The Collaborative International Dictionary of English v.0.48:

lepton \lepton\ n. (Physics)
   an elementary particle that participates in weak interactions
   but does not participate in the strong interaction; it has a
   baryon number of 0. Some known leptons are the electron,
   the negative muon, the tau-minus particle, and the
   neutrinos associated with each of these particles.
   [WordNet 1.5 +PJC]

WordNet (r) 3.0 (2006):

negative muon
    n 1: an elementary particle with a negative charge and a half-
         life of 2 microsecond; decays to electron and neutrino and
         antineutrino [syn: muon, negative muon, mu-meson]