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At the Schweizerisches Institut fiir Nuklearforschu~~g (SIN) we have measured the tnuon momentum in the pion decay T + -+p+ + v, at rest using a magnetic spectrometer. Our result is p,+ = 29.7877 f 0.0014 MeV/c. From our p, + value and the rest masses of the muon, mu+ = 105.65946$0.00024 MeV/c 2, and the pion, m,-= 139.5679 k0.0015 McV/c ', we derive a new value for the squared muon-neutrino rest mass, m, -0.13 f 0.14 (MeV/c 2)2. From this we obtain m,, 5 0.57 MeV/c (90% C.L.), which is at present tge lowest experimental upper limit for the muon-neutrino rest mass. Here the validity of the CPT theorem (m,+ = m,.) and energy and momentum conservation in pion decay were assumed. Our result can be interpreted in a second way: From cosmological arguments using the big-bang hypothesis an upper limit of about 40 eV/c has been derived for the neutrino rest mass. With this limit and the abovt: value of the muon mass we car1 calculate the pion-muon mass difference or the rest mass of the T + with strongly iniproved precision from our measurement. The results are 6m = m,+m,+ = 33.9063+0.0018 MeV/c or m,+ = 139.5658 f 0.018 MeV/c ' . This positive-pion mass is consistent with the present world average for the negative-pion mass, in agreement with the CPT theorem.

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