Search for Neutrinoless Conversion of Muon into Electron

Abstract

A search has been made for the hypothetical reaction in which a muon near a nucleus is converted into an electron without production of neutrinos. Negative muons were stopped in a copper target. A magnetic spectrometer at right angles to the beam transmitted particles of the momentum expected for the electron (about 90 Mev/c on entry into the spectrometer). A long scintillation counter at the output of the spectrometer gave a pulse corresponding to the emerging particle's energy loss. Selection by both momentum and pulse-height eliminated particles heavier than the electron and greatly reduced the accidental background. In the main run, three events meeting the selection criteria were recorded, while the expected number of accidentals is 0.23±0.04. Various alternative processes that would produce accepted events are considered and found to give expectation values even smaller than that for accidentals. Without further experimentation, we cannot decide whether the hypothetical reaction does or does not occur, but we can set an upper limit of ($4_{-2\mathrm{}}^{+3\mathrm{}}$)×${10}^{-6\mathrm{}}$ on the ratio $R$ of the reaction rate to that of normal absorption.