolated using dimensional regularization and are shown to cancel precisely against the divergent parts of the virtual loop corrections, following the standard Mo-Tsai procedure. A significant component of the bremsstrahlung corrections arises from the interference between lepton and proton radiation, which, together with the analytically computed NLO two-photon exchange (TPE) contributions, constitutes the charge-odd radiative effects. These corrections are particularly relevant for charge asymmetry observables extracted from elastic lepton--anti-lepton scattering off the proton. Accordingly, we also provide a theoretical analysis of the charge asymmetry at NLO accuracy. All results are derived beyond the ultra-relativistic limit by retaining finite lepton mass effects in light of the low-energy regime accessible by the MUSE experiment.

Speaker: Ms Bhoomika Das (Department of Physics, Indian Institute of Technology Guwahati)

16:34 Nuclear structure studies close to shell closure N = 126 using quasiparticle-phonon plus rotor method

Abstract
The analysis of the heavy and very heavy nuclei with, particularly, an extension into the domain of exotic and superheavy nuclei is in the center of the contemporary research in low energy subatomic physics. This research program, currently going on the biggest laboratories in the world, is motivated from the theoretical calculations which predict the existence of an island of stability for superheavy nuclei beyond Z = 82 for protons and N = 126 for neutrons. To study the structure of heavy and superheavy nuclei, we need to produce a handful of events and therefore provide a detailed spectroscopic data. Owing to the Key words extreme limit of current capabilities and weak production cross sections close to 1 nb, the structure of heavy and very heavy nuclei could help us to understand the structure and stability of superheavy nuclei since such shell properties may be a consequence of nuclear deformation.

Speaker: Youssra Elabssaoui (Physique Nucléaire)

16:35 Precision Spectroscopy of Low-Lying States in Muonic Boron with MMC Detectors

The spectroscopy of light muonic atoms offers a powerful tool for probing nuclear structure with high precision. By studying X-ray transitions, particularly low-lying states such as the 2p–1s transitions, it is possible to extract absolute nuclear charge radii with high accuracy.
However, measuring these transitions for low-Z nuclei in the 20–150 keV energy range remains challenging, primarily due to the limited energy resolution of conventional solid-state detectors. These measurements are essential for improving theoretical models, validating predictions of bound-state QED, and exploring physics beyond the SM.
To overcome these limitations, the QUARTET collaboration employs metallic magnetic calorimeters (MMC) to perform high-precision X-ray spectroscopy of muonic atoms. This presentation will describes the spectroscopy methods developed for light muonic atoms, details the experimental setup, and presents preliminary results for muonic boron obtained with MMC detectors.

Speaker: Aziza Zendour (PSI)

PSI2025_pdf.pdf

16:37 Study on the Radiation Damage of VUV-sensitive MPPC in Liquid Xenon

A Multi-Pixel Photon Counter (MPPC) sensitive to vacuum ultraviolet (VUV) light, called VUV-MPPC, is used in the liquid xenon (LXe) gamma-ray detector for the MEG II experiment. In the MEG II runs with high intensity muon beam, the degradation of VUV-MPPC's photon detection efficiency (PDE) to VUV light was observed. The cause of PDE degradation is considered due to a surface damage of VUV-MPPC by the irradiation of the muon beam. The plausible candidate radiation sources make the radiation damage are gamma-ray and VUV photon but we couldn’t specify which radiation sources make that damage yet. To elucidate the cause of the PDE degradation, we irradiated a VUV-MPPC with the comparable amount of VUV photons as in the MEG II experiment in LXe. In this presentation, we present the results of the reproduction test of PDE degradation for the VUV-MPPC.

Speaker: Ryusei Umakoshi (University of Tokyo)

PSI2025_poster_umakoshi.pdf

16:38 Neutron-to-mirror-neutron oscillations in an ultracold neutron beam

The concept of “mirror matter” has been postulated in various terms since the 1950s. The modern formulation supposes that every Standard Model particle has a partner with opposite chirality, in order to restore parity symmetry in the weak interaction. Neutrons are of particular interest because their lack of electric charge allows for the possibility of mixing between the ordinary and mirror forms. Observation of such a phenomenon would have implications for baryogenesis, dark matter, and even cosmic rays. We present an overview of an experiment performed in May–July 2024 at the PF2 ultracold neutron (UCN) facility of the Institut Laue-Langevin. The experiment aims to search for evidence of neutron-to-mirror-neutron oscillations in a UCN beam, under the influence of a magnetic field in the range 1–10 mT. Neutrons were counted using one of the fast-response gaseous detectors designed and built for the n2EDM experiment, known as GADGET.

Speaker: Daniel Galbinski (LPC Caen)

2025_09_09 - PSI2025 Poster.pdf

16:39 Characterization of the Swiss Spallation Neutron Source as a Site for Neutrino Experiments

Medium-energy, short-baseline neutrino experiments play a crucial role in testing both the Standard Model and physics beyond it. In recent years, pulsed neutron spallation sources have emerged as promising venues for such investigations. The Swiss Spallation Neutron Source (SINQ) at PSI presents another bright neutrino source, however, with almost continuous neutrino production due to the 50 MHz time structure of the proton beam. We outline a comprehensive roadmap toward a state-of-the-art computational characterization of SINQ as a potential neutrino experiment facility using Geant4.

Speaker: Sergey Konstantin Ermakov (ETH Zürich)

16:40 Gravity induced CP violation in neutral mesons experiments

The impact of earth's gravity on neutral mesons dynamics is analyzed. The main effect of a Newtonian potential is to couple the strangeness and bottomness flavor oscillations with the quarks zitterbewegung oscillations (arXiv:2503.09465). This coupling is responsible of the observed CP violations in the three types of experiments analyzed here: (i)indirect violation in the mixing, (ii) direct violation in the decay to one final state and (iii) violation in interference between decays with and without mixing. The three violation parameters associated with these experiments are predicted in agreement with the experimental data. The amplitude of the violation is linear with respect to the strength of gravity so that this new mechanism allows to consider matter dominated cosmological evolutions providing the observed baryon asymmetry of the universe.

Speaker: jean-marcel rax (University of Paris)

PSI 2025 gravity induced CPV.pdf

PSI2025raxCPVgravity.pdf

16:41 Status of the magnetic shield for the muEDM experiment

The muEDM collaboration is aiming at measuring the electric dipole moment of the muon with unprecedented sensitivity of $\sigma (d_\mu) = 6 \cdot 10 ^{23} \space \mathrm{e\space cm}$ at the Paul Scherrer Institute. The experiment uses the frozen-spin technique inside a 3-T superconducting solenoid magnet. One of the key parts of the experiment is the superconducting injection channels. They create magnetic field free regions, which are used to transport muons through the fringe field into the uniform magnetic field inside the bore of the solenoid. The injection channels are using superconducting Nb-Ti/Nb/Cu layered sheets rolled into a cylinder with a slit. During operation, the injection channels must be cooled to around 4.5 K temperature. We present the current status of the superconducting magnetic shield for the phase I experiment, aiming at the first demonstration of the frozen spin technique with a sensitivity of $ 4 \cdot 10^{-21} \space \mathrm{e} \space \mathrm{cm} $, which is currently being assembled at PSI.

Speaker: Pranas Juknevicius (PSI - Paul Scherrer Institut)

16:42 Testing QED and Beyond with ortho-Positronium

Positronium, a bound state composed of an electron and a positron, is a pure lepton system. Depending on the total spin, there are para-positronium (p-Ps) and ortho-positronium (o-Ps), which eventually annihilate into two photons and three photons, respectively. Due to minimal hadronic effects, experimental measurement of the continuous spectrum of three-photon annihilation of o-Ps can be used to verify the accuracy of high-order perturbation corrections in bound-state quantum electrodynamics (QED). In terms of theoretical calculations, Adkins in his two articles from 2005 [1] and 2000 [2] theoretically calculated the first-order correction O(𝛼) and the second-order correction O($𝛼^2$) of the three-photon annihilation energy spectrum of o-Ps. In 2014, an experiment at the University of Tokyo obtained a result [3,4] consistent with theoretical calculations [1], but with large uncertainties. This experiment aimed to measure the three-photon annihilation energy spectrum of o-Ps with higher experimental accuracy. At the same time, through Geant4 simulation, the theoretical energy spectrum, including O(𝛼) and O($𝛼^2$) corrections, was generated and compared with the experimental data to verify the degree of conformity. We will present the current status of simulations and experiments, as well as the future goals of research and measurements.
[1] G. S. Adkins, Analytic Evaluation of the Amplitudes for Orthopositronium Decay to Three Photons to One-Loop Order, Phys. Rev. A 72, 032501 (2005)
[2] G. S. Adkins, R.N. Fell, J Sapirstein, Order A2 Corrections to the Decay Rate of Orthopositronium, Phys. Rev. Lett. 84, 22 (2000)
[3] S. Adachi, First Verification of Higher-Order Corrections in the Energy Spectrum of Orthopositronium Decay Gamma Rays, Master Thesis, University of Tokyo (2015)
[4] S. Adachi et al., J. Phys.: Conf. Ser. 618 012007 (2015)

Speaker: Beining Rao (Shanghai Jiao Tong university)

16:43 Application of Transformer-Based Machine Learning Architecture to the Positron Spectrometer Data Analysis in the MEG II Experiment

The MEG II experiment, conducted at PSI from 2021 and planned through 2026, targets a $\mu\to e\gamma$ search with a sensitivity of $6\times 10^{-14}$ to the muon branching ratio. While the experiment has already demonstrated the potential to reach the target sensitivity, improvements in the reconstruction and analysis techniques will enhance the sensitivity beyond the target value.

This study focuses on the positron spectrometer, designed to detect 52.8 MeV positrons from muon decays. A major challenge in the positron reconstruction has been the performance degradation due to the high pileup environment. To address this, a Transformer-based machine learning algorithm was introduced, implemented as a classifier to filter out noise hits. By improving the signal hit purity, the Transformer led to an O(10%) increase in the positron tracking efficiency.

This presentation will detail the design and integration of the Transformer within the positron tracking framework. This will then be followed by an evaluation of the resulting tracking improvements and their expected impact on the sensitivity of the experiment.

Speaker: Atsushi Oya (University of Tokyo)

PSI2025.pdf

16:44 Impact of NNLO QED vs. TPE corrections in lepton-proton scattering at MAMI

McMule (Monte Carlo for Muons and other Leptons) is a powerful tool for fully differential higher-order QED calculations of scattering and decay processes involving leptons. It provides different type of observables such as cross-sections and branching ratios.
In this work, we use McMule to study the process of lepton-proton scattering up to and including next-to-next-to-leading order (NNLO) QED corrections. One important contribution at next-to-leading order (NLO) is the two-photon-exchange (TPE) correction, which is the main focus of this work. We present results for both elastic and inelastic TPE, including associated uncertainties, and compare their size to the subleading NNLO corrections. Finally, we make a comparison between the McMule prediction and older experimental data from electron-proton scattering experiments conducted in Mainz by the A1 collaboration.

Speaker: Matteo Ronchi (JGU)

PosterPSI2025FinalVersion.pdf

16:45 Status of the neutron decay experiment PERC

The decay of free neutrons is a powerful tool for precision tests of the Standard Model of particle physics. Correlation coefficients - such as the beta asymmetry $A$ and the Fierz interference term $b$ - serve as input for the determination of the CKM matrix element $V_{ud}$ and for searches for (effective) scalar and tensor as well as right-handed couplings.
The neutron decay spectrometer PERC (Proton Electron Radiation Channel), which is set up at the research reactor FRM II in Garching, Germany, aims to improve the accuracy of several correlation coefficients by up to one order of magnitude. PERC consists of a 12 m long superconducting magnet system, in which the neutron beam is contained by a non-depolarizing neutron guide. The magnetic field guides electrons and protons produced in the neutron decay towards the main detector, which will initially be a scintillation detector with photomultiplier tube readout. A second detector system, which consists of a scintillator read out by silicon photomultipliers, is installed in the upstream area of PERC and allows to identify backscatter events.
The poster gives an overview of PERC and presents the current status.

Speaker: Lilli Löbell (Technical University of Munich)

PSI2025_Lilli_Loebell.pdf

16:46 Recent results from CUORE and path towards CUPID

The search for neutrinoless double beta decay (0νββ) is fundamental for investigating lepton-number violation, probing new physics beyond the Standard Model, and determining whether neutrinos are Majorana particles. CUORE, a cryogenic calorimetric experiment at LNGS, studies 0νββ in $^{130}$Te using 988 TeO₂ crystals, reaching a tonne-scale mass and operating below 15 mK. Since 2017, CUORE has accumulated over 2.5 tonne-years of exposure, constraining 0νββ in $^{130}$Te and achieving one of the most precise two-neutrino double beta decay (2νββ) half-life measurements and a detailed background reconstruction across a broad energy range. These results provide essential nuclear physics benchmarks for 0νββ searches. Building on CUORE’s success, CUPID (CUORE Upgrade with Particle ID) aims to significantly enhance its 0νββ discovery sensitivity to 10$^{27}$ yr in $^{100}$Mo, covering the Inverted Hierarchy of neutrino masses. It will deploy in total 240 kg of $^{100}$Mo in 1596 enriched Li₂MoO₄ crystals. 1710 light detectors with Neganov-Trofimov-Luke amplification will enable simultaneous heat and light readout for enhanced background rejection, particularly against $\alpha$ surface contamination and ββ pileup. CUPID will reuse CUORE’s cryostat and infrastructure. Current efforts focus on detector performance validation, sensitivity studies, and finalizing the experimental design to maximize physics reach. This work presents the latest CUORE results and outlines the key milestones towards CUPID’s realization.

