Abstract
The TWIST Collaboration has performed new measurements of two of the parameters that describe muon decay: ρ, which governs the shape of the overall momentum spectrum, and δ, which governs the momentum dependence of the parity-violating decay asymmetry. This analysis gives the results ρ=0.750 14±0.000 17(stat)±0.000 44(syst)±0.000 11(η), where the last uncertainty arises from the correlation between ρ and the decay parameter η, and δ=0.750 67±0.000 30(stat)±0.000 67(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 Pμπξ of 0.995 24<Pμπξ≤ξ<1.000 91 at 90% confidence are determined, where Pμπ is the polarization of the muon when it is created during pion decay, and ξ 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.7 MoreReceived 7 July 2008DOI:https://doi.org/10.1103/PhysRevD.78.032010This article is available under the terms of the Creative Commons Attribution 3.0 License. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.
Highlights
In the standard model (SM) of particle physics, the charged-current weak interaction violates parity maximally—only left-handed particles are affected
Limitations of the reconstruction and physical aspects of the detector required the imposition of fiducial cuts, which were applied after the data and simulation were calibrated
We find that all but ð5:7 Æ 0:2Þ Â 10À4 [ð5:12 Æ 0:05Þ Â 10À4] of the upstream stops events in the data [simulation] that contain a reconstructed downstream track within the fiducial region contain a reconstructed track in the upstream half of the detector; all but ð8:2 Æ 0:2Þ Â 10À4 [ð7:87 Æ 0:06Þ Â 10À4] of the events that contain a reconstructed upstream track within the fiducial region contain a reconstructed track in the downstream half of the detector
Summary
In the standard model (SM) of particle physics, the charged-current weak interaction violates parity maximally—only left-handed particles (or right-handed antiparticles) are affected. Sets a model-independent limit on any muon right-handed couplings [1,2]. The quantities , , , and , often called the Michel parameters, are bilinear combinations of the weak coupling constants and describe the shape of the decay spectrum. Àe) and the polarization of the decay positron, to determine limits on the weak coupling constants. Left-right symmetric (LRS) models [7] comprise an interesting class of extensions to the SM These models include a right-handed weak coupling, which is suppressed by the mass of the associated gauge boson. The values of and can be combined to provide a modelindependent limit on right-handed couplings in muon decay: QR 1⁄4 121⁄21 þ 13 À 196:.
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