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## Mass limits for

These measurements apply to , the dominant mass eigenstate in . The decay of pions is the most appropriate process to search for this neutrino mass. In case of pion decay at rest the square of the muon momentum is: . A massive neutrino would reduce the muon momentum. In a recent paper the up to now most precise measurement of from stopped pions has been reported. The group at PSI (Switzerland) improved former limits by using a beam of muons originating from the decay of -mesons stopped at the surface of a pion production target. The muon momenta are analysed by a magnetic spectrometer, the muons are detected by a silicon microstrip counter and a single silicon surface barrier detector. The muon momentum was measured with great accuracy to be MeV/c yielding an upper limit for the neutrino mass of MeV at 90% CL.

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