MEASUREMENT OF THE MUON MOMENTUM IN PION DECAY AT REST USING A SURFACE MUON BEAM

We have measured the momentum of muons from the decay pi+ --> mu+nu(mu) at rest to be p(mu+) = 29.79207 +/- 0.00012 MeV/c. This result leads to a laboratory upper limit of 0.16 MeV (CL = 0.9) for the muon-neutrino mass. The cosmological upper limit of the neutrino mass, the muon mass and the new p(mu+)-value yield the pion mass m(pi+) = 139.57031 +/- 0.00016 MeV. The mean kinetic energy of the pions stopped in the isotropic-graphite production target immediately before their decay is found to be T(pi+)BAR = 0.415 +/- 0.027 eV.