Further Observation of Dimuon Production by Neutrinos

Using a quadrupole focused neutrino beam, 61 events with two muons in the final state have been observed at Fermilab. These include seven ${\ensuremath{\mu}}^{\ensuremath{-}}{\ensuremath{\mu}}^{\ensuremath{-}}$ events. A comparison of the event rate in two targets of different hadron absorption length indicates that attributing the events to $\ensuremath{\pi}$ or $K$ leptonic decay is ruled out by 4.0 standard deviations. No trimuon events were observed which, combined with lepton conservation, indicates an unobserved neutral lepton is present in most of the events.