Intense Muon Physics Working Group Summary

Over many years, the muon has provided important input to the standard model: the value of the weak coupling constant GF , strong proof of the V −A nature of the weak interaction, information on the induced weak pseudoscalar form factor gp, strong contraints on new physics from its anomalous magnetic moment, and sensitive limits on the presence of new physics which would cause lepton flavor violation in the muon’s decay. In addition to its contributions to particle physics, the muon has become a useful tool in condensed matter physics. We heard about all of these topics in our working group sessions, and the projections from these very nice talks are on the NuFact website. In this summary we will focus on the “muon trio”: the anomalous magnetic dipole moment (MDM)[1] aμ = (g − 2)/2; the search for a permanent electric dipole moment (EDM) of the muon[2] which would signify / P , / T and by implication CP violation if CPT is valid; and the search for lepton flavor violation (LFV)[3,4,5]. We focus on these experiments, and specifically on the latter two because they require the highest possible muon flux, which would only be available at a high-intensity muon source such as a neutrino factory. Useful reviews can be found for: muon physics at a neutrio factory[6]; theory of muon (g − 2)[7,8]; electric dipole moments[9]; and lepton flavor violation[10,11]. The muon anomalous magnetic moment has now been measured by BNL E821 to a relative precision of 0.5 parts per million (ppm),[12,13,14] and it is proposed to improve this experiment to 0.2 ppm in an upgraded experiment E969 at Brookhaven. E969 has scientific approval but is not yet funded[15]. Since the first precise result from E821 became available[12], there has been an approximate 2.5 standard deviation discrepancy between theory (presently known to about 0.6 ppm) and experiment, when the hadronic contribution is taken from ee data. There is much activity worldwide in improving our knowledge of the hadronic contribution[16,17]. This increased precision available to E969, combined with the expected improvements in the knowledge of the hadronic contribution, eventually should give at least a factor of two reduction in the combined experiment-theory uncertainty when comparing the two. It is possible to improve on the experiment further[18], but to fully realize the potential of the improved experimental measurement, the hadronic contribution would need to be known