Neutron Lifetime Puzzle

Saturday 13 Sept 2025, 00:00 → 21:30 Europe/Zurich

WHGA/001 - Auditorium (PSI)

Anita Govaerts Van Loon / Tanja Muelhaupt (PSI - Paul Scherrer Institut), Bernhard Lauss (PSI - Paul Scherrer Institut), Dieter Achim Ries (PSI - Paul Scherrer Institut), Noah Gabriel Yazdandoost Khosravi (PSI - Paul Scherrer Institut)

Satellite Workshop "Neutron Lifetime Puzzle — Measurements, Systematics, and Theoretical Implications at PSI2025"

We look forward to a dynamic workshop focused on recent and ongoing efforts to measure the neutron lifetime, addressing experimental techniques, systematic uncertainties, and implications for the Standard Model of Particle Physics. 

Beyond lifetime measurements, we also encourage discussions on neutron beta decay parameters, and connections to precision tests of weak interactions. Looking ahead, we want to assess future experimental strategies and theoretical perspectives that could refine our understanding of neutron decay.

There are no conference fees for attending the satellite workshop.

 

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09:00 → 10:10 Opening Session: Welcome and Introduction

09:00 Welcome & Introduction

Speakers: Bernhard Lauss (PSI - Paul Scherrer Institut), Dieter Achim Ries (PSI - Paul Scherrer Institut), Noah Gabriel Yazdandoost Khosravi (PSI - Paul Scherrer Institut)

09:10 The Neutron Lifetime: Why do we keep measuring it?, Why do we keep getting it “wrong”?

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Speaker: Geoffrey Greene (University of Tennessee)

09:40 Addressing the neutron lifetime anomaly with correlation parameters

Combining measurements of the lifetime with measurements of (angular) correlation parameters in neutron beta decay, we strive to most precisely determine the matrix element $V_\mathrm{ud}$ of the CKM quark mixing matrix. The goal is to test CKM unitarity on the $10^{-4}$ level.

Alternatively, combining the determination of $V_\mathrm{ud}$ from superallowed nuclear decays and neutron correlation parameters, allows for a rather precise determination of the neutron lifetime.

In this talk, I will present the current status and prospects of correlation measurements (which exhibit their own anomaly), and discuss the connection to the neutron lifetime.

Speaker: Bastian Märkisch (Technische Universität München)

10:10 → 10:40 Break

10:40 → 12:25 Recent Advances in Neutron Lifetime Measurements

10:40 Current status and future activities of UCNtau+

Speaker: Susan Seestrom

11:10 The NIST Beam Neutron Lifetime Experiments

A series of neutron lifetime measurements using the beam method has been conducted at the National Institute of Standards and Technology (USA) for more than 30 years. In these experiments, a cold neutron beam passes through a quasi-Penning trap where protons from free neutron decay are captured and subsequently counted by a silicon detector. We will review the previous BL1 experiment (2005, 2013); discuss the current BL2 experiment and systematic issues we have focused on, in particular proton charge exchange with residual gas in the trap; and describe BL3, a new next-generation version of the experiment with improved systematics and capable of a high precision result at the <0.3 s level.

Speaker: Fred Wietfeldt (Tulane University)

11:55 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)

12:25 → 12:30 Photo

12:30 → 13:30 Lunch

13:30 → 14:30 Systematic Effects, Physics beyond the Standard Model

13:30 The Bound Beta Decay of the Free Neutron - A Possible Solution to the Neutron Lifetime Puzzle

The Bound Beta Decay of the Free Neutron - A Possible Solution to the Neutron Lifetime Puzzle

Erwin Gutsmiedl

Technische Universität München, Physik Department, James-Franck-Str.1,
D-85748 Garching, Germany

