The first measurement of the elementary process $\mu^- p \rightarrow
\nu_{\mu} n \gamma$ is reported. A photon pair spectrometer was used to measure
the partial branching ratio ($2.10 \pm 0.22) \times 10^{-8}$ for photons of k >
60 MeV. The value of the weak pseudoscalar coupling constant determined from
the partial branching ratio is $g_p(q^{2}=-0.88m_{\mu}^2) = (9.8 \pm 0.7 \pm
0.3) \cdot g_a(0)$, where the first error is the quadrature sum of statistical
and systematic uncertainties and the second error is due to the uncertainty in
$\lambda_{op}$, the decay rate of the ortho to para $p \mu p$ molecule. This
value of g_p is $\sim$1.5 times the prediction of PCAC and pion-pole dominance.Comment: 13 pages, RevTeX type, 3 figures (encapsulated postscript), submitted
to Phys. Rev. Let
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.
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