Physics at the low-energy, high-precision frontier

24 February 2014

The PSI2013 workshop focused on fundamental symmetries and interactions.

Isochronous cyclotron

More than 180 physicists from around the world gathered at the Paul Scherrer Institut (PSI) last year for the 3rd workshop on the “Physics of fundamental Symmetries and Interactions” at low energies and the precision frontier – PSI2013. Broadly speaking, the focus was on high-precision experiments, with results complementary to those at the LHC, often covering a parameter space in physics beyond the Standard Model that is inaccessible to direct searches at the LHC or even at future colliders.

PSI’s particle-physics laboratory fosters cutting-edge research using the unmatched high power of its 590 MeV, 2.2 mA proton cyclotron to produce the brightest low-momentum beams of muons and pions and, since 2011, ultracold neutrons. This environment set the scene for lively discussions on the latest results and the future direction of worldwide low-energy precision experiments. Among the many workshop contributions, there were several major topical areas of interest.

Fundamental physics probed with antiprotons and antihydrogen featured prominently, with recent results from experiments at CERN’s Antiproton Decelerator. The now regular production of antihydrogen has moved these experiments closer to final physics measurements. Among the main goals are sensitive tests of CPT symmetry and measurements in antihydrogen spectroscopy, such as determination of the ground-state hyperfine splitting, together with tests of antihydrogen free fall. A recent result is the Penning-trap measurement by the ATRAP collaboration of the antiproton’s magnetic moment to 5 ppm precision. A further highlight, involving Penning traps but with ordinary matter, is determination of the electron’s mass with unprecedented precision by the MPI-Heidelberg group, achieving an order-of-magnitude improvement.

Many presentations covered experiments using cold (CN) or ultracold (UCN) neutrons. A full session was devoted to the neutron lifetime and worldwide progress on improving its precision, to resolve the significant outstanding discrepancy between results from neutron-storage experiments and those using beams. For the latter, a new result from the National Institute of Standards and Technology in the US was presented, consolidating the existing discrepancy.

Neutron-decay parameters and spin correlations of the decay particles are sensitive to physics beyond the Standard Model. Competing CN and UCN experiments using improved experimental techniques such as precision neutron polarimetry at the 100 ppm level were presented, with future plans for UCNs at Los Alamos National Laboratory (LANL) and the Proton Electron Radiation Channel project at the FRM II neutron source at the Technische Universität München. Other parity-violation experiments were also discussed, with a new result for neutron capture on hydrogen by the NPDG experiment at the Spallation Neutron Source (SNS) at Oak Ridge, trapped radium ions at KVI Groningen, and neutron spin rotation in helium.

UCN production with new-generation sources – either in existence or under construction – was extensively covered, including the use of superfluid helium (at Institut Laue–Langevin (ILL) and TRIUMF) and solid deuterium (Mainz, LANL and PSI) as superthermal converters. UCN densities are steadily increasing, despite experimental and technical difficulties that have slowed down the expected progress. The main thrust for these high-intensity UCN sources comes from the search for a permanent electric dipole moment (EDM) of the neutron. Because it is the focus of an experiment at PSI, there was intensive discussion on this topic at the workshop. Several talks elaborated on efforts to search for the neutron EDM by international collaborations at various institutions. These are mainly based on UCN-storage measurements that employ either Ramsey’s Oscillatory Field method (at ILL, SNS, PSI, the Petersburg Nuclear Physics Institute, TRIUMF, Osaka University and FRM II) or crystal diffraction (at ILL).

Complementary atomic (Fr, Ra, Xe) and molecular (YbF, ThO) EDM searches have even higher experimental sensitivities, but sometimes suffer from being more difficult to interpret in terms of the fundamental EDMs. Diamagnetic atoms are usually interpreted in terms of searches for nuclear EDMs, whereas measurements in polar molecules and paramagnetic atoms give limits on the electron EDM. However, the workshop was a little too early to see the result of the new ThO experiment ACME, by a Harvard/Yale University group, which appeared shortly afterwards. Proposed storage-ring-based EDM measurements with protons and deuterons are also being pursued actively.

Common to all of the EDM searches are the many challenging experimental difficulties, especially in terms of magnetic shielding and the control and measurement of the magnetic field. Presentations from the theoretical side underlined that EDM studies in different systems are complementary and necessary in helping to identify the underlying models of CP or T violation. Also in this context, recent results on CP violation were presented from the NA62 experiment at CERN, on the kaon system, and from LHCb at the LHC.

UCNs also allow study of the quantization of gravitational bound states of the neutron, which are sensitive to non-Newtonian gravity and hypothetical extra forces, mediated by, for example, axions, axion-like particles, or chameleons. Such forces can also be probed in clock-comparison experiments, as explained at the workshop for the 3He/129Xe case. These are sensitive to possible Lorentz violations, which can be accommodated in the framework of the so-called Standard Model Extension (SME). In the SME, Lorentz violation stems from an underlying background field in the universe, resulting, for example, in day/night or annual variations of fundamental parameters. Recently calculated effects in neutron decay, as well as in muonium and positronium spectroscopy, were also discussed, with experimental efforts.

Charged-lepton flavour violation was another key topic where increasing worldwide efforts are under way. Lepton flavour violation involving muons is predicted by various models that go beyond the Standard Model, at levels that might be within reach of the next generation of experiments. Nevertheless, major progress is needed, both in experimental techniques and in increased muon-beam intensities, and is being pursued actively.

The international PSI-based MEG collaboration presented its new limit of 5 × 10–13 on the μ → eγ branching ratio. The project to search for the decay μ → 3e at a sensitivity level of 10–16 was presented by the Mu3e collaboration. Impressive efforts towards the construction of the Muon Campus at Fermilab were also shown, with the goal of a new, more precise (g–2)muon measurement to help solve or confirm the present discrepancy with the Standard Model calculation. There are also plans to search for μ → e conversion within Project-X, at a sensitivity of 10–17 and beyond. Similar efforts in Japanese projects that are ongoing at Osaka University and the Japan Proton Accelerator Research Complex (J-PARC) were also detailed. These involve huge efforts in the muon sector towards, for example, μ → e conversion and muon g–2 experiments. The progress shown at J-PARC following repairs of the extensive earthquake damage was impressive.

There was great encouragement on the part of all participants to meet again at PSI for PSI2016

The new result on the pseudoscalar coupling between the muon and the proton from the MuCAP experiment at PSI was presented and discussed, finally solving a long-standing puzzle and providing the first precise value of this Standard Model parameter. Interpretations within recent calculations based on effective field theory were presented, together with relevant ongoing precision measurements in the deuterium system.

In the light of the current construction of the free-electron laser – SwissFEL – at PSI, the possible use of such high photon-intensities or electron beams for particle-physics experiments attracted much interest, for example in using high-intensity lasers for “light shining through the wall experiments”, which search for weakly interacting sub-electronvolt particles. The final session of the workshop – held with the detector workshop of the Swiss Institute of Particle Physics, CHIPP – provided an overview of state-of-the-art detector technology, which is under development to cope with future high-intensity experiments.

Aside from fundamental science, the Hochrhein Bigband jazz concert delighted participants, as did the workshop dinner featuring a “fundamental classic” of Swiss cusine – raclette. A workshop summary of the year 2034 provided an amusing outlook from a theoretician’s point of view of what might be important in particle physics 20 years from now. In the meantime, there was great encouragement on the part of all participants to meet again at PSI for PSI2016.

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