The discrepancy in neutron lifetime measurements between storage experiments with ultracold neutrons and beam experiments with cold neutrons has persisted as a significant puzzle in particle physics. Storage experiments typically involve trapping neutrons and observing their decay over time, while beam experiments measure the decay products, such as electrons and protons, as neutrons pass through the decay volume. Interestingly, the average lifetimes obtained from these two methods differ by nearly 10 seconds, despite reduced statistical and systematic errors.
A promising yet controversial explanation for this discrepancy involves the bound beta decay of the neutron into a hydrogen atom and a anti-neutrino. Traditional theoretical estimates suggested a branching ratio of approximately $\sim10^{-6}$, which seemed too small to account for the observed differences. However, recent theoretical developments propose a significantly larger branching ratio of $\sim 10^{-2}$, potentially bridging the gap between the two experimental findings.
In this presentation, I will explore the implications of these new theoretical predictions and present my own calculations, which question their validity. By critically evaluating these findings, I aim to shed light on whether bound beta decay could indeed resolve the neutron lifetime puzzle or if alternative explanations need to be considered.

Speaker: Gutsmiedl Erwin (Technische Universität München)

13:50 How Small Heating of Ultracold Neutrons could be related to the Neutron Lifetime Puzzle

After cooling and trapping, Ultracold Neutrons (UCNs) reacquire kinetic energy over a relatively long period of time. This process is called Small Heating and it is not yet fully understood. Most intriguingly, this process wears off to some extent.

In 2024 we have proposed a dressed quark model that could explain the Neutron Lifetime Puzzle (NLP). The neutrons in the beam could be in so-far undetected low-energy excited states, while neutrons in the bottle are in the ground state. Neutrons in excited states have more restricted beta decay channels and are thus longer lived than their ground-state versions.

We revisit the Small Heating phenomenon in the light of the excited-state hypothesis. We suppose there is a chain of electromagnetic decays from a neutron in one of the excited states to the ground-state neutron. If this chain is sufficiently long-lived that cold neutrons arrive in the bottle still being in one of the excited states, they may undergo a part of the decay chain inside the bottle. The recoil of each decay event could contribute to the Small Heating of UCNs at an early stage after trapping before wearing off. From the time evolution of the statistical moments of the kinetic energy of UCNs during Small Heating we could infer information on the decay chain and the average photon energy.

Speaker: Felix Hummel (TU Wien)

14:10 Experimental limits on an excited neutron state as an explanation of the beam-bottle neutron lifetime discrepancy.

Koch and Hummel[1] suggest a new solution to the neutron lifetime enigma[2]. The neutron lifetime enigma arises from the 4.4 standard deviation difference between the lifetime measured for bottled neutrons[3] and measurements of lifetime from a beam of cold neutrons[4]. Koch and Hummel point out the beam experiments measure the decay rate of neutrons very close in time to their source whereas the bottle measurement use neutrons ~1000 s after their production. They postulate the existence of an excited state of the neutron, n*, that has a longer $\beta$-decay lifetime than the ground state, n, and that a transition could occur between these two states by $\gamma$-ray emission with a decay time shorter than the holding time used for bottle lifetime measurements. Here, we will present an analysis of the UCN$_ \tau$data aimed a searching for an explanation of this difference using the model proposed by Koch and Hummel
Acknowledgments
The authors would like to thank Dr. Ben Koch for his comments on the paper. This work is supported by the LANL LDRD program; the U.S. Department of Energy, Office of Science, Office of Nuclear Physics under Awards No. DE-FG02-ER41042, No. DE-AC52-06NA25396, No. DE-AC05-00OR2272, and No. 89233218CNA000001 under proposal LANLEDM; NSF Grants No. 1614545, No. 1914133, No. 1506459, No. 1553861, No. 1812340, No. 1714461, No. 2110898, No. 1913789, and No. 2209521; and NIST precision measurements grant.
1. Koch, B. and F. Hummel, Exciting hint toward the solution of the neutron lifetime puzzle. Physical Review D, 2024. 110(7): p. 073004.
2. Greene, G.L. and P. Geltenbort, The neutron enigma. Scientific American, 2016. 314(4): p. 36-41.
3. Musedinovic, R., et al., Measurement of the free neutron lifetime in a magneto-gravitational trap with in situ detection. Physical Review C, 2025. 111(4): p. 045501.
4. Yue, A., et al., Improved determination of the neutron lifetime. Physical review letters, 2013. 111(22): p. 222501.