Speaker: Nicola Manenti (University of Pavia - INFN Pavia)

16:47 Beamline and magnetic field preparation for correlation coefficients measurements in the BRAND experiment

BRAND is a precision experiment that will investigate free polarized neutron beta-decay [1] at the PF1B cold neutron beamline of the Institut Laue-Langevin (ILL), which offers the world’s highest cold neutron flux [2]. The experiment will perform simultaneous measurements of 11 correlation coefficients in neutron beta-decay [3], including five that have never been measured before. This enables a sensitive search for possible scalar or tensor interactions beyond the Standard Model through the analysis of transverse elec-tron polarization. The first beamtime for the BRAND-2 setup is planned in the first half of 2026, and will require both uniform magnetic field and targeted beamline preparations.

Neutrons are polarized by PF1B’s supermirror polarizer which provides beam-averaged polarization up to 99.7% [4]. A highly uniform magnetic field of low magnitude is required within BRAND’s fiducial volume in order to, on one hand, align and preserve the neutron polarization, and, on the other hand, to minimize deflections of charged decay products, facilitating accurate reconstruction of their trajectories. To achieve this, an Active Magnetic Shielding system (AMS), similar as [5,6], will fully enclose the BRAND detection apparatus. The AMS will actively compensate for static magnetic perturbations – such as the Earth's mag-netic field – as well as dynamic fluctuations that can be caused by nearby instruments at the ILL. Addi-tionally, efficient spin transport is necessary between the polarizer, which operates with a transversal field, and the decay chamber, where a longitudinal field is needed.

BRAND aims for precision measurements of correlations between the momenta of decay products and the spins of both the neutron and the electron, requiring a high and precisely known neutron polarization. Measuring the polarization of the neutron, and conversely, deriving it from the measured beta asymmetry A, allows for cross-checks of the beam polarization. This makes neutron polarization one of the critical tools to validate the BRAND-2 setup, and assess systematic effects.

This poster presents the simulation results of the final design of the Active Magnetic Shielding system built for the BRAND experiment. It also includes results from McStas simulations of the PF1B beamline, aimed at improving the accuracy of the beamline model. Finally, the preliminary design of the beamline for the BRAND-2 setup, and the polarization measurement plans will be presented.

References
[1] K. Bodek et al., EPJ Web Conf. 262, 01014 (2022).
[2] H. Abele et al., Nucl. Instr. Meth. A 562, 407 (2006).
[3] J.D. Jackson et al., Physical Review 106.3, 517 (1957).
[4] A. K. Petukhov et al., Rev. Sci. Instrum. 94, 023304 (2023).
[5] N. J. Ayres et al., EPJ C 81.6, 512 (2021).
[6] S. Afach et al., J. Appl. Phys. 116, 084510 (2014).

Speaker: Clément Desalme (Institut Laue-Langevin)

Beamline_and_magnetic_field_preparation_for_correlation_coefficients_measurements_in_the_BRAND_experiment.pdf

16:48 Performance evaluation of LYSO crystals for energy and timing measurements in a gamma-ray pair spectrometer for future $\mu\to e\gamma$ experiment

The process $\mu \to e\gamma$ is a charged lepton flavor violating (CLFV) decay that is forbidden in the Standard Model, but its measurable branching ratio is predicted by several new physics models. The current experimental limit on this decay has been set by the MEG II experiment at PSI, which will continue data collection until the end of 2026 with the target sensitivity of $Br < 6 \times 10^{-14}$ (90% C.L.). With the increased muon beam rate available at PSI starting in 2028, discussions are underway for a future experiment aiming to search for $\mu \to e\gamma$ with a sensitivity of $\mathcal{O}(10^{-15})$.
Achieving this level of sensitivity requires the development of photon detectors with excellent resolution and high rate capability. The number of background events increases with decreasing photon energy resolution, making this a critical factor. Therefore, the use of a pair spectrometer for photon detection, which offers better resolution and higher rate capability than traditional calorimeters, is under consideration.
The photon pair spectrometer works by converting photons into electron-positron pairs in a converter, followed by measurement of their momentum, position, and timing. To achieve optimal resolution with reasonable conversion efficiency, an active material for the converter is essential to allow energy deposit measurement. LYSO:Ce is a promising candidate due to its excellent properties, such as large light yield and fast response.
To evaluate the performance of a converter prototype consisting of LYSO and SiPM readout, a 3 GeV electron beam test was conducted at the KEK PF-AR Test Beam Line. Furthermore, to optimize the design of the active converter, comparisons were made between different LYSO sizes and SiPM readout methods, focusing on light yield and timing resolution.
The results show that the active converter using LYSO achieved excellent timing resolution (20–25 ps) across the entire crystal region, as well as detection of several thousand photoelectrons, exceeding the requirements for an active converter (40 ps and 700 photoelectrons). Therefore, LYSO is identified as a suitable material for the active converter of the photon pair spectrometer in the next-generation $\mu \to e\gamma$ search experiment.

Speaker: Rei Sakakibara (the University of Tokyo)

PSI2025poster_sakakibara.pdf

16:49 Mott Polarimeter for Neutron Correlation Coefficients Measurement

The BRAND experiment is designed to search for hints of physics beyond the Standard Model. This objective is achieved through the precise study of neutron beta decay. Generally, in neutron beta decay, the emitted electron is longitudinally polarized due to the vector and axial couplings inherent in the Weak decay theory. Any deviation from the electron's longitudinal polarization (except electromagnetic corrections) would indicate the presence of new physics. Various correlation coefficients can be constructed from the spin, momentum, and energy of the decay products and the neutron itself. These correlation coefficients exhibit varying sensitivities to scalar and tensor couplings. The 2nd phase of experiment (BRAND-II) will simultaneously measure 11 correlation coefficients: a, A, B, D, H, L, N, R, S, U, and V. Seven of these (H, L, N, R, S, U, V) depend on the transverse polarization of the electron; among these, only N and R have been previously addressed experimentally. The remaining five will be determined for the first time by the BRAND-II experiment. The simultaneous measurement of coefficients a, A, B, and D will help in controlling systematic errors. Furthermore, it will be insightful to extract these coefficients using an experimental technique different from those previously employed.

In this experiment the transverse electron polarization of electrons emitted in the decay of cold neutrons will be measured by leveraging the Mott scattering process. The BRAND-II Mott polarimeter consists of a Multi-wire Drift Chamber (MWDC) to track an electron, and a plastic scintillator to measure the energy of it. A mixture of helium and isobutane will be used in the MWDC at atmospheric pressure. Currently, one segment of the BRAND-II experiment is under construction. The results from the characterization of the BRAND-II Mott polarimeter, such as the 3D position resolution of the MWDC and the energy resolution of the plastic scintillator, will be presented.

Speaker: Mr Shantanu Bhalerao (Jagiellonian University, Institute of Physics)

16:50 The Mu3e Commissioning Run at PSI in 2025

The Mu3e experiment at the Paul Scherrer Institute (PSI) will search for the charged lepton flavour violating decay µ⁺ → e⁺e⁻e⁺, improving the current best limit set by the SINDRUM experiment by four orders of magnitude.

Mu3e will be conducted in two phases. Phase I, currently under construction at the πE5 beamline at PSI, will utilise an intense DC surface muon beam of 10⁸ µ⁺/s to reach a sensitivity of 2 × 10⁻¹⁵. Phase II will exploit the future High-Intensity Muon Beam (HIMB) to push this further to the 10⁻¹⁶ level. This improvement is made possible by combining high-intensity muon beams with a low-material-budget tracking system based on ultra-thin HV-MAPS silicon pixel detectors, fast scintillating fibre and tile detectors for sub-ns timing resolution, and a high-rate data acquisition system. Operating in a 1 T solenoidal magnetic field, the detector is optimised for the µ⁺ → e⁺e⁻e⁺ signature, enabling precise reconstruction of the decay vertex and invariant mass of the three final-state particles.

A commissioning run campaign was conducted in June 2025 at the PSI πE5 beamline as a key step in preparations for Phase I data-taking. This campaign successfully validated critical detector components - including vertex, scintillating fibre, and tile modules - and demonstrated their integration with the high-intensity muon beamline under a 1 T magnetic field. These results represent a major milestone towards readiness for Phase I measurements.

This contribution will present updates and the first results from the recent commissioning run campaign at PSI.

Speaker: Mikio Sakurai (University College London)

PSI2025_Mu3eCommissioningRun2025_MikioSakurai.pdf

16:51 Towards optimal data flow in high-energy physics: achieving determinism and real-time in the Mu3e experiment

The Mu3e experiment, searching for charged lepton flavor violation in the µ⁺ → e⁺e⁻e⁺ channel with 2*10^-15 sensitivity in Phase I, is under commissioning at PSI PiE5 beamline. To achieve this, Mu3e, operating with the world’s most intense continuous muon beam, must handle multi-terabit-per-second data streams from millions of detector channels. Meeting this challenge requires a triggerless DAQ system that achieves extreme throughput, strict determinism, and zero data loss, known as the “impossible triangle,” which has never been achieved in the history of networking or high-energy physics.

We present an FPGA datapath architecture with enhanced data flow dynamics, designed to sustain line-rate processing while maintaining temporal order. This design directly addresses timestamp bursts caused by recurling particles, which can strike multiple detector layers within nanoseconds and generate clusters of nearly simultaneous events. Such bursts, when handled with conventional round-robin arbitration, lead to timestamp reordering and bufferbloat due to micro-burst traffic. Our approach provides a generic and scalable solution to maintain strict temporal ordering in high-throughput environments, offering a robust path forward for future high-rate experiments, such as Mu3e Phase II.

Our work raises broader questions at the frontier of data acquisition: 1) How can true line-rate, i.e. O(1), sorting and scheduling be achieved in hardware—and extended, perhaps, even to software? 2) Why is determinism, i.e., data arriving with precise timing at designated ports, fundamental for correctness in high-energy physics experiments? 3) Why do data loss and reordering persist despite the absence of throughput bottlenecks? By addressing these challenges, the Mu3e DAQ system aims not only to enable Phase I and Phase II physics goals, but also to inform the wider academic community, bridging high-energy physics and computer networking, in the pursuit of optimal, real-time data flow.

Speaker: Yifeng Wang (ETH Zurich)

PSI2025_poster_YifengW.pdf

16:52 Spin-selective detection and manipulation of ultracold neutrons

Ultracold neutron (UCN) experiments suffer from low counting statistics, especially in precision measurements such as searches for the neutron electric dipole moment (nEDM). In-situ experimental designs, where all measurement and detection steps occur within a superthermal UCN source, have the potential to significantly increase the usable UCN density. Such approaches require novel detector concepts adapted to the in-situ environment.
In this work, we describe a spin-selective detector concept that combines localized magnetic fields, generated by superconducting microstructures, with a neutron-absorbing multilayer. In the presence of a magnetic field, high-field-seeking neutrons have an increased probability to penetrate a reflecting layer and reach a neutron absorber underneath. The corresponding probability for absorbing low-field-seeking neutrons is significantly lower, which creates an effective window for spin discrimination.
This detector concept has the potential to contribute to overcoming certain limitations in current nEDM experiments and enables new strategies in quantum sensing, UCN storage, and quantum computation with neutrons.
We present the current development status, initial characterization results, and outline future measurement plans using polarized neutron reflectometry.

Speakers: Josef Tremmel, Tim Sandmann

16:53 The Tracking Detector for the P2 Experiment

The P2 Experiment at the new Mainz Energy-Recovering Superconducting Accelerator (MESA), which is currently under construction in Mainz, will measure the weak mixing angle in elastic electron-proton scattering at low momentum transfer with unprecedented precision.

A key parameter for the analysis, the momentum transfer Q², is measured by a tracking detector designed for high rates, radiation hardness, and a low material budget.
It consists of 4 identical modules arranged in two layers, with each module containing two sensor planes, populated with novel HV-Monolithic Active Pixel Sensors glued and wire-bonded on rigid-flex strips.
The mechanical, electrical, and cooling designs have been developed and are currently undergoing testing. For this purpose, a scaled-down prototype has been constructed.
The readout features a radiation-hard frontend built around CERN's lpGBT ASIC and an FPGA-based backend.

The poster presents the design of the tracker and the associated readout electronics.

Speaker: Lucas S. Binn

poster_p2_tracking_detector.pdf

16:54 Simulation study of background and systematic effects for the muEDM experiment at PSI

At PSI a high-precision experiment is being set up to search for the muon electric dipole moment (muEDM) employing the frozen-spin technique. A muEDM larger than the Standard Model prediction would be a sign of new physics. The search will eventually improve the current best direct limit by three orders of magnitude to $6\cdot 10^{-23}$ e$\cdot$cm. The EDM signal is measured by detecting the change in emission asymmetry of decay positrons from stored muons in the bore of a magnet. Muons that cannot be stored are stopped after injection, giving rise to background events. A simulation was set up to study the discrimination of signal events against background events. Furthermore, systematic effects must be controlled and are investigated in simulations, such as an apparent EDM due to a non-zero electric field $E_z$ along the axis of the magnet.

Speaker: David Hoehl (PSI - Paul Scherrer Institut)

PSI2025 - Study of Signal-to-Background Discrimination in the muEDM experiment.pdf

16:55 Efficiency of the neutron spin transport system in the n2EDM experiment

The n2EDM experiment of an international collaboration at the Paul Scherrer Institute (PSI) aims to improve the sensitivity of the measurement of the neutron electric dipole moment by a factor of ten with respect to its predecessor. A most efficient neutron spin transport is vital to achieve such a sensitivity. We present an overview of the spin transport coil system, the three main channels of neutron depolarization in the apparatus, and preliminary results on the performance.

Speaker: Gian Caratsch (ETH Zürich)

16:56 The Mu3e Data Acquisition System

The Mu3e experiment is designed to search for
the lepton flavor violating decay $\mu^+ \rightarrow e^+e^-e^+$.
The aim of the experiment
is to reach a branching ratio sensitivity of $10^{-16}$.
The experiment is located at the Paul Scherrer Institute (Switzerland)
and an existing beam line providing $10^8$ muons per second will allow
to reach a sensitivity of a few $10^{-15}$ in the first phase of the experiment.
The detector utilizes thin High-Voltage Active Monolithic Pixel Sensors
for precise position measurement
and scintillating fibre and tile detectors for precise time measurement.
In a first phase of experiment the total data rate will reach 100 Gbit/s.
This work will present Mu3e DAQ system
where this large stream of data from all detectors is sorted and merged.
The final stream of data is passed to GPU filter farm
where full track and vertex reconstruction is performed
to allow for reduction in data rate by factor of 100
for subsequent permanent storage.

