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Register a new userMeasurement of the Michel Parameter xi in Polarized Muon Decay and Implications on Exotic Couplings of the Leptonic Weak Interaction
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The Michel parameter xi has been determined from a measurement of the longitudinal polarization of positrons emitted in the decay of polarized and depolarized muons. The result, xi = 0.981 +- 0.045stat +- 0.003syst, is consistent with the Standard Model prediction of unity, and provides an order of magnitude improvement in the relative precision of this parameter. This value sets new constraints on exotic couplings beyond the dominant V-A description of the leptonic weak interaction.
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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 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.
We review the recent measurements of the rare pion decays: Pi+ -> Pi0 e+ Nu [pion beta, Pi_(e3), or Pi_beta decay], radiative decay Pi+ -> e+ Nu Gamma [Pi_(e2Gamma) or RPD], and Pi+ -> e+ Nu [Pi_(e2)] decay, as well as the radiative muon decay, Mu -> e Nu Nu-bar Gamma, their theoretical implications, and prospects for further improvement.
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.
We present the first measurement of the Michel parameters $bar{eta}$ and $xikappa$ in the radiative leptonic decay of the $tau$ lepton using 703 f$mathrm{b}^{-1}$ of data collected with the Belle detector at the KEKB $e^+e^-$ collider. The Michel parameters are measured by an unbinned maximum likelihood fit to the kinematic information of $e^+e^-rightarrowtau^+tau^-rightarrow (pi^+pi^0 bar{ u})(l^- ubar{ u}gamma)$ $(l=e$ or $mu)$. The preliminary values of the measured Michel parameters are $bar{eta} = -2.0 pm 1.5 pm 0.8$ and $xikappa = 0.6 pm 0.4 pm 0.2$, where the first error is statistical and the second is systematic.
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