Precision measurement of the muon decay parametersρandδ

Abstract

The TWIST Collaboration has performed new measurements of two of the parameters that describe muon decay: , which governs the shape of the overall momentum spectrum, and , which governs the momentum dependence of the parity-violating decay asymmetry. This analysis gives the results ¼ 0:750 14 AE 0:000 17ðstatÞ AE 0:000 44ðsystÞ AE 0:000 11ðÞ, where the last uncertainty arises from the correlation between and the decay parameter , and ¼ 0:750 67 AE 0:000 30ðstatÞ AE 0:000 67ðsystÞ. 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 of 0:995 24 < P < 1:000 91 at 90% confidence are determined, where P is the polarization of the muon when it is created during pion decay, and 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.

Precision muon decay measurements and improved constraints on the weak interaction

“…The array of low mass chambers was installed in a frame referred to as the cradle, filled with helium to further reduce the amount of material traversed by the muons and positrons. Two different target foils were used over two run periods to study the effects of the target material on the decay parameters measurement: a (30:9 AE 0:6) m thick silver foil and the (71:6 AE 0:5) m thick aluminum foil used for the intermediate TWIST measurement [8]. Both metal targets had purity exceeding 99.999% and featured minimal depolarization of the muons after stopping [14].…”

“…We studied the stability of the decay parameters with respect to the definition of the regions by varying by a few percent all the fiducial boundaries. These boundaries were slightly modified for this analysis compared to the ones used for the intermediate measurement [8].…”

“…Initial and intermediate measurements of and have already been published [6][7][8]. This paper presents a detailed description of the final measurement of the and decay parameters by the TWIST Collaboration reported in [9].…”

CPviolating dimuon charge asymmetry in general left-right models

“…From the global analysis of muon decay measurements [16], the lower bound on g can be obtained without imposing discrete symmetry as follows: (15) The new gauge boson mass M W 0 is severely constrained from K L À K S mixing if the model has manifest [6]. Instead, if we take the following form of V R , the limit on M W 0 may be significantly relaxed to approximately 300 GeV, and the W 0 boson contributions to B dðsÞ " B dðsÞ mixings can be large [18]:…”

Experimental Constraints on Left-Right Symmetric Models from Muon Decay

“…Intermediate TWIST results have already reduced those uncertainties to ð0:7-3:8Þ Â 10 À3 [6,7]. TWIST has now realized its goal of making about an order of magnitude improvement in each of the parameters.…”