Exploring the low-energy precision frontier

23 February 2011

PSI2010 was a showcase for the low-energy route to new physics.


PSI2010, the 2nd International Workshop on the Physics of Fundamental Symmetries and Interactions at low energies and at the precision frontier, brought together experimentalists and theoreticians, united by a common quest for experimental precision using probes as diverse as neutrons, antiprotons, muons, atoms, molecules and even condensed-matter samples. The meeting, which was aimed at consolidating recent results and planning future directions in the field, took place at the Paul Scherrer Institut (PSI) on 11–14 October and was supported by PSI and the Swiss Institute for Particle Physics (CHIPP).

With 146 participants from 17 countries, the form of the workshop led to lively discussions, helping to promote the transfer of information within the community. Results were presented in 65 plenary talks and some 30 posters, most of which related to experiments. PSI being a world-leading centre for muon, pion and neutron physics, many presentations were related to investigations with neutrons (40%) and pions or muons (30%). This reflected both the high local interest as well as the strength of the worldwide community – about three-quarters of the presentations were on work at facilities other than PSI.

Gearing up for new physics

The workshop began with a talk on “How to look at low-energy precision physics in the era of the LHC” given by Daniel Wyler of the University of Zurich. He described how low-energy precision physics is complementary to the search for new physics at the LHC and how it can even answer specific questions that reach beyond the LHC – a theme that was highlighted in other talks. The final results from the TRIUMF Weak Interaction Symmetry Test (TWIST) experiment on muon decay demonstrate the impact of precision results on, for example, left–right symmetric models or sterile neutrinos, as TRIUMF’s Glen Marshall explained.

Fundamental neutron physics, introduced by Torsten Soldner of Institut Laue-Langevin (ILL), cropped up in several sessions. These covered recent controversial results on neutron-lifetime measurements in storage bottles and results on neutron decay at ILL and the Los Alamos National Laboratory (LANL), as well as new proposals for measurements with higher sensitivity. Peter Geltenbort of ILL provided a special twist to the topic with his results on the efficient guiding capabilities for ultracold neutrons (UCNs) using coated commercial Russian water hoses.

The search for permanent electric dipole moments (EDMs) of fundamental particles was discussed by several speakers, who covered the majority of the present worldwide efforts. Michael Ramsey-Musolf of the University of Wisconsin discussed the paramount importance of permanent EDMs and their cosmological implications, and he set the scene for several talks on the experimental searches for a neutron EDM at ILL, the Spallation Neutron Source, PSI, Osaka and TRIUMF. Ben Sauer of Imperial College showed new data on the search for the electron EDM in ytterbium fluoride, while Blayne Heckel reported on activities to improve on the present world record in the experiment on mercury at the University of Washington. Future directions for EDM searches and co-magnetometers using 129Xe or neutron crystal-diffraction were introduced in further talks, as well as in posters.

Part of the workshop was devoted to violations of space–time symmetry. Ralf Lehnert of Universidad Nacional Autónoma de México outlined the theoretical framework of the extension to the Standard Model that causes oriented universal fields, which could typically manifest themselves in daily or yearly time variations of physics observables. On the experimental side, Michael Romalis of Princeton University and Werner Heil of the University of Mainz presented impressive new limits from searches for violations in Lorentz symmetry in clock-comparison experiments using, respectively, the K-3He system and 129Xe and 3He.

Searches for extra forces were introduced by Hartmut Abele of the Technical University Vienna, who described using gravitational states of UCNs, while Anatoli Serebrov of the Petersburg Nuclear Physics Institute (PNPI) discussed the potential of stored UCNs for detecting dark matter. Several presentations also covered the search for tensor-type weak currents in nuclear beta-decay, using the WITCH experiment at ISOLDE at CERN and the LPCTrap facility at GANIL. Seth Hoedl of the University of Washington showed new results of an axion search based on a torsion pendulum. There were also reports on the status of the ALPHA and ASACUSA experiments at CERN, which aim at atomic spectroscopy of antihydrogen and related CPT tests and CERN’s Michael Doser explained the AEGIS experiment to probe gravity with antihydrogen.

On the facilities side, a special session provided an excellent overview of the present status of UCNs – a flourishing global area. This included reports on the performance of UCN sources in operation at LANL and the University of Mainz, as well as on the status of construction at the Technical University Munich and commissioning at PSI. Proposals for future UCN sources at the Japan Proton Accelerator Research Complex (J-PARC), TRIUMF and the PNPI were also shown at the workshop.

Several sessions were devoted to muon physics. Peter-Raymond Kettle of PSI reported on the latest results of the MEG experiment searching for the lepton-flavour violating μ → e + γ decay. The community is currently planning ahead for the next generation of searches for rare muon decays, as became clear when Bob Bernstein from Fermilab explained the Mu2e proposal, which will search for the neutrinoless conversion of muons to electrons, and Andre Schöning of Heidelberg University suggested a new μ → 3e search at PSI. Efforts towards considerably higher muon beam intensities were presented for the Research Centre for Nuclear Physics at Osaka, J-PARC and PSI, and Harry Van der Graaf of Nikhef presented new silicon-gas detectors that could be used at such future facilities.

In one of the highlights, Dave Hertzog from University of Washington presented the newly released final result on the muon lifetime from the MuLan experiment at PSI, which gives a new determination to 0.6 ppm of the Fermi weak coupling constant. The competing muon lifetime experiment at PSI, FAST, was presented by Eusebio Sanchez of CIEMAT, who showed the current status of the analysis and gave the outlook for results expected soon.

Laura Marcucci of the University of Pisa explained the motivations for precision measurements in muon capture in the context of theoretical efforts in effective field theory, while Peter Winter of the University of Washington detailed the on-going MuSun experiment to determine precisely the rate of muon capture in deuterium. Results and opportunities from pion decays were discussed by Dinco Pocanic of Virginia.

The new proton charge radius result from the muonic hydrogen Lambshift experiment, presented by Aldo Antognini of the Swiss Federal Institute of Technology (ETH) Zurich, revived a heated discussion about the results published earlier in 2010. Theory still struggles to explain the discrepancy between the muonic and ordinary hydrogen Lambshift results, both of which involve QED calculations. While optical hydrogen spectroscopy and QED appear to be in agreement with electron scattering data, the muonic hydrogen result, which is far more precise, is 5 σ from the CODATA value. Antognini went on to explain how all systematic errors in the muonic experiment are found to be far below the observed difference.


Aside from the programme of talks, the poster session provoked lively discussions among participants, enhanced by locally brewed draught beer and grilled specialities. There was also the opportunity to gather at organized evening events. In particular, a special trumpet concert linked music to physics through the performance of modern interpretations of Baroque master-works and through the demonstration of acoustic phenomena in a special quadrophenia opus composed by one of the performers, Eckhard Kopetzki. The workshop dinner took place at the local historic grape-pressing cellar (Trotte), an easy stroll from the workshop site. The Swiss speciality of raclette cheese was served freshly melted accompanied by the sounds of alphorns.

Many participants expressed their wish for a repeat of this low-energy precision physics workshop at PSI – the best indication of the workshop’s success. This also showed the growing interest in the field, in which various experiments and particle sources will soon come online.

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