On the muon neutrino mass

During the runs of the PS 179 experiment at LEAR of CERN, we photographed an event of antiproton-Ne absorption, with a complete π⁺ → μ⁺ → e⁺ chain. From the vertex of the reaction a very slow energy π⁺ was emitted. The π⁺ decays into a μ⁺ and subsequently the μ⁺ decays into a positron. At the first decay vertex a muon neutrino was emitted and at the second decay vertex an electron neutrino and a muon antineutrino. Measuring the pion and muon tracks and applying the momentum and energy conservation and using a classical statistical interval estimator, we obtained an experimental upper limit for the muon neutrino mass: m_ν < 2.2   MeV at a 90% confidence level. A statistical analysis has been performed of the factors contributing to the square value of the neutrino mass, to deduce the possibility to reach experimentally the lowest muon neutrino mass limit from the π → μ ν decays.