Speaker: Dr Alexandr Kozlinskiy (Mainz University KPH)

16:57 Towards Charge conjugation symmetry test in Electromagnetic Interaction using J-PET

Charge conjugation symmetry (C symmetry) still remains a fundamental symmetry in the realm of physics. It is well-known to be maximally violated in weak interactions. However, its validity is yet to be tested in Electromagnetic (EM) and Strong interactions. With the aim to test this symmetry in EM interactions, the forbidden decay channel of the triplet Positronium state - the ortho-Positronium (oPs) shall be explored. The C symmetry forbids this state from decay into anything other than an odd number of photons; henceforth a search for four-photon decay extends the feasibility of testing the C symmetry in EM interaction using a J-PET detector. Furthermore, the bosonic nature of photons hints at a distinct configuration in the event of a C-symmetry violation. Known for its outstanding timing ($\sim$ $ 250$ ps) and angular ($\sim$1 $^{\circ}$) resolutions, J-PET offers a viable and substantial platform to perform this symmetry test. J-PET series of detectors has previously established its credibility in the tests of discrete symmetries, further supporting the feasibility of the aforementioned test. In this presentation, the motivation behind the study, the theoretical assumptions, and recent advancements in the test of C symmetry using J-PET shall be discussed.

Speaker: Pooja Tanty (Jagiellonian University)

16:58 Bayesian Optimization and Real-Time Control in the Muon EDM Experiment at PSI

Designing high-precision particle physics experiments involves optimizing over complex, computationally expensive simulations, often under significant uncertainty—particularly in inputs such as magnetic field maps. In the Muon EDM experiment at PSI – which aims to measure the Electric Dipole Moment (EDM) of the muon using the frozen spin technique – the injection of muons into the experiment is very sensitive, requiring expensive simulations to be optimized.

We apply Bayesian optimization with Gaussian processes to efficiently optimize the experimental parameters to maximize injection efficiency. This approach enables sample-efficient optimization, quantifies uncertainty, and provides tolerance estimates for the experimental parameters. However, due to potential deviations in the magnetic field, a purely feed-forward design strategy is insufficient. To bridge this sim-to-real gap, we are developing real-time feedback control strategies that adaptively tune operational parameters—such as coil currents and injection timing—during runtime.

This hybrid approach enables optimization of the experimental geometry offline, while robustly compensating for real-time fluctuations, improving the reliability and performance of the experiment.

Speaker: J. Alexander Jaeger (PSI - Paul Scherrer Institut)

16:59 Search for the Fierz Interference Term with PERKEO III

Observables of neutron decay are, among others, the $\beta$-asymmetry $A$ and the Fierz interference term $b$. Through precision measurements of $A$ we have access to the CKM matrix element $V_{ud}$, while a non-zero Fierz term $b$ would imply the existence of scalar or tensor interactions beyond the V-A theory of the Standard Model.

The currently most precise direct determinations of $A$ and $b$ from $\beta$-spectrum measurements were obtained with the PERKEO III experiment at the ILL PF1b facility. These results are based on the 2009 measurement campaign, using a combined fit to the experimental $\beta$-asymmetry. To control major systematic effects, we used a pulsed beam of cold neutrons. This beam is guided into the 2 m long decay volume of the experiment, in which some of the neutrons decay. The charged particles from the decay follow the magnetic field toward one of two scintillation detectors with PMT readout. 

A subsequent measurement campaign in 2019/20 aimed to measure the electron spectrum from unpolarized neutrons to extract an improved limit for the Fierz interference term $b$. This method offers higher statistical sensitivity but is systematically challenging. We present experimental details as well as the status of the ongoing analysis. Our current focus lies on characterizing the readout electronics and incorporating a comprehensive model of secondary particles induced by our calibration device into the analysis chain.

Speaker: Anna Schubert (TUM)

17:00 Muonic Atom Spectroscopy of U-238 for the Extraction of Nuclear Properties

Nuclear charge radii can be determined utilizing muonic atom spectroscopy. Muonic atoms are easily formed by directly stopping negative muons inside a material. Muons are 207 times heavier than electrons and consequently orbit 207 times closer to the nucleus, making them highly sensitive to nuclear properties.

The muX experiment aims to determine the absolute nuclear charge radius of Radium-226 using muonic atom spectroscopy. However radioactive isotopes are usually only available in microscopic quantities. To address this, the muX collaboration developed a novel technique based on transfer reactions in a high-pressure hydrogen/deuterium gas mixture.

Once captured by the target material, the muons cascade down to their ground state, emitting characteristic X-rays. The energies of these X-rays reveal the muonic energy level scheme, which provides insights into properties such as the nuclear charge radius, quadrupole moment, and magnetic moment.

In the case of Uranium-238 ($^{238}$U), muonic atom spectroscopy was performed to extract its nuclear properties. The muonic $^{238}$U spectrum was analyzed, with a focus on studying cascade behaviors associated with both direct and transfer muon capture. Notably, direct muon capture exhibited a preference for transitions from $(n, l=n-1)$ to $(n-1, l=n-2)$ states compared to transfer muon capture, consistent with cascade simulations.

Speaker: Anastasia Doinaki (PSI - Paul Scherrer Institut)

17:01 The PanEDM Experiment: Commissioning Progress and Subsystem Overview

The PanEDM experiment, coupled to the new ultracold neutron source SuperSUN at the Institut Laue-Langevin, aims to measure the neutron electric dipole moment (nEDM) with a sensitivity of $4 \times 10^{-27}$ e·cm after 100 beam-days in its first phase.

The search for a CP-violating electric dipole moment is among the most powerful and long-standing precision tests of the Standard Model, and stringently constrains new physics scenarios. A finite nEDM violates time-reversal symmetry and, under CPT conservation, implies CP violation - an essential ingredient for explaining the observed baryon asymmetry of the Universe. The nEDM remains today among the best-motivated experiments in particle physics, delivering constraints at and above the energy scales accessible with colliders.

PanEDM employs trapped ultracold neutrons (UCN) that can be stored for hundreds of seconds, during which coherent spin precession will be driven within a tightly controlled magnetic environment. Key elements of the apparatus include: a compact magnetically shielded room with a mHz shielding factor of 6 million; a nonmagnetic vacuum chamber made from laminated glass-fiber composite; a low-energy UCN spectrum enabling long storage times with high density; and low-noise Cs magnetometers with stability below 50 fT for integration times between 70 and 600 s. PanEDM will operate without a comagnetometer in phase I, exploring the limits of magnetic field control and stabilization.

The high-density UCN source SuperSUN is based on a superfluid-helium conversion medium that, unlike flux sources of UCN, enables storing high neutron densities for long times on the order of several hundred seconds. Its recently demonstrated in-situ storage density of 273 cm$^{-3}$ is the highest achieved to date, and opens a path towards several further generations of statistics improvements based on the same production mechanism.

Speaker: Dr Kseniia Svirina (Institut Laue-Langevin)

poster_Svirina_PSI2025_vFinal_.pdf

17:02 The Mu3e Scintillating Fiber Detector

The goal of the Mu3e experiment is to search for charged LFV in the muon decay $\mu^+ \to e^+ e^- e^+$. The improvement of the sensitivity by 4 orders of magnitude compared to the limit set by the PSI SINDRUM collaboration 40 years ago, drives the need to suppress all sources of backgrounds to a level well below $10^{-16}$. Accidental backgrounds can be strongly rejected by requiring very precise timing.
This will be achieved with a scintillating fiber detector (SciFi) in conjunction to scintillating tiles. The SciFi detector consists of 12 fiber ribbons made by staggering three layers of 250 $\mu\text{m}$ round scintillating fibers, each with a time resolution of $\sim 250\,\text{ps}$, an efficiency of $\sim 98\%$, and a thickness of $\sim0.2\% \,X_{0}$ to minimize multiple scattering.
Each fiber ribbon is read out at both ends by 128 channel SiPM arrays coupled to MuTRiG ASICs, which are able to withstand single channel rates of up to $10^{6}$ hits per second expected at $10^{8}$ muon stops per second during the phase I of the experiment.
This poster will provide an overview of the features of the SciFi detector.

Speaker: Gentian Shatri

posterPSI2025_GentianShatri.pdf

17:03 The free neutron lifetime experiment τSPECT

The neutron storage experiment $\tau$SPECT aims to measure the free neutron lifetime, an essential input for precision tests of the Standard Model of particle physics and the Big-Bang nucleosynthesis, by confining ultracold neutrons (UCNs) in a three-dimensional magnetic trap. In contrast to material bottles, magnetic storage avoids interactions with the trap wall, eliminating systematic biases on the measured lifetime related to wall interactions. The magnetic bottle is generated using a cylindrical Halbach octupole array of permanent magnets combined with superconducting coils for axial confinement. A spin-flip based loading scheme is used to fill the trap. After a variable storage period, surviving UCNs are counted in-situ using a SiPM based detector capable of operating in high magnetic fields. This poster provides an overview of the $\tau$SPECT experiment, highlighting its most critical components and preliminary results from the 2024 measurement campaign at Paul Scherrer Institute.

Speaker: Julian Auler (Johannes Gutenberg University Mainz)

17:04 Calibration of the MEG II Pixelated Timing Counter for 2023-2024 Dataset

The MEG II experiment at PSI searches for the charged lepton flavor violating decay, $\mu\to e\gamma$. The physics run commenced in 2021 and is planned to continue until the end of 2026, aiming at a target sensitivity on the branching ratio of $6\times 10^{−14}$. Based on data from 2021 and 2022, we have set the most stringent upper limit to date on the branching ratio at BR($\mu\to e\gamma$) < 1.5 × 10$^{-13}$ at a 90% confidence level.
Positron timing is measured by pixelated Timing Counter (pTC), a detector consisting of 512 plastic scintillator tiles, each with an intrinsic time resolution of 80 - 120 ps. By combining signals from multiple hits, the pTC achieves an excellent average time resolution of 43 ps.
While the pTC performance during the 2021-2022 run was robust, a bias in the measurement of time difference between photon and positron, dependent on the number of hit tiles in the pTC, was observed. This presentation will report on the calibration of the pTC using 2023-2024 dataset and detail the investigation into the origin of the hit-dependent timing bias found in the earlier data.

Speaker: Weiyuan Li (University of Tokyo)

PSI2025_weiyuan.pdf

17:05 Normalization detectors for the neutron lifetime experiment τSPECT

The $\tau$SPECT experiment measures the free neutron lifetime by confining ultracold neutrons (UCN) in magnetic field gradients and counting the remaining neutrons after varying storage times. There are statistical and systematical changes in the yield and energy of the UCN produced by the neutron source, therefore the amount of neutrons filled into the trap in each filling cycle has to be monitored. A neutron detector has been built and installed into the experimental beamline to monitor the flux of UCN during the filling process. The charged particles resulting from the neutron capture reaction $^{10}\text{B}$ (n,$\alpha$)$^7$Li cause scintillation in a $^{10}\text{B}$-coated ZnS:Ag layer. This light is detected by silicon photomultipliers coupled in coincidence to the scintillator. A second version suited for a larger beamline diameter and with some design improvements is currently under development. Since the two detectors will be at different heights this will also give an insight in the UCN energy spectrum.
This poster will cover the detector’s design, results of measurements, as well as the idea for the improved version.

Speaker: Viktoria Ermuth (Johannes Gutenberg University Mainz)

17:06 Polarizers for ultra-cold and very cold neutrons based on reflection off iron-cobalt foils

Commonly used UCN beam polarizers employ the longitudinal Stern-Gerlach effect due to a strong magnetic field, or spin dependent neutron transmission through a thin magnetized foil, exploiting the combination of the spin independent neutron optical potential and the spin dependent magnetic potential. In contrast to commonly used iron foils, the commercial alloy Hiperco50, consisting of 49% iron, 49% cobalt and 2% vanadium, has nearly matched nuclear and saturated magnetic potentials of 130 neV and +/-139 neV, respectively. It should therefore be able to polarize UCN down to zero energy even in reflection. First tests of this concept were performed using various prototypes in the standard polarizer/spin-flipper/analyzer configuration installed at ILL’s UCN sources PF2 and SUN. At PF2, time-of-flight analysis with a chopper demonstrated neutron polarization for a wide spectrum, including neutron energies far beyond the UCN range. At SUN, spectrally integrated flipping ratios larger than five were measured using annealed foils. A second generation of devices is being prepared for a second beam time in autumn 2025.

Speaker: Oliver Zimmer (Institut Laue Langevin)

17:07 Offline Characterization of a Highly Uniform Magnetic Field for the n2EDM Experiment

The n2EDM experiment at the Paul Scherrer Institute (PSI) aims to improve the sensitivity to the neutron electric dipole moment (nEDM) by an order of magnitude relative to the current best limit ($1.8 \times 10^{-26}~e\cdot\mathrm{cm}$). A key requirement to achieve this goal is the generation and precise control of a highly homogeneous static magnetic field $ B_0 $ within the precession volume. Magnetic field non-uniformities directly affect the statistical sensitivity and introduce systematic errors into the measurement.

Consequently, the magnetic field requirements are stringent. In particular, to suppress the so-called false EDM effect, a systematic shift arising from the use of a mercury co-magnetometer, higher-order odd gradients (e.g., $ G'_{3,0} $, $ G'_{5,0} $, $ G'_{7,0} $) must be reproducible at the level of $ \lesssim 23~\mathrm{fT/cm} $, thereby limiting the associated false EDM contribution to below $ 3\times10^{-28}~e\cdot\mathrm{cm} $ [1].

