Search for right-handed currents in muon decay

Abstract

Limits are reported on charged right-handed currents, based on precise measurement of the end-point $e^{+}$ spectrum in ${\mu }^{+}$ decay. Highly polarized ${\mu }^{+}$ from a TRIUMF ‘‘surface’’ muon beam were stopped in pure metal foil and liquid-He targets selected to minimize depolarization effects. In the stopping target region either a spin-precessing transverse field (70 or 110 G) or a spin-holding longitudinal field (0.3 or 1.1 T) was applied. Data collected with the spin-precessing field were used for the momentum calibration of the spectrometer. The spin-held data were used to measure the relative $e^{+}$ rate at the momentum end point in a direction opposite to the ${\mu }^{+}$ spin. In terms of the standard muon-decay parameters this rate is given by (1-ξ$P_{\mu }$δ/ρ) where $P_{\mu }$ is the muon polarization. The combined 90% confidence lower limit from the analysis presented in this paper and our earlier analysis of the spin-precessed data by means of the muon-spin-rotation (μSR) technique is ξ$P_{\mu }$δ/ρ>0.9975. For models with manifest left-right symmetry and massless neutrinos this result implies the 90% confidence limits m($W_2$)>432 GeV/$c^2$ and -0.050<ζ<0.035, where $W_2$ is the predominantly right-handed boson and ζ is the left-right mixing angle. Limits are also deduced on the ${\nu }_{\mu L}$ mass and helicity in ${\pi }^{+}$ decay, non-(V-A) couplings in helicity projection form, the mass scale of composite leptons, and the branching ratio for μ→e+f where f (familon) is the neutral massless Nambu-Goldstone boson associated with flavor-symmetry breaking.