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Register a new userPrecise measurement of parity violation in polarized muon decay
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We present a new high precision measurement of parity violation in the weak interaction, using polarized muon decay. The TWIST collaboration has measured $P_mu^pi xi$, where $P_mu^pi$ is the polarization of the muon in pion decay and $xi$ describes the intrinsic asymmetry in muon decay. We find $P_mu^pi xi = 1.00084 pm 0.00029,(textrm{stat.})_{-0.00063}^{+0.00165},(textrm{syst.})$, in good agreement with the standard model prediction of $P_mu^pi=xi=1$. Our result is a factor of 7 more precise than the pre-TWIST value, setting new limits in left-right symmetric electroweak extensions to the standard model.
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The quantity P_{mu}^{pi}xi, where xi is one of the muon decay parameters and $P_{mu}^{pi}$ is the degree of muon polarization in pion decay, has been measured. The value P_{mu}^{pi}xi = 1.0003 +- 0.0006 stat. +- 0.0038 syst. was obtained. This result agrees with previous measurements but is over a factor of two more precise. It also agrees with the Standard Model prediction for P_{mu}^{pi}xi and thus leads to restrictions on left-right symmetric models.
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.
The alpha decay parameter in the process Omega-minus -> Lambda + K-minus has been measured from a sample of 4.50 million unpolarized Omega-minus decays recorded by the HyperCP (E871) experiment at Fermilab and found to be [1.78 +/- 0.19(stat) +/- 0.16(syst)]{times}10^{-2}. This is the first unambiguous evidence for a nonzero alpha decay parameter, and hence parity violation, in the Omega-minus -> Lambda + K-minus decay.
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.
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