Speaker: Christopher Morris (LANL)

14:30 → 15:00 Future Measurements and Experimental Strategies I

14:30 τSPECT- towards a new measurement of the free neutron lifetime in a full-3D magnetic trap

Neutron physics can provide cornerstone ingredients for a high-precision test of the Cabibbo-Kobayashi-Maskawa (CKM) matrix unitarity without nuclear structure corrections. The matrix element Vud is extracted from the combination of a high-accuracy determination of λ, the ratio of axial-vector and vector coupling strength of the weak interaction, a commensurate theoretical description of neutron beta decay, and a high accuracy determination of τn. In a first step, τSPECT aims to determine τn with an uncertainty of < 0.3 s to illuminate the neutron lifetime puzzle, a significant disagreement of τn measurements using complementary methods. The τSPECT experiment confines ultracold neutrons (UCNs) in a full 3D magnetic gradient field trap. We have operated the instrument at the UCN source of the Paul Scherrer Institute since 2023. After filling and holding UCNs for hundreds of seconds in the trap, τSPECT counts the surviving UCNs and extracts τn from the storage curve. Quasi-trapped
UCNs could leave the trap on the timescale of τn, and a strict control of these marginally trapped UCNs is required to avoid a bias towards a low value of τn. In 2023, τSPECT was transferred from JGU Mainz to PSI and reassembled. In 2024, τSPECT has performed a first blinded science data run with an anticipated statistical precision of 1 s on τn. The most important systematic bias effects could be investigated for the first time with high statistics, and dedicated background studies were performed. We will present the current instrument’s performance, selected aspects of the ongoing data analysis, and near-term performance upgrade opportunities. The τSPECT setup provides an indispensable test bed for future full-magnetic trap setups featuring larger volume and deeper trapping potential.

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

15:00 → 15:30 Break

15:30 → 16:10 Future Measurements and Experimental Strategies II

15:30 UCNProBe a beam-type experiment using ultracold neutrons

UCNProBe is an upcoming experiment at Los Alamos National Laboratory designed to measure the lifetime of a free neutron, τₙ. Currently, there is a significant discrepancy of about 10 seconds (4.5 sigma) between the two primary methods used for this measurement. One approach involves using a cold neutron beam to detect the charged products of neutron beta decay. The other relies on ultracold neutrons (UCN), which are stored in physical or magnetic traps. UCNProBe will follow the beam-type measurement method but uniquely incorporate UCN characteristics. In this setup, UCN will be confined within a material bottle made of deuterated polystyrene box that will also serve as an in-situ detector for electrons from the beta decay. For neutron counting, a boron-coated YAP:Ce crystal scintillator will be used. This presentation will provide an overview of the experiment and the latest updates on its development.

Speaker: Martin Krivos (Los Alamos National Laboratory)

15:50 The path to first measurements with the superconducting magnetic storage "PENeLOPE"

Over the past decades we have designed and constructed a superconducting magnetic trap for ultra cold neutrons (PENeLOPE). With the large storage volume of about 550 liters as well as the design allowing for real time detection of decay protons, we aim at a high precision measurement with unprecedented systematic and statistical uncertainties. The system was constructed and set up at the Technical University of Munich in 2020. Following initial tests, PENeLOPE was transferred to TRIUMF in 2024 to perform first measurements at the new TUCAN UCN source.
I will outline our setup and present data from the cryostat tests performed ealier this year.

Speaker: Moritz Gundel (TUM)

16:10 → 17:00 Round Table Discussion

16:10 Round Table Discussion

Discussion on the future.
Where is / should be neutron lifetime 10 years from now:

Moderator: Peter Geltenbort

Panelists:
Felix Hummel
Kenji Mishima
Chris Morris
Dieter Ries
Fred Wietfeldt

Speaker: Peter Geltenbort (Institut Laue-Langevin)

16:50 Closing Words

Speakers: Bernhard Lauss (PSI - Paul Scherrer Institut), Dieter Achim Ries (PSI - Paul Scherrer Institut), Noah Gabriel Yazdandoost Khosravi (PSI - Paul Scherrer Institut)