To verify that these criteria are fulfilled under operating conditions, we conducted a comprehensive magnetic field mapping campaign inside the magnetically shielded room (MSR) of the n2EDM experiment, employing a robotic field mapper that had recently undergone hardware and software upgrades.

We present recent results confirming that the design goals for field uniformity and gradient control, as outlined in [2], are met. In addition, we report field maps of the "magic field" configuration, at which the total false EDM is reduced by an order of magnitude. These data provide important input for the physics data-taking phase of n2EDM and confirm the compatibility of the magnetic field environment with the targeted experimental sensitivity of $ 1\times10^{-27}~e\cdot\mathrm{cm} $.

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References

[1]: C. Abel et al., "Generating a highly uniform magnetic field inside the magnetically shielded room of the n2EDM experiment", Eur. Phys. J. C 85, 202 (2025). https://doi.org/10.1140/epjc/s10052-025-13902-x

[2]: N. J. Ayres et al., "The design of the n2EDM experiment", Eur. Phys. J. C 81, 512 (2021). https://arxiv.org/abs/2101.08730

Speaker: Valentin Czamler (LPSC)

17:08 An optically pumped Cesium magnetometry array for the n2EDM experiment

The n2EDM experiment at the Paul Scherrer Institute aims to measure the neutron electric dipole moment with a sensitivity of below 1E−27 e⋅cm by observing neutron spin precession in a near perfectly uniform magnetic field. Precise control of systematic effects, particularly those caused by magnetic field non-uniformities, is crucial for achieving this sensitivity. To address this, an array of 112 optically pumped Cesium vapor magnetometers, capable of picotesla-level precision measurements, will be deployed. This system will provide real-time measurements of magnetic field gradients, enabling the control and reduction of systematic uncertainties arising from magnetic field non-uniformities. In this contribution, the design, performance, and integration of this system into the experimental setup will be presented.

Speaker: Lea Segner

17:09 Calibration and commissioning of the Mu3e Vertex Detector

The Mu3e experiment aims to search for the charged lepton flavour violating decay μ⁺ → e⁺e⁻e⁺ with an ultimate sensitivity of 10⁻¹⁶. Its Vertex Detector employs ultra-thin
MuPix11 sensors to provide precise tracking with minimal material. During our beam time at PSI this year, we successfully commissioned the detector.
Through Time over Threshold calibration, signal transmission tuning, and in-pixel threshold adjustment, we achieved efficient operation and recorded the first positron
tracks from muon decays. This milestone marks a major step toward physics data taking in 2026.

Speaker: Thomas Senger (University of Zürich)

PSI2025_Thomas_Senger.pdf

17:10 Monte Carlo simulation framework for the neutron lifetime experiment τSPECT

The $\tau$SPECT experiment aims to measure the free neutron lifetime
with an uncertainty goal of sub-second by storing ultra-cold neutrons (UCNs)
in a fully magnetic bottle using a spin-flip loading technique. Monte Carlo (MC) simulations of neutron dynamics in the experiment are a key element to study and understand systematic effects, reduce uncertainties, and improve the experimental design. Based on pre-existing MC UCN software packages, we significantly enhanced their capabilities and set up a comprehensive simulation framework for our experiment. We accurately simulate the production, transport, storage, and detection of UCNs in $\tau$SPECT.

This poster presents the simulation framework as well as the latest simulation results.

Speaker: Niklas Pfeifer (Johannes Gutenberg University Mainz)

17:11 Demonstration of LYSO crystals as a high resolution calorimeter option for the new PIONEER experiment

The PIONEER experiment intends to measure rare pion decays at the PSI PiE5 beamline to achieve the most precise test of lepton flavor universality to date. To achieve the necessary statistics for PIONEER, a pion stop rate of more than 300 kHz must be employed; such high beam rates result in significant pileup in the calorimeter used to differentiate between pion and muon decay products, which motivates calorimeter segmentation as a means to curtail pileup. LYSO crystals are an intrinsically segmented calorimeter option that has been demonstrated at a previous 2023 PiM1 beam time to meet PIONEER calorimeter specifications, where a 1.52% energy resolution at 70 MeV was measured with rectilinear crystals. The desired azimuthal symmetry of the calorimeter can be achieved using larger, tapered crystal geometries. Six LYSO crystals with this design have been grown and tested at low energies using sealed radioactive sources, for which excellent energy resolution and uniformity has been confirmed. Ongoing tests at the PSI PiM1 beamline are evaluating crystal performance using positrons at the PIONEER energy scale.

Speaker: Mr Omar Beesley (University of Washington)

17:12 Storing muons and freezing spin: development of a fast kicker magnet and electrode system

Precision storage ring experiments, such as those testing fundamental symmetries and investigating nuclear structure, rely on precise control of electric and magnetic fields to guide, focus and probe charged particles. This is achieved using various techniques, generally involving distributed systems across large scale rings [1-4].
The frozen-spin technique is a yet-undemonstrated approach to search for electric dipole moments (EDMs) of charged particles decaying via the parity-violating weak interaction [5]. A precisely tuned electric field is applied to particles in cyclotron orbits about a magnetic field, cancelling precession induced in the orbital plane by the anomalous magnetic moment, $a=(g-2)/2$. This permits accumulation of spin precession out of the orbital plane, as would be induced by a non-zero EDM. Competitive precision EDM searches are thus rendered feasible using lower momentum in a compact storage ring where fields are localised, presenting both advantages and novel challenges.

The muEDM Collaboration is undertaking such a search with muons [6], for which a small $a$ necessitates an electric field of only $1.8\,\mathrm{kV/cm}$ for $p=22\,\mathrm{MeV}/c$ muons in a magnetic field of $2.4\,\mathrm{T}$. This will be applied by concentric cylindrical electrodes proximate to the $r=30\,\mathrm{mm}$ orbit of the muon. Material must be minimised, to reduce multiple Coulomb scattering of emitted positrons, without comprising cylindrical uniformity beyond stringent limits due to systematic effects, especially stray axial electric fields [7].

A pulsed magnetic field will be applied as the muons traverse the centre of the solenoid, reducing their axial momentum and trapping them inside a weakly-focusing magnetic field. The axial confinement permits betatron oscillations up to $\pm40\,\mathrm{mm}$ due to the wide momentum acceptance of the pulse and absence of an axial cooling scheme. This approach requires a fast kicker magnet to supply current up to $200\,\mathrm{A}$ in a $\sim50\,\mathrm{ns}$ pulse, with subsequent oscillations suppressed below $10\,\mathrm{A}$ to avoid systematic effects and maximise spin precession time prior to muon decay.

The motivation and technical development of these localised systems for spin control and trapping on rapid sub-microsecond timescales will be summarised in this poster. In particular, the features of the pulsed power system, developed for characterisation and tests of the kicker magnet, which enable fast-switching will be explained. The electrodes have been designed in parallel to minimise shielding of the pulsed magnetic field and satisfy constraints from systematic effects. These systems will be commissioned in 2025-26, enabling the first attempt to implement the frozen-spin technique, a potential milestone in the field of EDM searches. Moreover, extending the reach of precision trapping and spin control technologies towards shorter timescales offers synergies with precision searches for new physics harnessing polarized short-lived radionuclides increasingly available at modern rare isotope beam facilities [8-10].

[1] The Muon 𝑔 − 2 Collaboration, arxiv:2506.03069 [hep-ex] (2025).
[2] J. Alexander et al., arxiv:2205.00830 [hep-ph] (2022).
[3] S. Karanth et al., Phys. Rev. X 13, 031004 (2023).
[4] B. Franzke et al., Nucl Instr. and Meth. B 24-25, p. 18-25 (2023).
[5] F.J.M. Farley et al., Phys. Rev. Lett. 93, 052001 (2004).
[6] A. Adelmann et al., Eur. Phys. J. C 85, 622 (2025).
[7] G. Cavoto et al., Eur. Phys. J. C 84, 262 (2024).
[8] W. Gins et al., Nucl Instr. and Meth. A 925, p. 24-32 (2019).
[9] K. Minamisono et al., Nucl Instr. and Meth. A 709, p. 85-94 (2013).
[10] C. Dutsov et al., arxiv:2506.13588 [hep-ex] (2025).

Speaker: Timothy David Hume (PSI - Paul Scherrer Institut)

17:13 Development of a cryogenic device for a polarized target nuclei to search for the time-reversal symmetry violation in compound nucleus

Understanding the matter-dominated universe requires the discovery of CP violation beyond the Standard Model. A promising approach is to search for time-reversal invariance violation (TRIV), which is equivalent to CP violation, using polarized neutron and polarized target nuclear reactions. Neutron transmission experiments are expected to be a particularly sensitive probe of TRIV effects by exploiting the same enhancement mechanisms that produce large parity violation (PV) in neutron-induced compound nuclear reactions [1]. The NOPTREX collaboration aims to realize the measurement at J-PARC MLF, with Lanthanum (La)-139 selected as the first target nucleus. For this nuclei, TRIV-to-PV cross-section ratio of 0.59 ± 0.05 was theoretically predicted, making it highly promising for the measurements of TRIV effects [2]. TRIV measurement using compound nuclear reactions requires the polarized nucleus target. Lanthanum-139 possesses a nuclear quadrupole moment and can be efficiently polarized up to about 50% through Dynamic Nuclear Polarization (DNP) using Nd-doped LaAlO₃ crystals [3]. To enable DNP under experimental conditions, we are developing a cryogenic equipment that incorporates a dilution refrigerator capable of reaching temperatures below 0.1 K and is compatible with an existing superconducting magnet. In this presentation, we will report on the current progress of the cryogenic system development for nuclear polarization.

[1] R. Nakabe, et al., Phys. Rev. C 109, L041602 (2024).
[2] T. Okudaira, et al., proceedings of J-PARC symposium 2024.
[3] P. Hautle and M.Iinuma, Nucl. Instrum. Methods Phys. Res. A 440, 638-642 (2000).

Speaker: Ms Shiori Kawamura (Nagoya university)

poster_kawamura_ver2.pdf

17:14 A spatially resolving detector for ultracold neutrons

Ultracold neutrons (UCNs) can be stored in material vessels and magnetic field gradients. This property allows for long observation times and thereby precision measurements of fundamental neutron properties. In the presented detector design, UCNs are converted into an electrical signal by employing a $^{10}\text{B}$ conversion layer stacked with a ZnS(Ag) scintillation layer. The neutron capture reaction in the conversion layer generates a light pulse in the scintillation layer, which is then guided onto an array of silicon photomultipliers by a 3D printed light guide. This setup is well suited for in-situ detection of UCNs in strong magnetic fields and compatible with vacuum environments. In a first test beamtime, the detector was compared to a commercial UCN detector and its spatial resolution was evaluated. This poster will present the detector setup as well as the test beamtime results.

Speaker: Konrad Franz (Johannes Gutenberg University Mainz)

17:15 Performance of a Magnetically Shielded Room for the Xenon EDM Experiment

We present the performance characterization of a Magnetically Shielded Room (MSR) designed to meet the stringent magnetic requirements of next-generation $^3$He/$^{129}$Xe co-magnetometer experiments, in particular the search for a permanent electric dipole moment (EDM) of $^{129}$Xe. The Xenon EDM experiment aims to probe new sources of CP violation beyond the Standard Model with unprecedented sensitivity, requiring a magnetically quiet environment with residual fields well below the nanotesla range and ultra-low magnetic field gradients. The MSR features a cubical walk-in volume with an edge length of 2560 mm and comprises three layers of 3 mm thick Mu-metal, complemented by a 8 mm thick copper-coated aluminum layer. An optimized magnetic equilibration (degaussing) procedure is introduced, consisting of a frequency sweep at constant amplitude followed by an exponential amplitude decay. This complete degaussing cycle requires 21 minutes and reliably reduces the residual magnetic field at the center of the MSR to below 1 nT. The choice of this procedure is supported by in-situ hysteresis measurements of the fully assembled MSR and eddy current simulations, both indicating saturation at the center of the Mu-metal layers. Furthermore, shielding factors can be improved by approximately a factor of 4 in all spatial directions through low-frequency (0.2 Hz), low-amplitude (1 A) magnetic shaking applied to the outermost Mu-metal layer. Finally, we present a gradient compensation system installed inside the MSR and report high-precision measurements of magnetic field gradients. These gradients are extracted from transverse relaxation rates of precessing $^{129}$Xe spin samples, achieving a resolution below 1 pT/cm.

Speaker: Fabian Allmendinger (Physikalisches Institut)

17:16 Enhanced antihydrogen accumulation with laser-cooled Be+

The study of cold antihydrogen for CPT symmetry tests began in 2010 with the first successful demonstration of trapping individual antihydrogen atoms [1]. In the ALPHA experiment, antihydrogen is produced via a three-body recombination process involving one antiproton and two positrons [2]. Antihydrogen is formed by combining cold plasmas of positrons and antiprotons in a specialized Penning-Malmberg trap, which spatially overlaps with a magnetic minimum trap designed to confine antihydrogen atoms [3]. Due to the shallow depth of the magnetic potential - capable of trapping only atoms with kinetic energies corresponding to temperatures below 0.5 K - early experiments typically confined ~20 antihydrogen atoms per production cycle. In 2017 the technique was advanced to allow continuous synthesis and accumulation of antihydrogen [4, 5], enabling key milestones such as the first high-precision measurement of the 1S–2S transition [6] and the first observation of gravity’s influence on antimatter [7].
Antihydrogen production through the three-body recombination process depends on the thermal energy of the positrons; both the production and trapping rates increase as the positron temperature decreases. So far the temperature of positron plasma in ALPHA-2 trap was limited to around 20K, which was achieved via the cyclotron cooling mechanism in the high magnetic field. To reduce the temperature of the positron plasma even further, an active cooling mechanism is required.
Inspired by pioneering work at NIST [8], a sympathetic cooling of positrons with laser-cooled beryllium ions (Be+) was proposed [9]. The Be+ ions are generated via laser ablation of a solid beryllium target [10], then confined within a Penning-Malmberg trap and Doppler cooled using a 313 nm laser. Upon merging with the positron plasma, the laser-cooled Be+ ions carry away thermal energy from the positrons through Coulomb interactions. Sympathetic cooling technique allowed to achieve ~2.5 times lower temperatures of positron plasma than before [11].
Early development of Be+ laser-cooling technique suffered from irreproducibility of the number of ablated beryllium ions and inefficient laser-cooling scheme. Several laser system upgrades were performed, most importantly to allow for simultaneous laser-cooling and Be+ cloud compression using Rotating Wall technique [12]. This improved laser-cooling technique was successfully integrated into the standard antihydrogen synthesis cycle. An eight-fold enhancement in antihydrogen trapping efficiency per synthesis cycle has been demonstrated, enabling the accumulation of over 15,000 antihydrogen atoms in less than seven hours. The implementation of sympathetic cooling of positrons method into antihydrogen production cycle has not only accelerated the experimental timeline but also opened new opportunities for detailed investigations of fundamental symmetries. These include potential searches for sidereal variations and other precision tests of antimatter and its interactions, which were previously inaccessible due to limited sample sizes and extended accumulation times.

