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.

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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.

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