The Collaborative International Dictionary of English v.0.48:

mu-meson \mu-meson\, mu meson \mu meson\n. (Physics)
   an elementary particle with a negative charge and a half-life
   of 2 microseconds; the muon. It is a lepton, not a true
   meson, and decays to an electron and neutrino and
   antineutrino.

   Syn: muon, negative muon.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

mu-meson
    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]