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Register a new userA Precision Measurement of the Muon Decay Parameters $rho$ and $delta$
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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 $rho=0.75014pm 0.00017(text{stat})pm 0.00044(text{syst})pm 0.00011(eta)$, where the last uncertainty arises from the correlation between $rho$ and the decay parameter $eta$, and $delta = 0.75067pm 0.00030(text{stat})pm 0.00067(text{syst})$. 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 $P_mu^pixi$ of $0.99524 < P_mu^pixi leq xi < 1.00091$ 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.
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The muon decay parameter delta characterizes momentum dependence of the parity-violating muon decay asymmetry. A new measurement of delta has been performed using the first physics data recorded by the TWIST experiment at TRIUMF. The obtained value, delta=0.74964+-0.00066(stat.)+-0.00112(syst.), is consistent with the Standard Model expectation delta=3/4. This is the first determination of delta performed using a blind analysis technique. Combined with other data, the measurement sets new model-independent limits on effective right-handed couplings of the muon. Improved limits on the product of another muon decay parameter, xi, and the muon polarization in pion decay, Pmu, are obtained in the form: 0.9960<Pmu*xi<=xi<1.0040, at 90% confidence level. Implications for left-right symmetric models are discussed.
The TWIST experiment has made a precision measurement of three of the decay parameters in muon decay. The newest results are rho = 0.75014 +-0.00017(stat) +-0.00044(sys) +-0.00011(eta) and delta = 0.75067 +-0.00030(stat) +-0.00067(sys). Together with previously published results, improved constraints on possible extensions of the electroweak Standard Model are derived.
The muon decay parameter delta has been measured by the TWIST collaboration. We find delta = 0.74964 +- 0.00066(stat.) +- 0.00112(syst.), consistent with the Standard Model value of 3/4. This result implies that the product Pmuxi of the muon polarization in pion decay, Pmu, and the muon decay parameter xi falls within the 90% confidence interval 0.9960 < Pmuxi < xi < 1.0040. It also has implications for left-right-symmetric and other extensions of the Standard Model.
The TWIST Collaboration has measured the Michel parameter $rho$ in normal muon decay, $mu^+ to e^+ u_e bar{ u}_{mu}$. In the Standard Model, $rho$ = 3/4. Deviations from this value require mixing of left- and right-handed muon and electron couplings in the muon-decay Lagrangian. We find $rho$ = 0.75080 $pm$ 0.00044(stat.) $pm$ 0.00093(syst.) $pm$ 0.00023, where the last uncertainty represents the dependence of $rho$ on the Michel parameter $eta$. This result sets new limits on the $W_L-W_R$ mixing angle in left-right symmetric models.
To measure the muon decay parameters with high accuracy, we require an array of precision drift detector layers whose relative position is known with very high accuracy. This article describes the design, construction and performance of these detectors in the TWIST (TRIUMF Weak Interaction Symmetry Test) spectrometer.
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