Workshop on Muonic Atom Spectroscopy

Friday 21 Oct 2016, 09:00 → 18:10 Europe/Zurich

Auditorium / WHGA001 (Paul Scherrer Institut)

The workshop aims to bring together the physics community interested in performing high resolution muonic X-ray spectroscopy of medium and high-Z elements. While muon beam intensities and quality have been improved in recent years at PSI, still muonic X-rays have never been studied with highly efficient multi-Ge-detector arrays covering large solid angles. Such advancements open the way to the measurements of nuclear charge radii in radioactive elements and of atomic parity violation effects in muonic atoms. From the fruitful exchange between participants, the workshop aims to strengthen the physics case of these measurements and to discuss future plans and ideas.
 

1 Welcome opening

Muon spectroscopy for nuclear charge radii

2 Nuclear charge radii measurements by collinear laser spectroscopy and Penning trap g-factor experiments

Collinear laser spectroscopy provides access to nuclear ground-state properties via the hyperfine structure (including the isotope shift) of atomic spectra. This is still the only technique to access information on nuclear charge radii along long isotopic chains from neutron-deficient up to very neutron-rich isotopes and lifetimes down to the millisecond scale. Also nuclear moments can be addressed with very high accuracy. The basic principles will be shortly summarized, the extraction of nuclear properties and their impact for the understanding of nuclear physics is highlighted on a few recent examples. These include, e.g. the determination of absolute charge radii for the lightest elements, changes in the shell structure and connected disappearance of magic numbers as well as the influence of the monopole interaction on the evolution of the shell-structure. In the case of g-factor measurements of the electron bound in hydrogen-like ions by using a single ion confined in a Penning trap a comparison of the experimental value with the state-of-the-art theoretical value, which includes nuclear structure corrections, allows for a determination of the nuclear charge radius of the isotope of interest. Although the extracted values are not yet competitive with the established ones, our current proof-of-principle experiment demonstrates the feasibility of this method of nuclear radius determination. The technique of Penning-trap based g-factor measurements of the bound electron in highly charged ions as well as recent results will be presented.

Speaker: Prof. Klaus Blaum (Max Planck Institut für Kernphysik)

Slides Muon_Spectroscopy_WS_Blaum_2016_Web.ppt

3 Muonic atoms to measure charge radii of stable and soon unstable nuclei?

-ray transitions in muonic atoms have been used to derive absolute charge radii of many stable nuclei. Presently, the obtained radii, together with those provided by electron scattering, are used extensively as input to connect the optical isotope shifts with changes in charge radii for radioactive nuclei. Laser spectroscopy on radioactive nuclei which provides these optical isotope shifts is at present a very active field (3 setups at CERN-ISOLDE alone) and thus many radii of stable nuclei are used every year as calibrations. And because changes in charge radii are very small, it is important that the reference radii of stable isotopes, or at least their differences, are very precise and free of large systematic errors. Unfortunately, radii derived so far from muonic atom transitions – measured mostly in 1970’ and 1980’s – have large systematic uncertainties. This is partly due to quite uncertain nuclear polarization corrections, which were calculated with methods available at that point. And these uncertainties propagate to differences in radii of unstable nuclei. In addition, the calibration of optical isotope shifts requires at least 3 isotopes with already known charge radii, so for chemical elements with fewer stable or extremely long-lived isotopes the connection between optical isotope shifts and changes in charge radii has to rely on atomic theory which in most cases has still too large uncertainties. For the above reasons, in order to derive precise radii of radioactive nuclei, it would be extremely interesting to investigate again muonic transitions in some of the stable nuclei as well as in long-lived isotopes for elements where only 2 radii have been measured before. In addition, if the studies can be extended to more unstable isotopes, this would allow studying charge radii of unstable nuclei with a new method and could even provide results for nuclei which are difficult to access with laser spectroscopy. These new measurements have to be accompanied with new calculations of nuclear (and other) corrections, using modern approaches and the available computing power. This presentation will show briefly how charge radii were derived from Muonic-atom data in the past, how they are used to calibrate optical isotope shifts and why the uncertainties are too large, and finally how the situation can be improved with new muonic-atom measurements and calculations.

