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Register a new userA precision measurement of the muon decay parameter delta
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Andrei Gaponenko
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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.
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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 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.
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 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.
Using 482 pb$^{-1}$ of data taken at $sqrt{s}=4.009$ GeV, we measure the branching fractions of the decays of $D^{*0}$ into $D^0pi^0$ and $D^0gamma$ to be $BR(D^{*0} to D^0pi^0)=(65.5pm 0.8pm 0.5)%$ and $BR(D^{*0} to D^0gamma)=(34.5pm 0.8pm 0.5)%$ respectively, by assuming that the $D^{*0}$ decays only into these two modes. The ratio of the two branching fractions is $BR(D^{*0} to D^0pi^0)/BR(D^{*0} to D^0gamma) =1.90pm 0.07pm 0.05$, which is independent of the assumption made above. The first uncertainties are statistical and the second ones systematic. The precision is improved by a factor of three compared to the present world average values.
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