Search for right-handed currents in the decay chain ${\mathit{K}}^{+}$→${\mathrm{\mu }}^{+}$ν, ${\mathrm{\mu }}^{+}$→${\mathit{e}}^{+}$νν¯

Positron asymmetry in the decay of muons produced in ${\mathit{K}}^{+}$→${\mathrm{\mu }}^{+}$ν has been precisely measured in a search for right-handed currents. A novel technique utilizing a ‘‘${\mathit{K}}_{\mathrm{\mu }2}$ muon beam’’ from the at-rest decay of positive kaons in a primary-proton-beam target was employed. A limit of ‖ξ${\mathit{P}}_{\mathrm{\mu }}$‖>0.990 (90% C.L.) was obtained from the energy-integrated positron asymmetry observed in muon spin precession under a transverse magnetic field. This limit sets a new constraint on the mass of a second weak boson (${\mathit{M}}_{\mathit{W}2}$) as well as on the Cabibbo angle of the right-handed sector.