We have determined the momentum p ϩ of muons from the decay ϩ → ϩ at rest, by analyzing a surface muon beam in a magnetic spectrometer equipped with a silicon microstrip detector. The result p ϩϭ (29.792 00Ϯ0.000 11) MeV/c leads to a squared muon-neutrino mass of m 2 ϭ(Ϫ0.016Ϯ0.023)MeV 2 , which corresponds to a ''laboratory'' upper limit of 0.17 MeV ͑C.L.ϭ0.9͒ for the muon-neutrino mass.The cosmological upper limit of the neutrino mass ͑65 eV͒, the muon mass, and the new value of p ϩ yield the pion mass m ϩϭ (139.570 22Ϯ0.000 14) MeV. Alternatively, if one does not use the cosmological upper limit of m , then a combined fit including the new p ϩ value, and the m ϩ and m Ϫ values from other experiments and the CPT theorem (m ϩϭ m Ϫ) leads to m Ϯϭ (139.570 37Ϯ0.000 21) MeV. As a side result, the mean kinetic energy of the pions stopped in the production target, made of isotropic graphite, immediately before their decay is found to be T ϩϭ (0.425Ϯ0.016) eV. This is consistent with the hypothesis that the pions are trapped in the potential well of a spherical harmonic oscillator, V(r)ϭV 0 ϩ 1 2 k s r 2 , with k s ϭ(1.144Ϯ0.088)ϫ10 17 eV/cm 2 .
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