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 ${\ensuremath{\mu}}^{+}$ decay. Highly polarized ${\ensuremath{\mu}}^{+}$ 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 ${\ensuremath{\mu}}^{+}$ spin. In terms of the standard muon-decay parameters this rate is given by (1-\ensuremath{\xi}${P}_{\ensuremath{\mu}}$\ensuremath{\delta}/\ensuremath{\rho}) where ${P}_{\ensuremath{\mu}}$ 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 (\ensuremath{\mu}SR) technique is \ensuremath{\xi}${P}_{\ensuremath{\mu}}$\ensuremath{\delta}/\ensuremath{\rho}>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<\ensuremath{\zeta}<0.035, where ${W}_{2}$ is the predominantly right-handed boson and \ensuremath{\zeta} is the left-right mixing angle. Limits are also deduced on the ${\ensuremath{\nu}}_{\ensuremath{\mu}L}$ mass and helicity in ${\ensuremath{\pi}}^{+}$ decay, non-(V-A) couplings in helicity projection form, the mass scale of composite leptons, and the branching ratio for \ensuremath{\mu}\ensuremath{\rightarrow}e+f where f (familon) is the neutral massless Nambu-Goldstone boson associated with flavor-symmetry breaking.