Speaker: Joanna Peszka

17:17 The ¹⁹⁹Hg Co-magnetometer System for the n2EDM Experiment

The n2EDM experiment at the Paul Scherrer Institute searches for the electric dipole moment (EDM) of the neutron with a baseline sensitivity of approximately $1×10^{−27}$ e·cm. Precise monitoring of the average magnetic field experienced by the neutrons is essential to prevent systematic shifts in the EDM measurement that cannot be otherwise mitigated. This magnetic field monitoring is achieved using optically pumped $^{199}$Hg co-magnetometers, which operate in the same storage volumes as the neutrons. The improved neutron statistical sensitivity requires the co-magnetometers to measure the magnetic field with an uncertainty of 25 fT.
This poster presents the design, implementation, and performance of the mercury co-magnetometer system.

Speakers: Nikolaus Stephan Edler von Schickh (PSI - Paul Scherrer Institut), Wenting Chen (PSI - Paul Scherrer Institut)

17:18 Interferometric measurements for the LEMING experiment

The LEMING experiment is designing the next generation of laser spectroscopy and gravity experiments using a novel atomic beam of muonium (Mu = μ⁺ + e⁻). Cold atomic muonium beams are generated in vacuum and subsequently undergo self-interference using newly engineered, self-aligned diffraction gratings. The setup allows for nanometer-sensitive measurements of muonium displacements due to gravitational acceleration. Here we present the development of the atomic interferometer setup, detailing its construction, characterization, and performance validation using a combination of x-ray diagnostics and imaging techniques.

Speaker: Francesco Lancellotti (ETH Zurich)

17:19 The muEDM Experiment Technical Overview

The muEDM experiment aims to measure the electric dipole moment (EDM) of the muon with unprecedented sensitivity, providing a powerful probe for physics beyond the Standard Model. Utilizing the frozen spin technique, the experiment is designed to isolate EDM-induced spin precession while suppressing magnetic moment (g−2) effects. This poster presents a technical overview of the experimental design, focusing on key components currently under development and testing to meet the requirements for the first phase of the experiment.

Speaker: Diego Alejandro Sanz Becerra (PSI - Paul Scherrer Institut)

17:20 The ground state hyperfine splitting in muonic hydrogen experiment (HyperMu) at PSI

The HyperMu experiment at PSI aims at the first measurement of the ground state hyperfine splitting in muonic hydrogen (μp) with 1 ppm precision using pulsed laser spectroscopy. This accuracy allows for a precise extraction of the proton structure contributions, including the Zemach radius and the proton polarizability.

To measure the ground state hyperfine splitting in μp, we are developing a unique pulsed laser system designed to deliver 4 mJ pulses at a wavelength of 6.8 μm, randomly triggered upon muon detection. We report on the latest laser development within the experiment, the several developments of the detection system that was carried out and the optimization of the experimental parameters to obtain a successful resonance signal.

Speaker: Ahmed Ouf (johannes gutenberg universität)

17:21 Development of Key Components of the UCN-Optics System of PanEDM

The PanEDM experiment aims to measure the neutron electric dipole moment, using ultracold neutrons (UCN) produced by the superfluid-helium UCN source SuperSUN at the Institut Laue-Langevin (ILL). UCN will be stored in double-chamber spectrometer for spin-precession measurements, with a sensitivity of $4 \times 10^{-27}\,e \,\mathrm{cm}$ anticipated after 100 days of measurement time.
A UCN-optics system serves as the interface from SuperSUN to PanEDM; it is responsible for bringing polarised UCN through a polariser into the two precession chambers, and back to the detection system after the spin-precession period. Due to space limitations and dilution losses during extraction and transport, the interface components must be as compact as possible -- while also satisfying the competing requirement that UCN should survive in the guide system for long times without significant loss. Major sections of the guide system now employ metallic guides for improved mechanical stability, precision, and robust mounting.
We present a first characterisation of two key interface subsystems, carried out at the ILL using a prototype superfluid-helium UCN source SUN-2 and the PF2 UCN turbine user facility.

The guide-manifold (GM) splits UCN into two branches feeding the two spin-precession chambers, and consists of eight individual nickel-phosphorus (NiP) coated guides arranged in an asymmetrical Y-shape with a total length of $1.2 \, \mathrm{m}$. This design features a simple mechanism to connect and seal guides, and ensures that the two precession chambers can be filled simultaneously.
To determine the transport efficiency for low-energy UCN through the GM, we performed several transmission and storage measurements at SUN-2, reaching a spectrally-integrated transmission efficiency of more than $0.95$ and storage times of approximately $100 \, \mathrm{s}$. A corresponding wall-loss factor of $\eta \approx 4 \times 10^{-4}$ is consistent with literature.

PanEDM's UCN detection system relies on simultaneous spin detection, using magnetised iron-foil analysers and adiabatic radiofrequency (RF) spinflippers. While these well-established techniques are usually limited to non-conductive guides with at most thin metallic coatings, we have developed short RF spinflippers that can be used with thin-walled metal guides (coated internally with NiP).
For the first time, we demonstrate the efficient operation of such spin-flippers with metallic guides, obtaining average efficiencies of $1.002 \pm 0.008$ within the relevant UCN longitudinal-velocity range at PF2.

Having shown that these components function adequately in isolation, they will be commissioned as modular subsystems at SuperSUN to sequentially validate performance of the entire UCN interface.

Speaker: Luca Kaess (TU Munich)

17:22 qBounce: developments and implementation of new experimental techniques

The qBounce experiment investigates the quantum states of ultracold neutrons in the gravitational field of the Earth. This offers a unique opportunity to study gravity at a microscopic level with great accuracy. When neutrons are confined above a horizontal mirror, ultracold neutrons form discrete quantum energy levels arising from the interplay between gravitational and quantum effects. This poster outlines the experimental approach and highlights current and future developments, including the improvements to the setup to minimise downtime and to increase the capabilities. In addition, we present a method to detect neutrons with a high spatial sensitivity using boron-coated CMOS photodetectors.

Speaker: Dr Johann Marton (TU Wien)

17:23 The Gradiometer: A detection system for magnetic contamination in n2EDM

The n2EDM experiment at the Paul Scherrer Institut seeks to measure the neutron electric dipole moment with a sensitivity below 10^(−27) e . cm, which demands an extremely well-controlled magnetic environment. To track down tiny magnetic contaminants that could mimic an EDM signal, we built a mobile gradiometer based on optically pumped cesium magnetometers operating in the Mx configuration. Through differential phase-sensitive detection, the system achieves sub-picotesla gradient sensitivity and can resolve dipole moments as small as 0.1 nAm^2. Combining the precision and reliability of cesium magnetometry, the device enables material scans under realistic experimental conditions, providing vital diagnostics to safeguard the magnetic cleanliness of the n2EDM setup.

Authors: Luz Sanchez-Real Zielniewicz, Lea Segner, Judith van Keirsbilck, Victoria Kletzl, Georg Bison, Vira Bondar on behalf of the nEDM collaboration

Acknowledgement of grants: Swiss National Science Foundation 200441, 213222 and 236419.

Speaker: Luz Sanchez-Real Zielniewicz (ETH Zurich)

17:24 Determination of a Rate-Dependent Gain Correction for the Muon g-2 Calorimeters at Fermilab

The Muon $g-2$ experiment at Fermilab has measured the anomalous magnetic moment of the muon ($a_{\mu} \equiv (g_{\mu} -2)/2$) to a precision of 127 parts-per-billion. During the experiment, `fills' of $\mathcal{O}\left( 10^5 \right)$ $3.1\,\text{GeV}/\text{c}$ muons were injected into the $g-2$ storage ring, of which $\approx 5{\small,}000$ muons were stored and decay over the course of $700\,\mu\text{s}$. These stored muons decay into positrons which spiral inward and impact a series of PbF$_2$ crystal calorimeters read out by silicon photomultipliers (SiPMs) positioned around the inner radius of the storage ring. Over the course of each fill, calorimeter gain stability at the $5 \times 10^{-4}$ level was required to meet our systematics goals even as the instantaneous rate of positrons changed by 5 orders of magnitude. The gain of the calorimeter was monitored by a dedicated laser system, which corrected for the effect of the large flash of particles at injection and pulse-pair SiPM pixel effects with recovery on the $\mathcal{O}(15\,\text{ns})$ timescale. After the conclusion of the final running period, a series of tests were conducted at the University of Washington to resolve an observed residual gain-like effect after these corrections were applied. Here we describe these measurements, the discovery of the cause of the gain change --- the cumulative perturbations the positrons themselves have on the detector gains long after injection, each on the $\mathcal{O}(4-8\,\mu\text{s})$ timescale and with a relative amplitude of $\leq 10^{-4}$ --- and the application of the correction in the final publication.

Speaker: Dr Joshua LaBounty (University of Washington)

PSI_2025_Poster_JoshLaBounty.pdf

17:25 Mu3e Online Event Selection on GPU

The Mu3e experiment is designed to search for the Charged Lepton Flavor Violation (cLFV) through the rare decay μ+ → e+e−e+, targeting a branching ratio sensitivity of 10^-15 using the PSI piE5 beamline in Phase I, scheduled for 2026.

To cope with the exceptionally high muon rate of 10^8/s (equivalent to ~80 Gbps raw data rate), a triggerless, GPU-based online event selection algorithm is implemented on the computing farm to reconstruct tracks and vertices in real time and identify the Mu3e signal candidate, thereby reducing the data rate by two orders of magnitude.

This algorithm consists of four main stages: triplet selection, track reconstruction, vertex selection followed by SciFi selection, together enabling efficient suppression of background events while retaining high signal efficiency. Its feasibility and performance have been successfully demonstrated during the Mu3e commissioning test beam in June 2025.

Speaker: Chen Xie (ETH Zurich)

17:26 MCUCN simulations for n2EDM - study of UCN spin kinetics

for the nEDM Collaboration at PSI

The n2EDM project, aiming for a most sensitive measurement of the electric dipole moment of the neutron hosted by PSI, takes advantage of extensive Monte Carlo simulations of the ultracold neutron storage and transport. This includes modelling of the UCN spin transport system, which also allows the study of depolarization effects in the guides during filling, during storage in the precession chambers, in the emptying guides and in the polarization analysis system. For the understanding of transversal depolarization in the chambers, as a function of magnetic gradient fields, a method of calculation of autocorrelation functions for UCNs was implemented in the MCUCN code. In this presentation, we will give an update of the n2EDM model and most recent results on spin kinetics.

Speaker: Geza Zsigmond (PSI - Paul Scherrer Institut)

17:27 Physics with Ultra-Cold and Very-Cold Neutrons at the PF2

We provide an overview of the capabilities of the modernized Ultra-Cold and Very-Cold Neutron beam ports of the PF2 instrument at the Institut Laue-Langevin.

Experiments using UCN and VCN are important tools to investigate fundamental physics and beyond. Experiments range from dark sector searches over cross-section measurements to neutron instrumentation.
The PF2 instrument serves as platform enabling numerous successful UCN and VCN experiments.

We showcase selected user experiment performed at the PF2 using UCN and VCN during the last two years that did  benefit from a modernisation campaign at the instrument.

We also present these enhancements and modernizations, including a new NOMAD based DAQ- and experiment-control system, the outcome of a UCN guide system cleaning, and a comprehensive toolbox for beam shaping and instrumentation for the VCN beam line.
These improvements are on the one hand designed to reduce the threshold to perform a broader range of complex future experiments and on the other hand already lead to an increase in neutron flux and a colder spectrum for UCN and VCN.

Speaker: Dr Hanno Filter-Pieler (Institut Laue-Langevin)

17:28 A new view on Quantum Computers

We describe a concept for a quantum computer based on an abundant number of energy eigenstates. These states form Q-bits or, ad libitum, higher dimensional Q-Nits with N > 2, allowing gate operations according to the quantum computing requirements of DiVincenzo. This system with higher dimensional Q-Nits offers potential advantages over traditional Q-Bit-based quantum computing. It provides a larger state space for storing and processing information, which can reduce circuit complexity, simplify experimental setups, and enhance algorithm efficiency.

