Experimental Medium-Energy Physics
Precision measurements at low energies provide an alternate path to the frontier in particle physics, and provide complementary information to that obtained at the highest-energy colliders. The Intermediate Energy Physics Group has a long history of precision measurements, beginning with our study of charge-parity (CP) violation at the the Low Energy Antiproton Ring (LEAR) at CERN in Geneva, where among other things our collaboration observed direct time reversal violation for the first time. Since then we spent many years helping to lead the muon (g-2) experiment at the Alternating Gradient Synchrotron (AGS), a 30 GeV proton accelerator at Brookhaven National Laboratory. Our results from that experiment are at variance with the Standard Model predicion by 3.4 standard deviations, which could be the first indication of new physics at the TeV scale. More recently we have played a major role in the muon lifetime experiment at the Paul Scherrer Institut in Switzerland. Our final result should produce a new measurement of the strength of the weak interaction, the Fermi constant, that is 20 times more precise than it was known. We have also taken significant responsibility in the search for a permanent electric dipole moment (EDM) of the neutron that is being prepared for the Spallation Neutron Source at the Oak Ridge National Laboratory. The presence of an EDM would violate both parity and time-reversal symmetries, thus signifying a new source of CP violation, which is one of the elements needed to explain the baryon anti-baryon asymmetry of the universe.