Speaker: Dr Magdalena Kowalska (CERN PH Department)

Slides MKowalska-PSI16.pptx

4 (e,e) + muX: densities + moments

We discuss the information on nuclear densities that can be determined via elastic electron scattering. This is compared to the knowledge that results from the analysis of muonic X-ray data. The complementary nature of the two probes is emphasized. We also address the question how how more detailed properties of nuclear densities could in the future be investigated with muonic X-rays.

Speaker: Prof. Ingo Sick (University of Basel, Department of Physics)

Slides psiq.pdf

10:35 Coffee break

Muon spectroscopy for nuclear charge radii

5 Nuclear shape and QED effects in muonic atoms

In this talk I will describe QED calculations of transition energies in muonic atoms. I will particularly focus on the nuclear model dependence of the Coulomb and QED corrections. IN particular, energies evaluated using Fourrier-Bessel or Sum of Gaussian and Fermi models with identical mean-spherical radius will be compared. Effect on the hyperfine structure will be also be examined.

Speaker: Prof. Paul Indelicato (Laboratoire Kastler Brossel, CNRS)

Slides PSI-Nucl-Effects-atoms-2016-Indelicato.pdf

6 Muon spectroscopy with rare nuclei via transfer reaction in solid hydrogen

Muonic atom spectroscopy is a unique tool to determine the nuclear charge distribution and has successfully been used for many years to measure sable isotopes. Several years ago the cold hydrogen film method was proposed to expand muonic atom spectroscopy by utilizing nuclear beams, including, in the future, radioactive isotope beams. Muonic radioactive atoms would be produced by utilizing the transfer reaction in hydrogen/deuterium solid films for measuring muonic X-rays. An overview of the results of our experiment with stable isotopes at RIKEN-RAL and future prospects using radioactive materials will be presented.

Speaker: Dr Patrick Strasser (KEK)

Slides strasser_PSI_MuAtSpec2016.pdf

7 Toward the measurement of the nuclear charge radius of 226Ra

Muonic atoms as laboratories for fundamental physics provide crucial input to quantum electrodynamics, the weak interaction and the strong interaction. Many studies of muonic atoms have relied on the detection of X-ray from the muonic cascades. Most stable and a few unstable isotopes have been investigated with muonic atom spectroscopy techniques. In particular, muonic atoms have been used to extract the most accurate nuclear charge radii. However, experiments with muonic atoms have been limited by low muon rates, poor beam quality and large muon stop volumes, but also by available detector technology for this environment. While beam intensities and quality have been improved in recent years, still no higher multiplicity spectroscopy of muonic cascades has been performed. We are preparing an experiment to determine the charge radius of radium, which is one of the missing parameters for the measurement of atomic parity violation in radium. In this talk I will present the plans and status of the experiment and the results of our last beam time.

Speaker: Andreas Knecht (Paul Scherrer Institut)

Slides Andreas_Knecht_-_Radium_Charge_Radius.pdf

12:30 Lunch

Muon Spectroscopy for Atomic Parity Violation

8 Muon Beam Prospects - Present & Future

The intensity frontier has, in addition to technological developments, enabled a boom in muon physics allowing both mu- & mu+ beams to set some of the most stringent limits on fundamental parameters and decays, also reflected in the increased demand for muonic atom spectroscopy experiments. A general overview of muon beam production techniques and possibilities offered by PSI's High Intensity Proton Accelerator complex (HIPA) will be presented placing emphasis on mu- beams for muonic atom formation. Future prospects will also be addressed.

Speaker: Peter-R. Kettle (Paul Scherrer Institut)

Slides BeamsPRK.pdf

9 MINIBALL - Status and Perspectives

The high-resolution MINIBALL germanium detector array consists of 24 six-fold segmented, tapered, encapsulated high-purity germanium crystals. It was specially designed for low multiplicity experiments with low-intensity radioactive ion beams (RIB). Main features of the spectrometer are a very high photo peak efficiency for gamma or X-ray radiation and position sensitivity due to the electronic segmentation. The combination of these properties allows spectroscopic investigations in a wide energy range and to cope with huge Doppler effects. The Miniball array has been used in numerous Coulomb excitation and transfer reaction experiments with exotic RIBs with energies of 3 MeV/A produced at the ISOLDE facility. Recently the spectrometer was coupled to the super-conducting HIE-ISOLDE accelerator, which provides beams up to 5.5 MeV/A in a first stage. In the past, the array was successfully employed for studies with stable beams at various accelerator laboratories and RIBs provided at relativistic energies after fragmentation at GSI, Darmstadt.