Speaker: Daniel Aziz (Technische Universität Wien)

17:29 Towards a new generation split-crystal interferometer

Neutron Interferometry was introduced by H. Rauch and U. Bonse in 1974. It opened the path to matter wave interferometry, allowing many direct precision tests of quantum mechanics and fundamental physics and the precise measurement of scattering lengths. A thermal neutron interferometer uses perfect crystals as optical elements (beam splitters, mirrors, and recombiner), where all acting diffraction planes are machined out of a single silicon ingot. This technology allows splitting the neutron wave paths by several centimeters and achieving interference contrast up to 90%. However, it also imposes limitations on the size (given by the available size of perfect silicon ingots) and systematic errors (mainly due to intrinsic crystal imperfections).
To overcome these limitations, the concept of neutron interferometry was extended to the use of diffracting gratings. So far, grating interferometers have not yet allowed for achieving comparable contrast and/or path separation. Other, more recent efforts are based on using multilayer optics. While these devices achieve excellent contrast, their beam-path splitting remains very small, typically in the range of a few hundred micrometers.
To overcome the size limitations of crystal interferometers, we have recently demonstrated that it is possible to construct an interferometer using two separated crystals. We are currently developing a dedicated setup for crystal separation up to one meter, enabling the introduction of massive samples into one of the beam paths for the first time. The main idea consists of setting up a combined optical, X-ray, and neutron
interferometer. The optical interferometer allows for the control of the position of all crystals in space on a subnanoradian and subnanometer level with kHz readout. The Xray interferometer informs about the lattice constant properties in time. A neutron interferometry measurement consists of a spatial- and time-resolved neutron detection, where to every detection event a phase value from the optical and X-ray interferences is associated. This allows the reconstruction of the neutron quantum phase induced by interactions in the interferometer, despite time- and space-dependent instability and imperfection of the interferometer.
We present several proof-of-principle reconstructions of the interference, which allowed us to work out the new interferometer concept. Each of them was carried out using a conventional single-crystal interferometer, applying the perturbations that are expected to occur in the split-crystal version. We further present the first proof of principle of a split-crystal prototype as well as the full design of a next-generation split-crystal interferometer and its status of implementation, which is foreseen to happen until spring 2026.

Speaker: Michael Jentschel (Institut Laue Langevin)

17:30 A new results of neutron lifetime measurement with cold neutron beam at J-PARC

The ``neutron lifetime puzzle'' arises from the discrepancy between neutron lifetime measurements obtained using the beam method, which measures decay products, and the bottle method, which measures the disappearance of neutrons.
To resolve this puzzle, we conducted an experiment using a pulsed cold neutron beam at J-PARC. In this experiment, the neutron lifetime is determined from the ratio of neutron decay counts to $^3$He(n,p)$^3$H reactions in a gas detector. This experiment belongs to the beam method but differs from previous experiments that measured protons, as it instead detects electrons, enabling measurements with distinct systematic uncertainties. By enlarging the beam transport system and reducing systematic uncertainties, we achieved a fivefold improvement in precision. Analysis of all acquired data yielded a neutron lifetime of $\tau_{\rm n}=877.2~\pm~1.7_{\rm(stat.)}~^{+4.0}_{-3.6}{}_{\rm (sys.)}$~s. This result is consistent with bottle method measurements but exhibits a $2.3\sigma$ tension with the average value obtained from the proton-detection-based beam method.

We will present about the new results.

Speaker: Kenji Mishima (RCNP, Osaka university)

17:31 Spin dependent exotic interactions

Speaker: Lei Cong (University of Mainz)

17:32 Search for Exotic Forces

Speaker: Lei Cong

Wednesday 10 September

09:00 → 10:40 Session: Wed - 1

Convener: Claude Amsler

09:00 Feebly Interacting Particles in the MeV-GeV range: experimental landscape

Particle physics today faces the challenge of explaining the mystery of
dark matter, the origin of matter over anti-matter in the Universe, the origin of the neutrino masses, the apparent fine-tuning of the electro-weak scale, and many other aspects of fundamental physics.
Perhaps the most striking frontier to emerge in the search for answers involves new physics at mass scales comparable to familiar matter, in the MeV - GeV range.
I will review the current results and the experimental prospects for their searches at beam dump, fixed-target, and collider-based experiments.

Speaker: Mrs Gaia Lanfranchi (Laboratori Nazionali di Frascati INFN)

09:30 Searching for Ultra-Low-Mass Dark Matter with Precision Atomic Experiments

Ultra-low-mass bosonic particles produced non-thermally in the early Universe may form a coherently oscillating classical field that can comprise the observed cold dark matter. The very high number density of such particles can give rise to characteristic wave-like signatures that are distinct from the particle-like signatures considered in more traditional searches for WIMP dark matter. In particular, ultra-low-mass scalar dark matter may induce apparent variations of the fundamental “constants” of Nature, while ultra-low-mass pseudoscalar (axionlike) dark matter may induce time-varying spin-precession effects (including oscillating electric dipole moments). I discuss the basic principles, recent results and future possibilities in searches for ultra-low-mass dark matter using a variety of precision low-energy experiments, including atomic spectroscopy, optical cavities and interferometers, torsion pendula, magnetometry, neutron experiments and g-factor measurements.

References:
[1] Y. V. Stadnik and V. V. Flambaum, Physical Review D 89, 043522 (2014).
[2] Y. V. Stadnik and V. V. Flambaum, Physical Review Letters 114, 161301 (2015).
[3] Y. V. Stadnik and V. V. Flambaum, Physical Review Letters 115, 201301 (2015).
[4] Y. V. Stadnik and V. V. Flambaum, Physical Review A 94, 022111 (2016).
[5] Y. V. Stadnik and V. V. Flambaum, Physical Review A 93, 063630 (2016).
[6] C. Abel et al., Physical Review X 7, 041034 (2017).
[7] A. Hees, O. Minazzoli, E. Savalle, Y. V. Stadnik and P. Wolf, Physical Review D 98, 064051 (2018).
[8] H. Grote and Y. V. Stadnik, Physical Review Research 1, 033187 (2019).
[9] C. Smorra et al., Nature 575, 310 (2019).
[10] Y. V. Stadnik, Physical Review Letters 131, 011001 (2023).
[11] C. Abel et al., SciPost Physics 15, 058 (2023).
[12] Y. V. Stadnik, Nature Astronomy 8, 434 (2024).
[13] P. Fierlinger, M. Holl, D. Milstead, V. Santoro, W. M. Snow and Y. V. Stadnik, Physical Review Letters 133, 181001 (2024).
[14] P. Fierlinger, J. Sheng, Y. V. Stadnik and C.-Y. Xing, arXiv:2412.10832.
[15] A. Banerjee, I. M. Bloch, Q. Bonnefoy, S. A. R. Ellis, G. Perez, I. Savoray, K. Springmann and Y. V. Stadnik, arXiv:2502.04455.

Speaker: Yevgeny Stadnik (The University of Sydney, Australia)

10:00 Future perspectives for $\mu \to e \gamma$ searches

Searches for charged lepton flavor violation in the muon sector stand out among the most sensitive and clean probes for physics beyond the Standard Model. Currently, $\mu^+ \to e^+ \gamma$ experiments provide the best constraints in this field and, in the coming years, new experiments investigating the processes of $\mu^+ \to e^+e^+e^-$ and $\mu \to e$ conversion in the nuclear field are anticipated to surpass them. However, it is essential to maintain comparable sensitivities across all these processes to fully leverage their potential and differentiate between various new physics models if a discovery occurs. This talk will discuss ongoing efforts to develop a future experimental program aimed at improving the sensitivity of $\mu^+ \to e^+ \gamma$ searches by one order of magnitude within the next decade.

Speaker: Wataru Ootani (Univ. of Tokyo)

10:20 Progress towards precision measurement of muon-electron elastic scattering: the MUonE experiment

The MUonE experiment at CERN is motivated by longstanding questions surrounding the muon's anomalous magnetic moment, which would be sensitive to contributions from new physics. However, the precision of its Standard Model theory value is limited primarily by the leading-order hadronic vacuum polarization term. MUonE will determine this term using a new approach, by measuring the shape of the differential cross section for elastic scattering of 160 GeV muons on atomic electrons in a low-Z target; this process is sensitive to the hadronic running of the electromagnetic coupling α. A pilot run in summer 2025 is expected to provide the test data needed to establish control of systematic effects. We will present the status of the experiment, first preliminary results, and future plans.

Speaker: Frederick Gray (Regis University, Denver, Colorado (US))

10:40 → 11:10 Coffee Break

11:10 → 12:40 Session: Wed - 2

Convener: Hirohiko Shimizu (Nagoya University)

11:10 Fundamental physics with NMR: from high field to lower than low

Recent work of our group and collaborators will be described, including the progress towards measuring parity violation in chiral molecules [1] and a novel type of nuclear-spin oscillators [2].

[1] Erik Van Dyke, James Eills, Kirill Sheberstov, John Blanchard, Manfred Wagner, Robert Graf, Andrés Emilio Wedenig, Konstantin Gaul, Robert Berger, Rudolf Pietschnig, Denis Kargin, Danila A. Barskiy, and Dmitry Budker, Towards detection of molecular parity violation via chiral co-sensing: the 1H/31P model system, DOI: 10.1039/D5CP00126A; Phys. Chem. Chem. Phys., 2025, 27, 6092-6103, https://arxiv.org/abs/2412.20997

[2] Jingyan Xu, Raphael Kircher, Oleg Tretiak, Dmitry Budker, and Danila A. Barskiy, Quantum Magnetic J-Oscillators, https://arxiv.org/abs/2504.06498 (2025)

Speaker: DMITRY BUDKER (Helmholtz Institute Mainz)

Budker_PSI_2025.pptx

11:40 Fundamental symmetries violations in atoms: theory status, challenges, and outlook

Studies of fundamental symmetries violations in atoms and molecules provide some of the most confronting tests of the standard model and sensitive searches for new physics beyond. In this talk, I will review the current status and key challenges of the theory related to atomic parity violation and electric dipole moments. I will also discuss how atoms may be used to deduce improved nuclear physics properties, essential for reducing theory uncertainties and increasing the discovery potential of current and future experiments.

Speaker: Prof. Jacinda Ginges (The University of Queensland)

12:10 Testing CPT with the Lepton Symmetry Experiment LSym

One of the most striking mysteries in our visible universe is the origin of the large asymmetry between matter and antimatter which our standard model, despite the observed charge-parity (CP) violation, seems to be unable to explain. At MPIK we are currently developing the Lepton Symmetry (LSym) experiment. There, we will store a single positron and an electron in a deep-cryogenic Penning trap. Our double-trap system allows determining the spin states of both particles non-destructively and unambiguously in a magnetic bottle, while the extremely homogeneous magnetic field in our so-called cavity trap supports unperturbed precision measurements. After cooling to the ground state of motion, we compare the g-factor, mass and charge with orders of magnitude higher precision than previously possible in order to probe matter-antimatter (charge-parity-time, CPT) symmetry.

Speaker: Sven Sturm (MPIK)

12:40 → 14:00 Lunch

14:00 → 15:50 Session

Convener: Philipp Schmidt-Wellenburg (PSI - Paul Scherrer Institut)

14:00 Muonium Precision Experiments at J-PARC

Muonium is a pure leptonic binary system that consists of a bound state of a positive muon and an electron. Its energy level structure can be calculated with high precision within the framework of bound-state quantum electrodynamics (QED). Muonium serves as an ideal probe of the electroweak interaction to rigorously test the Standard Model and search for additional unknown interactions between leptons.
At the J-PARC Muon Science Facility (MUSE), two muonium precision experiments are currently being conducted. The first experiment involves precise measurements of the muonium ground-state hyperfine structure (HFS) by the MuSEUM collaboration, utilizing a microwave magnetic resonance technique. High-precision measurements of muonium HFS at high magnetic fields are recognized as one of the most sensitive tools for testing bound-state QED theory and simultaneously determining the fundamental constants of the positive muon magnetic moment and mass. MuSEUM aims to improve the precision of previous measurements by a factor of ten. The second experiment involves high-precision laser spectroscopy of the energy splitting between the 1s and 2s levels in muonium, led by a group at Okayama University. By comparing precise measurements with theoretical calculations, the muon-electron mass ratio will be determined at the ppb level, possibly more accurately than with muonium HFS. An overview of the different features of these new muonium precision experiments and the latest results will be presented.

Speaker: Patrick Strasser (KEK)

14:30 Latest results from the LEMING experiment

The LEMING experiment aims for measuring the gravitational acceleration of muonium, and to carry out next generation laser spectroscopy experiments in search for beyond SM physics. We developed a novel cold muonium source by converting conventional (sub)surface muons in a thin layer of superfluid helium, resulting in a high brightness atomic beam. Most recently, we verified the horizontal Mu source concept for interferometry. In this talk, the latest results and future plans are presented.

Speaker: Anna Soter (ETH Zurich)

15:00 New Advances in Muonic Atom Spectroscopy

Muonic atom spectroscopy is a well-established technique to measure absolute nuclear charge radii with exceptional precision, successfully utilized for nuclei throughout the entire nuclear chart – from protons and the lightest elements to medium-mass, heavy and radioactive isotopes. These precise measurements serve as critical benchmarks for ab initio nuclear theory, important input in atomic spectroscopy for high-precision QED tests, as well as the extraction of fundamental constants.
This presentation provides an overview of the muonic atom spectroscopy landscape at the Paul Scherrer Institute (PSI) and the variety of scientific questions explored within the area. A special focus will be placed on the latest advancements initiated by the QUARTET collaboration, which employs metallic magnetic calorimeters (MMCs) to access the low-Z region from lithium to neon. This latest advancement facilitates high-resolution x-ray spectroscopy of light muonic atoms, bridging a longstanding technological gap and opening up new prospects for precision studies at the intersection of atomic and nuclear physics.

Speaker: Katharina von Schoeler (ETH Zürich)

15:30 Quantum logic spectroscopy of the hydrogen molecular ion

I will present our latest results [1], implementing pure quantum state preparation, coherent manipulation, and non-destructive state readout of the hydrogen molecular ion H$_2^+$.

H$_2^+$ is the simplest stable molecule, and its structure can be calculated ab initio with high precision using quantum electrodynamics. By comparing the calculations with experimental data, fundamental constants can be determined, and the validity of the theory itself can be tested. However, challenging properties such as high reactivity, low mass, and the absence of rovibrational dipole transitions have thus far strongly limited spectroscopic studies of H$_2^+$.

