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

The asymmetry of positrons in the ${\mathit{K}}^{+}$→${\mathrm{\mu }}^{+}$ν, ${\mathrm{\mu }}^{+}$→${\mathit{e}}^{+}$ ${\nu }_{\mathit{e}}$ν${\mathrm{¯}}_{\mathrm{\mu }}$ decay chain was measured in a search for right-handed weak currents in ΔS=1 semileptonic decay. High-intensity low-background monoenergetic polarized muons of 236 MeV/c momentum resulting from kaon decay at rest were directly extracted from a primary production target which was hit by a proton beam of the KEK 12-GeV proton synchrotron. Muons were stopped in a pure-aluminum plate, and the energy-integrated asymmetry of the decay positrons with respect to the incoming muon direction was determined to high precision. The observed asymmetry yielded ξ${\mathit{P}}_{\mathrm{\mu }}$=-0.9996±0.0030(stat)±0.0048(syst). This result revealed no evidence of right-handed currents in this kaon-decay chain, and set a stringent bound on the mass of the right-handed weak boson.