A Precision Measurement of the Muon Decay Parameters $\rho$ and $\delta$

Abstract

The TWIST collaboration has performed new measurements of two of the parameters that describe muon decay: $\rho$, which governs the shape of the overall momentum spectrum, and $\delta$, which governs the momentum dependence of the parity-violating decay asymmetry. This analysis gives the results $\rho =0.75014\pm 0.00017(\text{stat})\pm 0.00044(\text{syst})\pm 0.00011(\eta )$, where the last uncertainty arises from the correlation between $\rho$ and the decay parameter $\eta$, and $\delta =0.75067\pm 0.00030(\text{stat})\pm 0.00067(\text{syst})$. 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 $P_{\mu }^{\pi }\xi$ of $0.99524<P_{\mu }^{\pi }\xi \le \xi <1.00091$ at 90% confidence are determined, where $P_{\mu }^{\pi }$ is the polarization of the muon when it is created during pion decay, and $\xi$ 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.

14 pages, 14 figs, accepted for publication in Phys. Rev. D

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• Measurement of the Muon Decay Parameter delta
• Supersymmetric Contributions to Weak Decay Correlation Coefficients
• Global analysis of muon decay measurements
• First Order Radiative Corrections to Polarized Muon Decay Spectrum
• Measurement of P_{mu}xi in Polarized Muon Decay
• Precision planar drift chambers and cradle for the TWIST muon decay spectrometer
• Hadronic Corrections at O(alpha^2) to the Energy Spectrum of Muon Decay
• Time expansion chamber system for characterization of TWIST low energy muon beams

Cited by in corpus (13)

• A search for two body muon decay signals
• The Fermi constant from muon decay versus electroweak fits and CKM unitarity
• Precise measurement of parity violation in polarized muon decay
• Precision muon decay measurements and improved constraints on the weak interaction
• Decay of negative muons bound in ${}^{27}$Al
• Question of Lorentz invariance in muon decay
• Observation of CP violation in $D^0\to K^{-}{\pi }^{+}$ as a smoking gun for New Physics
• CP violating dimuon charge asymmetry in general left-right models
• Charged particle spectra from ${\mu }^{-}$ capture on Al
• Calibration of the TWIST high-precision drift chambers
• Muon-Decay Parameters from COHERENT
• TWIST: Precision Measurement of the Muon Decay Parameters
• On spin asymmetry in muon and tau decays