We trap a single H$_2^+$ molecule together with a single beryllium ion using a cryogenic Paul trap apparatus, achieving trapping lifetimes of 11 h and ground-state cooling of the shared axial motion [2]. With this platform we have recently implemented Quantum Logic Spectroscopy of H$_2^+$. The H$_2^+$ molecule is produced in a chosen rovibrational state using resonance-enhanced multiphoton ionization. We use quantum-logic operations between the molecule and the beryllium ion for the preparation of single hyperfine states and non-destructive state readout. In the lowest rovibrational state of ortho-H$_2^+$ (rotation $L = 1$, vibration $\nu = 0$), we achieve a combined state-preparation and readout fidelity of 66.5(8)%. We demonstrate Rabi flopping on several hyperfine transitions using stimulated Raman transitions and microwaves. Utilizing a magnetic field insensitive hyperfine transition driven with a microwave, we observe sub-Hz linewidths and statistical uncertainties in the mHz range.

We are now performing a systematic measurement of the hyperfine structure which will provide a stringent test of state-of-the-art molecular theory and might enable putting an improved bound on a possible tensor force between the two constituent protons of the H$_2^+$ molecule [3].

Our results pave the way for high-precision rovibrational spectroscopy of single H$_2^+$ molecules, which would enable tests of quantum electrodynamics, metrology of fundamental constants such as the proton-electron mass ratio, and the implementation of an optical molecular clock based on the simplest molecule in nature.

[1] D. Holzapfel et al., Phys. Rev. X 15, 031009 (2025).
[2] N. Schwegler et al., Phys. Rev. Lett. 131, 133003 (2023).
[3] N. F. Ramsey, Physica 96A, 285 (1979).

Speaker: Fabian Schmid (ETH Zurich)

15:50 → 16:20 Coffee

16:20 → 17:20 Facility Tour - on request

Thursday 11 September

09:00 → 10:30 Session: Thu - 1

Convener: Torsten Soldner (Institut Laue Langevin)

09:00 Study of Neutron Optics and Physics in Japan

Neutrons exhibit significant quantum mechanical wave properties in the low-energy region. This optical characteristic not only makes neutron beam transport practical but also allows for precision measurements. One well-known example in fundamental physics is neutron confinement using the total reflection phenomenon of neutrons at a material surface, resulting in a well-known experiment measuring the neutron electric dipole moment. To pioneer neutron physics using neutron optics, the NOP beamline (Neutron Optics and Physics) has been installed at the BL-05 beam port of the MLF (Material and Life-science Facility) of J-PARC (Japan Proton Accelerator Research Complex). Using beam branching based on neutron reflection optics, neutron lifetime measurements using electromagnetic beam steering, pulsed ultracold neutron generation using a neutron reflector with an extremely large critical angle, and the development of a neutron interferometer using a set of multilayers compatible with pulsed neutron sources are currently in operation. Among these, neutron interferometers have dramatically improved the capability to measure neutron scattering length and are introducing possibilities of searches for new interactions. Furthermore, research is expanding into the optical properties of epithermal neutrons, and a new-physics search experiment (J-PARC E99 NOPTREX: Neutron Optical Parity and Time-Reversal EXperiment) is in preparation applying the amplification effect of space-time symmetry breaking based on detailed studies of the reaction mechanism of compound nuclear states. This new physics search opens up a different parameter space to conventional electric dipole moment searches, and is enabled by short-pulse neutron sources such as J-PARC.
In this presentation, I will explain the background and current status of the above, and discuss future prospects, including the possibility of using a steady-state neutron source brought about by a new research reactor, the construction of which is currently being planned.

Speaker: Hirohiko Shimizu (Nagoya University)

09:30 Particle physics possibilities at the ESS

The European Spallation Source (ESS), currently under construction in
Lund, Sweden, will soon become the world’s most powerful pulsed neutron
source and simultaneously the brightest pulsed neutrino source. Its unique capabilities open unprecedented opportunities for a precision particle physics program, complementing other facilities worldwide. Neutron sources, when combined with precision measurements and theoretical insight, can probe energy scales far beyond those accessible at the LHC or even future high-energy colliders. In January 2025 [1], a dedicated workshop at Lund University surveyed the landscape of ongoing and planned experiments at neutron sources, providing strategic input for the European Strategy Update.
Among the most ambitious initiatives, the HIBEAM/NNBAR collaboration
proposes a two-stage experimental program to investigate baryon number violation and other manifestations of physics beyond the Standard Model [3]. The first phase, HIBEAM (High-Intensity Baryon Extraction and Measurement), will operate as the fundamental physics beamline during the early phase of ESS operation. It is designed to explore a broad range of phenomena, including neutron–antineutron (n → ¯n) oscillations, transitions to sterile neutrons potentially connected to a hidden dark sector, axion-like particle (ALP) searches, as well as searches for a non-zero neutron electric dipole moment (EDM) and non-zero neutron charge [2]. This will mark the first-ever n → ¯n search at a spallation source, with world-leading sensitivity, and will serve as a pilot for the second phase, NNBAR. The second stage, NNBAR, will exploit a large dedicated beam port in the ESS target station monolith. Its goal is to improve the current n → ¯n sensitivity by three orders of magnitude compared to the previous limit set at the Institut Laue-Langevin (ILL) [4]. The observation of neutron to antineutron oscillations, violating baryon number B by two units, would have profound implications for open questions in fundamental physics, including the origin of the matter-antimatter asymmetry, the unification of forces, and the nature of neutrino mass.
To achieve this sensitivity, NNBAR will employ a state-of-the-art annihilation detector, highly efficient magnetic shielding, advanced neutron reflectors,and a novel moderator system optimized to maximize the flux of cold neutrons.The Conceptual Design Report for the experiment was delivered through the HighNESS project [5], funded by the European Framework for Research and Innovation Horizon 2020.
In this talk, I will present an overview of the proposed experimental program at the ESS, focusing on fundamental neutron and neutrino physics, with particular emphasis on recent developments in the HIBEAM and NNBAR projects.
References
[1] H. Abele, J. Amaral, W. R. Anthony, L. AAstrand, M. Atzori Corona,
S. Baessler, M. Bartis, E. Baussan, D. H. Beck and J. Bijnens, et al.
[arXiv:2506.22682 [nucl-ex]].
[2] V. Santoro, D. Milstead, P. Fierlinger, W. M. Snow, J. Amaral, J. Barrow, M. Bartis, P. Bentley, L. Bj¨ork and G. Brooijmans, et al. J. Phys.
G 52 (2025) no.4, 040501 doi:10.1088/1361-6471/adc8c2 [arXiv:2311.08326
[physics.ins-det]].
[3] A. Addazi et al., New high-sensitivity searches for neutrons converting into antineutrons and/or sterile neutrons at the HIBEAM/NNBAR experiment
at the European Spallation Source, J. Phys. G: Nucl. Part. Phys., 48, 070501 (2021).
[4] M. Baldo-Ceolin et al., A New experimental limit on neutron - anti-neutron oscillations, DFPD-94-EP-13, Z. Phys. C, 63, 409-416 (1994)
[5] V. Santoro, O. Abou El Kheir, D. Acharya, M. Akhyani, K. H. Andersen, J. Barrow, P. Bentley, M. Bernasconi, M. Bertelsen and Y. Beßler, et al. J.Neutron Res. 25 (2024) no.3-4, 315-406 doi:10.3233/jnr-230951

Speaker: Valentina Santoro (ESS)

10:00 Searching X17 with MEG

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Speaker: Angela Papa (PSI - Paul Scherrer Institut)

10:30 → 11:00 Coffee

11:00 → 12:50 Session: Thu - 2

Convener: Oliver Zimmer (Institut Laue Langevin)

11:00 The PIONEER experiment: searching for new physics with PSI’s low energy pions

The PIENU experiment at TRIUMF has provided, to date, the most precise experimental determination of R$^{e/\mu}_\pi$ , the ratio of pions decaying to positrons relative to muons. While more than an order
of magnitude less precise that the Standard Model (SM) calculation, the PIENU result is a precise test of the universality of charged leptons interaction, a key principle of the Standard Model (SM), constrains a large range of new physics scenario, and allows dedicated searches for exotics such as sterile neutrinos. I’ll go over a short overview of R$^{e/\mu}_\pi$ measurements and introduce the next generation precision pion decay experiment in the making: PIONEER. This newly proposed experiment aims at pushing the boundaries of precision on R$^{e/\mu}_\pi$ and expanding the physics reach by improving on the measurement of the very rare pion beta decay $\pi^+ \rightarrow \pi^0 e^+ \nu$.
This will provide a new and competitive input to the determination of |V$_{ud}$|, an element of the Cabibbo- Kobayashi-Maskawa (CKM) quark-mixing matrix.

Speaker: Chloe Malbrunot (TRIUMF)

11:30 Experimental Overview of Neutron Lifetime Measurements

The neutron lifetime is a key parameter in nuclear physics and cosmology, yet two world-leading techniques—the “bottle” and “beam” methods—disagree by almost 10 seconds. This talk will survey the experimental strategies, highlight recent advances, and discuss efforts to resolve this long-standing puzzle.

Speaker: Chen-Yu Liu (University of Illinois Urbana-Champaign)

12:00 Mirror beta transitions as an additional probe for $V_{ud}$

Since the potential of mirror beta transitions to provide an independent determination of the $V_{ud}$ quark-mixing matrix element was pointed out, about 15 years ago [1], many measurements of spectroscopic quantities have been performed and more detailed theoretical corrections have been addressed. These significantly improved the precision on the Ft-values of these transitions [2, 3]. In addition, several precise measurements of the asymmetry parameter have been reported and future measurements of other correlation parameters are being prepared. Although mirror transitions have (like neutron decay) the extra complication of requiring a correlation measurement to extract the axial vector-to-vector mixing ratio, the current precision on $V_{ud}$ from nuclear mirror transitions is still comparable to that from free neutron decay and is a factor of about 3 larger than the precision of the value extracted from pure Fermi transitions.
In recent years, important progress was made in the calculation of the nucleus-independent radiative correction ($∆_R$), in the nuclear structure-dependent radiative corrections ($δ_{NS}$) as well as in isospin-symmetry breaking effects ($δ_C$).
This progress will maintain nuclear mirror decays as a solid additional source for the determination of $V_{ud}$, along with superallowed pure Fermi transitions and free neutron decay. In this presentation we will discuss the current status and future prospects of these activities.

[1] O. Naviliat-Cuncic and N. Severijns, Phys. Rev. Lett. 102, 142302 (2009).
[2] N. Severijns, M. Tandecki, T. Phalet, I.S. Towner, Phys. Rev. C 78, 055501 (2008).
[3] N. Severijns, L. Hayen, V. De Leebeeck, S. Vanlangendonck, K. Bodek, D. Rozpedzik, I.S. Towner, Phys. Rev. C 107, 015502 (2023).

Speaker: Prof. Nathal Severijns (3 KU Leuven, Dept. of Phys. and Astron., Inst. for Nuclear and Radiation Physics)

12:30 Extraction of the weak magnetism and Fierz interference term from precision spectrum shape measurements in the miniBETA project

Precision spectrum shape measurements in nuclear beta decay can be used for testing the Standard Model and physics beyond it with accuracy being competitive with high-energy collider experiments. Such a comparison can be carried out in the framework of effective field theory. The most prominent and poorly known effect in the Standard Model is weak magnetism, the higher-order recoil correction induced by nuclear
pion exchange. Knowledge of this factor allows for study of the QCD influence on beta decay and plays an important role in determining the significance of the reactor neutrino anomaly. Searches for physics beyond the Standard Model can be realized by exploring the Fierz interference term, also modifying the beta spectrum shape.

This contribution will describe the experimental efforts in nuclear beta decay performed in the miniBETA project and will provide details on the systematic effects in the data analysis of the spectrum shape. The results from beta spectrum shape measurements on the allowed Gamow-Teller transition of 114In and 32P will be presented, including a first extraction of the weak magnetism form factor in the high nuclear mass range and a new value for the Fierz interference term. The measurements were performed with a plastic scintillator in combination with a multi-wire drift chamber, where the latter served as an effective background filter.

Speaker: Dagmara Rozpedzik (Jagiellonian University)

12:50 → 14:00 Lunch

14:00 → 15:30 Session: Thu - 3

Convener: Aldo Antognini (ETH)

14:00 Precision Penning-Trap Mass measurements for Fundamental Studies

Precision mass measurements of stable as well as long-lived nuclides have numerous applications among others in atomic-, nuclear-, neutrino- and particle physics. Technical developments in the manipulation and detection of radionuclides and stable species in high-precision Penning-trap mass spectrometry have boosted the field and allow for relative mass uncertainties at the level of 10-11 and below. These technical advances as well as the opening of new fields of applications like the measurement of not only nuclear but also electron binding energies of exotic species as well as tests of physics beyond the Standard Model will be presented.

Speaker: Klaus Blaum (Max Planck Institut für Kernphysik)

14:30 Progress towards a new measurement of the fine structure constant

We will report on our progress to measure the fine-structure constant 𝛼 with improved accuracy to resolve the currently unexplained discrepancy between recent 𝛼 measurements [1,2]. The fine-structure constant 𝛼 characterizes the strength of electromagnetic interactions. Precision measurement of 𝛼 with atom interferometry combined with precision measurement of 𝛼 with electron g - 2 enables one of the most powerful tests of the Standard Model and a broad search for new physics [3,4]. We have built a new four meter atomic fountain that suppresses the most challenging systematics related to the size and quality of the interferometry laser beam. Additionally, we are conducting measurements to experimentally validate our simulations of beam-related systematics by inducing controlled distortions to the interferometer beam and bragg beamsplitter process. We will report on the status and sensitivity goals of our current measurement campaign.

