Precision muon decay measurements and improved constraints on the weak interaction

Abstract

The TWIST Collaboration has completed its measurement of the three muon decay parameters , , and P . This paper describes our determination of , which governs the shape of the overall momentum spectrum, and , which controls the momentum dependence of the parity-violating decay asymmetry. The results are ¼ 0:749 77 AE 0:000 12ðstatÞ AE 0:000 23ðsystÞ and ¼ 0:750 49 AE 0:000 21ðstatÞ AE 0:000 27ðsystÞ. These are consistent with the value of 3=4 given for both parameters in the standard model, and each is over a factor of 10 more precise than the measurements published prior to TWIST. Our final results on , , and P have been incorporated into a new global analysis of all available muon decay data, resulting in improved modelindependent constraints on the possible weak interactions of right-handed particles.

Light sterile neutrino sensitivity at the nuSTORM facility

“…These decays all have the same neutrino content (50% ν e , 50%ν μ ), so any difference in the neutrino flavor would represent new physics. The energy spectrum of the muon decay positron has been measured to be consistent with the standard model at the level of a few parts in 10 4 [21]. The nuSTORM facility has been designed to inject 5 GeV pions into a muon storage ring [22], with a beam lattice in a race track configuration (Fig.…”

“…Rates of accessible neutrino oscillation channels for stored μ þ , assuming a simple (3 þ 1) sterile neutrino model [24] consistent with the LSND anomaly, are shown in Table I for 10 21 POT. The probability of observing a ν e → ν μ transition is given by…”

Measurement of the parameterξ′′in polarized muon decay and implications on exotic couplings of the leptonic weak interaction

“…A recent global analysis of muon decay data [7] has provided new limits on several of the couplings entering the expression of Δ in Eq. (27).…”

“…Most decay parameters are known with uncertainties below the percent level [5]. In particular, new results have recently been reported on the parameters P π μ ξ [6] and ρ, δ [7], for which the total errors reach respectively ðþ16.8; −6.9Þ × 10 −4 , 2.6 × 10 −4 , and 3.4 × 10 −4 . A notable exception among decay parameters is ξ 00 , which characterizes the angular and energy dependence of the positron longitudinal polarization in polarized muon decay.…”

Charged-particle spectra fromμ−capture on Al

“…There is an ongoing joint effort between Mu2e and COMET to study muon capture on aluminum in a dedicated AlCap experiment [8]. The present analysis, on the other hand, uses an opportunistic µ − dataset acquired by the TRIUMF Weak Interaction Symmetry Test (TWIST) experiment [9,10]. The same data were previously analyzed for the electron spectrum from muon decay in orbit (DIO) [11].…”