Speaker: Prof. Peter Reiter (University of Cologne)

Slides PSI_PR_2016.pptx

10 Three different ways to look for NC parity violation with muons

I will describe several conceptual schemes for detecting parity violating muon-nucleus interactions. 1. Muonic atoms, where Z=30 and higher atoms offer a possibility to look for 2S->1S parity violating transition. 2. Scattering of polarized muons, where muon spin is manipulated in a storage-ring type set-up. 3. Using optical activity type rotation of the transverse linear polarization of muons.

Speaker: Prof. Maxim Pospelov (Perimeter Institute for theoretical physics)

Slides PSImuAt2016.pdf

15:15 Coffee break

Muon Spectroscopy for Atomic Parity Violation

11 Early studies for PV experiments at SIN/PSI

Muonic atoms had been considered around 1974 as candidates for experiments to study neutral weak currents. Two main directions had been followed: M1(E1) mixing in s-p states and E2(E1) mixing in p-d states. After an introduction into the subject which outlines the special role of muonic atoms, the implications of the two approaches are discussed. In the M1(E1) mixing the influence of the electronic shell reduces the feasibility dramatically and was subject of intensive theoretical work which will be described in some detail. On the side of the experiment it could be shown at PSI that a high degree of ionization could be reached for low Z muonic atoms. The two-photon transition 2s-1s could be observed for the most interesting case of muonic boron but no M1 transition has been measured so far. In case of the E2(E1) mixing the 3d-1s transition had been observed in earlier experiments at CERN/SC for high Z muonic atoms. In a collaboration of Mainz/PSI muonic thulium could be worked out as most promising case combining some attractive features. An experiment to check the assumptions about the parameters of the mixing states has still to be performed. For both cases the order of magnitude for parity violating effects is given

Speaker: Dr Leopold Simons (Paul Scherrer Institute)

Slides Simons_PV.pdf

12 Atomic Parity Violation to Search for New Physics

The observation Atomic Parity Violation in atoms has been crucial for the acceptance of the Standard Model as a general theory in physics. Today APV has been measured best in Cs atoms and provides for precise value of the weak mixing (Weinberg) angle at the lowest energies with sub % accuracy. A deviation of this value from predictions based on measurements in all accessible ranges of momentum transfer would clearly indicate New Physics outside the present Standard Model. Possibilities of such physics could be for example Z’ bosons heavier than they can be limited at LHC or dark Z bosons at very low energies. It appears that APV provides for an excellent opportunity to observe these dark Z bosons or set the lowest mass bounds on them. For the extraction of the weak mixing angle from APV measurements precise calculations of atomic structure are indispensable. They are by far best possible for alkali atoms and alkali-earth ions. The weak effects scale stronger than Z3, where Z is the nuclear charge of an atom. Therefore heavy atoms or ions such as Fr or Ra+ are well suited for such research. In Groningen a Ra+ project using a single trapped ion is under way. It uses Ba+ as a non-radioactive precursor. It appears that for atomic theory the knowledge of the absolute value of the nuclear radius will become the limiting factor in the theoretical description of the ion. Muonic atom spectroscopy has a high potential to provide the required nuclear parameters.

Speaker: Prof. Klaus Jungmann (University of Groningen)

Slides PSI_Oct_2016_KJ.pptx

13 Atomic Parity Violation in muonic atoms

Searching for neutral current effects in muonic atoms is an old idea. A muon around a nucleus in the 2S state gets a small parity violating admixture from the 2P state, which then allows E1-M1 interference in the 2S-1S. For nuclei around Z=30, the experimental challenge is to deal with the intense background from nP-1S transitions and electrons from Michel decays. We investigate the feasibility of a parity violation experiment, utilizing large solid-angle germanium detectors to get the background from the atomic cascade under control.

Speaker: Dr Frederik Wauters (Johannes Gutenberg University Mainz)

Slides PSIworkshop2016.pdf

14 Closing remarks

Visit of the PSI HIPA facility