[1] Parker et al., Science 360, 191-195 (2018)
[2] Morel, et al., Nature 588, 61–65 (2020)
[3] Hanneke et al., Phys. Rev. Lett. 100, 120801 (2008)
[4] Fan et al., Phys. Rev. Lett. 130, 071801 (2023)

Speaker: Madeline Bernstein (UC Berkeley)

14:50 Measurements of charge exchange cross section of antiprotons with positronium in GBAR

The GBAR (Gravitational Behaviour of Antihydrogen at Rest) experiment seeks to measure the gravitational acceleration of antimatter with precision better than 1% using ultracold antihydrogen atoms. To obtain ultracold antihydrogen atoms, a multi-step process is used: first, antihydrogen ions are formed and sympathetically cooled using $\mathrm{Be^+}$ ions. Afterward, the extra positron is photo-detached from cooled antihydrogen ions to obtain ultracold antihydrogen atoms with temperatures on the order of 10 $\mathrm{\mu K}$.

Currently, the GBAR experiment is working towards the first-ever synthesis of the antihydrogen ions using the following scheme: first, antiprotons ($\mathrm{\bar{p}}$) and positronium (Ps = $\mathrm{e^+ e^-}$ ) are mixed in the so-called reaction chamber to obtain (hot) antihydrogen atoms ($\mathrm{\bar{H}}$). These $\mathrm{\bar{H}}$ atoms then undergo a second charge exchange with Ps, producing $\mathrm{\bar{H}^+}$ ions.

The first step, the production of (hot) $\mathrm{\bar{H}}$ atoms, has been achieved for the first time in 2022, and at the end of the 2024, we have improved the production rate of $\mathrm{\bar{H}}$ by an order of magnitude. The increased rate and better mastery of systematic effects enabled the measurement of the charge exchange cross section for the first reaction for ground state positronium and at two different antiproton kinetic energies: 4 and 6 keV. This cross section was measured only once in the past with protons for energies between 11 and 15 keV. Currently, GBAR is also in the process of measuring separately the cross section for the second charge-exchange reaction but with an energy-tunable pulsed hydrogen beam obtained from photo-neutralisation of $\mathrm{H^-}$ provided by ELENA. In this talk we will report on the results of both cross section measurements.

Speaker: Ivana Belosevic (CEA Saclay)

15:10 Towards the study of antiprotonic atoms and their annihilation fragments at AEgIS

On behalf of the AEgIS collaboration

The Antimatter Experiment: Gravity, Interferometry, Spectroscopy (AEgIS) at CERN's antimatter factory has achieved remarkable performance in trapping antiprotons for the pulsed creation of antihydrogen, as well as other antimatter-bound systems, such as positronium. Currently, a new technique is being developed using the AEgIS infrastructure to synthesize antiprotonic atoms, where an antiproton (1836 times the mass of the electron) is captured in close orbits around the atomic nucleus. This synthesis, performed within a Penning-Malmberg trap, enables novel studies of antiprotonic atoms in an ultra-high vacuum environment. It facilitates the creation of highly excited Rydberg antiprotonic atoms and, following annihilation, the capture of the resulting highly charged nuclear fragments. The study of these fragments offers unique insight into the annihilation mechanism, nuclear structure properties, and provides a new tool to synthesize radioactive highly charged ions (HCIs) in a trapped environment. In this talk, I will describe the proof-of-principle studies and the ongoing efforts towards the development of this new technique.

Speaker: Fredrik Parnefjord Gustafsson (CERN)

15:30 → 16:00 Coffee

16:00 → 17:20 Session: Thu - 4

Convener: Malte Hildebrandt (PSI - Paul Scherrer Institut)

16:00 Measurement of the EDM of the muon and betadecaying nucleis using the frozen-spin technique in a compact stroage trap

Electric dipole moments (EDM) of fundamental particles inherently violate the combined symmetry of charge-conjugation and parity inversion (CP) .
At PSI we plan to measure the EDM of the muon using the frozen-spin technique within a compact storage trap. This method exploits the high effective electric field, E = 165 MV/m, experienced in the muon’s rest frame with a momentum of about 23 MeV/c when passing through a solenoidal magnetic field of B=2.5 T. The same trap may be exploited for a first measurement of the EDM of Li8,providing an indirect measurement of the proton EDM.

The poster will outline fundamental considerations for a muon and a Li8 EDM search and present an overview of the demonstration experiment bein mounted at a secondary muon beamline of the Paul Scherrer Institute.

In an initial phase the expected sensitivity to a muon EDM is 4E-21 ecm, assuming 200 days of data.
In a subsequent phase, Phase 2, we propose to improve the sensitivity to 6E-23 ecm using a dedicated instrument installed on a different beamline that produces muons of momentum 125 MeV/c.
The search for the Li8 EDM could be conducted at the VITO beamline of ISOLDE, CERN.

Speaker: Philipp Schmidt-Wellenburg (PSI - Paul Scherrer Institut)

16:20 Measuring the electron electric dipole moment using ultracold YbF molecules

The Standard Model of Particle Physics is a versatile and well-tested theory. However, it does not explain the extreme imbalance of matter over antimatter observed in our Universe. A possible mechanism that could explain this asymmetry includes new sources of CP violation, which could result in the existence of a permanent electric dipole moment in fundamental particles such as the electron.
The most precise measurements of the electron electric dipole moment (eEDM) all use molecules [1,2]. The molecules are spin-polarized, and the eEDM is determined by measuring the spin precession frequency in an applied electric field. At Imperial’s Centre for Cold Matter, we have set up an experiment to measure the electron EDM using ultracold YbF molecules [3,4]. The electron is exposed to an exceptionally large effective electric field due to the heavy polar nature of the YbF molecule. To reach high sensitivity, the molecules are laser cooled in the two transverse directions [5], and the spin precession frequency is measured as the molecules fly through a beamline setup. This experiment is currently operational, and I will present the sensitivity that we reach and our efforts to control systematic effects.
[1] V. Andreev et al., Nature 562, 355 (2018)
[2] T. S. Roussy et al., Science 381, 46 (2023)
[3] N. J. Fitch et al., Quantum Sci. Technol. 6, 014006 (2021)
[4] Collings F. et al,arXiv:2503.21725v1 (2025)
[5] Alauze X. et al, Quantum Sci. Technol., 6, 044005 (2021)

Speaker: Elise Wursten (Imperial College London)

16:40 Status and progress of the NL-eEDM experiment

Status and progress of the NL-eEDM experiment
Jelmer Levenga, on behalf of the NL-eEDM collaboration

Within the Standard Model fundamental particles are predicted to have a permanent electric dipole moment (EDM). An EDM is a CP-violating signature which is small within the standard model, but can be greatly enhanced in Beyond the Standard Model (BSM) physics. EDM experiments are therefore powerful tools in constraining BSM models. The NL-eEDM experiment aims to be a competitive determination of the electron EDM (eEDM) with measurements on a spin 1 system, barium monofluoride molecules $^{138}\text{Ba}^{19}\text{F}$ [1]. The experiment uses a spin precession technique within the electronic and rovibrational ground state of the molecule, the $X^2\Sigma$ state. Using short optical pulses, within this state a superposition is be created in the angular momentum $F = 1$ state, eg: |$\psi$> = $\frac{1}{\sqrt{2}}$(|1,-1> + |1,1>). This superposition then evolves in time in well-controlled electric and magnetic fields resulting in an interference signal, which can also be read out with an optical pulse. The shape and behavior of the signal as a function of these parameters is theoretically well understood, allowing for a characterization of systematic effects without the need for additional auxiliary measurements [2]. Our experiment is uniquely powerful in its ability to provide measurements and data on the state of the experimental parameters such as the intensity and detuning of the pulses and strength of applied fields during operations, which are captured in the interference spectrum. Our setup has been moved to a new lab which provided an opportunity to perform upgrades to improve its sensitivity and diagnostic capabilities. We will discuss these upgrades and the future of the experiment.

[1] P. Aggarwal, H. L. Bethlem, A. Borschevsky, M. Denis, K. Esajas, P. A. B. Haase, Y. Hao, S. Hoekstra, K. Jungmann, T. B. Meijknecht, M. C. Mooij, R. G. E. Timmermans, W. Ubachs, L. Willmann, A. Zapara. “Measuring the Electric Dipole Moment of the Electron in BaF.” The European Physical Journal D 72, (2018): 197. https://doi.org/10.1140/epjd/e2018-90192-9.

[2] A. Boeschoten, V. R. Marshall, A. Borschevsky, S. Hoekstra, T. B. Meijknecht, A. Touwen, J. W. F. van Hofslot, H. L. Bethlem, K. Jungmann, M. C. Mooij, W. Ubachs, R. G. E. Timmermans, L. Willmann. “Spin-Precession Method for Sensitive Electric Dipole Moment Searches.” Phys. Rev. A 110, (2024): L010801. https://doi.org/10.1103/PhysRevA.110.L010801.

Speaker: Jelmer Levenga (University of Groningen, Nikhef)

17:00 NuPECC Long Range Plan 2024 for European Nuclear Physics

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Speaker: Eberhard Widmann (Stefan Meyer Institute)

18:00 → 21:00 Workshop Dinner

Friday 12 September

09:00 → 10:30 Session: Fri - 1

Convener: Wataru Ootani (Univ. of Tokyo)

09:00 70 years of muon radiography: a biased historical overview

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Speaker: Christopher Morris (LANL)

09:30 NREC - the Nuclear Radius Extraction Collaboration

The characterization of size and structure of hadrons and nuclei has been the focus of a large work effort over multiple decades. Recently, this topic garnered new interest for various reasons, including the proton charge radius puzzle and advances in determinations of the gravitational form factors. The accurate extraction of quantities does not only rely on precise data, but also on well-developed analysis techniques.
NREC is a group of physicists from multiple fields interested in these topics, with the aim to facilitate the exchange of ideas, the development and dissemination of tested analysis methods and their application on published data, as well as the discussion of new results. The charter has been signed by more than 110 physicists from all over the world.

In the talk, I will discuss the physics motivation, our goals, and the current activities.

Speaker: Jan Bernauer (Stony Brook University and RIKEN-BNL Research Center)

10:00 Charge radii of the lightest nuclei (Z=1 and 2)

The charge radii of the isotopes of hydrogen and helium serve as important benchmarks for understanding of nucleon and nuclear structure, and are essential input the QED tests or determinations of fundamental constants via high-precision laser spectroscopy of ordinary atoms.
Our laser spectroscopy of muonic H through He-4 has provided precision measurements of the 4 lightest nuclei. I will highlight some of the results and give an outlook on future experiments.

Speaker: Randolf Pohl (Uni Mainz)

10:30 → 11:00 Coffee

11:00 → 12:40 Session: Fri - 2

Convener: Efrain Patrick Segarra (PSI - Paul Scherrer Institut)

11:00 Kaon physics: present status and prospects

Kaon physics has played a foundational role in particle physics, from the discovery of strangeness and CP violation to some of the most precise tests of the Standard Model (SM). Today, it continues to provide a uniquely sensitive probe of potential new physics through the study of rare processes and flavor-changing neutral currents. This talk will present an overview of the current status and future prospects of the field, with a focus on experimental efforts. I will highlight recent progress from dedicated kaon experiments such as NA62 at CERN and KOTO in Japan, which are leading the search for the ultra-rare decays K+ to pi+ nunubar and KL to pi0 nunubar, respectively. These processes are highly suppressed in the SM and theoretically clean, making them powerful indirect probes of heavy new physics, including models with new sources of flavor or CP violation. I will also discuss the feasibility and ongoing studies exploring the potential for rare kaon decay measurements at LHCb, as well as prospects for future kaon experiments that aim to further push the intensity frontier. Together, these efforts reinforce kaon physics as a precision frontier with strong discovery potential in the coming decade.

Speaker: Radoslav Marchevski (EPFL - EPF Lausanne)

11:30 Recent developments in the theoretical description of $\mu \to e$ conversion in nuclei

The rate for $\mu\to e$ conversion in nuclei is set to provide one of the most stringent tests of lepton-flavor symmetry and a window into physics beyond the Standard Model. However, to disentangle new lepton-flavor-violating interactions, in combination with information from $\mu\to e\gamma$ and $\mu\to 3e$, it is critical that uncertainties at each step of the analysis be controlled and fully quantified. In particular, nuclear response functions related to the coupling to neutrons are notoriously problematic, since they are not directly constrained by experiments. These shortcomings can be addressed by combining ab initio calculations with charge distributions from elastic electron scattering by exploiting strong correlations among charge, point-proton, and point-neutron radii and densities. In this talk, I want to discuss these recent developments, which now allow, for the first time, for a comprehensive consideration of nuclear structure uncertainties in the interpretation of $\mu\to e$ experiments.

Speaker: Frederic Noël (ITP, AEC, Uni Bern)

11:50 The Mu3e Experiment

The Mu3e experiment searches for the charged lepton-flavour violating decay of a muon into two positrons and one electron. Due to the negligible Standard Model branching ratio, any observation would provide unequivocal evidence of new physics. A first phase of the experiment aims for a single-event sensitivity of one in $2 \cdot 10^{15}$ muon decays.

To reach this goal, the collaboration developed a low-mass pixel tracker based on high-voltage monolithic active pixel sensors, complemented by timing detectors, a system designed to fully reconstruct the kinematics of candidate $\mu^+ \rightarrow e^+ e^+ e^-$ events. A streaming data-acquisition system and online filter farm allow for the processing of over $10^8$ muons on target per second.

The experimental apparatus is currently being commissioned at the $\pi E5$ secondary muon beamline at the Paul Scherrer Institute. In the June 2025 campaign the vertex detector, timing detectors, the data-acquisition system, and all slow control systems were integrated inside our 1T superconducting magnet, and operated at a beam rate of over $10^7$ $\mu/s$. Next year the outer layers of the pixel tracker will be added, which enables first physics data taking before the long HIPA shutdown.

Speaker: Frederik Wauters (Johannes Gutenberg University Mainz)

12:10 Status of the High Intensity Muon Beamline at PSI

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Speaker: Andreas Knecht (Paul Scherrer Institut)

12:30 Final remarks

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12:40 → 13:50 Lunch