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Lepton family number violation is tested by searching for µ + → e + X 0 decays among the 5.8×10 8 positive muon decay events analyzed by the TWIST collaboration. Limits are set on the production of both massless and massive X 0 bosons. The large angular acceptance of this experiment allows limits to be placed on anisotropic µ + → e + X 0 decays, which can arise from interactions violating both lepton flavor and parity conservation. Branching ratio limits of order 10 −5 are obtained for bosons with masses of 13 -80 MeV/c 2 and with different decay asymmetries. For bosons with masses less than 13 MeV/c 2 the asymmetry dependence is much stronger and the 90% limit on the branching ratio varies up to 5.8 × 10 −5 . This is the first study that explicitly evaluates the limits for anisotropic two body muon decays.

The TWIST Collaboration has completed its measurement of the three muon decay parameters , , and P . This paper describes our determination of , which governs the shape of the overall momentum spectrum, and , which controls the momentum dependence of the parity-violating decay asymmetry. The results are ¼ 0:749 77 AE 0:000 12ðstatÞ AE 0:000 23ðsystÞ and ¼ 0:750 49 AE 0:000 21ðstatÞ AE 0:000 27ðsystÞ. These are consistent with the value of 3=4 given for both parameters in the standard model, and each is over a factor of 10 more precise than the measurements published prior to TWIST. Our final results on , , and P have been incorporated into a new global analysis of all available muon decay data, resulting in improved modelindependent constraints on the possible weak interactions of right-handed particles.

The TWIST Collaboration has performed new measurements of two of the parameters that describe muon decay: , which governs the shape of the overall momentum spectrum, and , which governs the momentum dependence of the parity-violating decay asymmetry. This analysis gives the results ¼ 0:750 14 AE 0:000 17ðstatÞ AE 0:000 44ðsystÞ AE 0:000 11ðÞ, where the last uncertainty arises from the correlation between and the decay parameter , and ¼ 0:750 67 AE 0:000 30ðstatÞ AE 0:000 67ðsystÞ. These are consistent with the value of 3=4 given for both parameters in the standard model of particle physics, and are a factor of two more precise than the measurements previously published by TWIST. A new global analysis of all available muon decay data incorporating these results is presented. Improved lower and upper limits on the decay parameter P of 0:995 24 < P < 1:000 91 at 90% confidence are determined, where P is the polarization of the muon when it is created during pion decay, and governs the muon decay asymmetry. These results set new model-independent constraints on the possible weak interactions of right-handed particles. Specific implications for left-right symmetric models are discussed.

This paper introduces a new approach to measure the muon magnetic moment anomaly a µ = (g − 2)/2, and the muon electric dipole moment (EDM) d µ at the J-PARC muon facility. The goal of our experiment is to measure a µ and d µ using an independent method with a factor of 10 lower muon momentum, and a factor of 20 smaller diameter storage-ring solenoid compared with previous and ongoing muon g − 2 experiments with unprecedented quality of the storage magnetic field. Additional significant differences from the present experimental method include a factor of 1,000 smaller transverse emittance of the muon beam (reaccelerated thermal muon beam), its efficient vertical injection into the solenoid, and tracking each decay positron from muon decay to obtain its momentum vector. The precision goal for a µ is statistical uncertainty of 450 part per billion (ppb), similar to the present experimental uncertainty, and a systematic uncertainty less than 70 ppb. The goal for EDM is a sensitivity of 1.5 × 10 −21 e • cm.