New precision measurement of the muon momentum in pion decay at rest

Abstract

We have continued the measurement of the muon momentum p μ + in the decay π + → μ + v μ of pions at rest. The new result p μ + = 29.79179±0.00053 MeV/ c, and the assumption of a vanishing neutrino mass m νμ lead to a charged pion mass of 139.56996±0.00067 MeV/ c 2, 3.5 standard deviations higher than the current world average of the mass obtained from pionic X-rays. For m ν μ > 0 the discrepancy would be even larger.