A fundamental particle, identical to the electron, but approximately 200 times heavier. It is commonly found in cosmic rays.

Elementary charged particle of about 207 electron masses with a half-life of about 1.5-millions of a second. It decays into an electron and a neutrino. Also know as mu-meson.

A lepton that appears identical to the electron except for its greater mass.

an elementary particle with a negative charge and a half-life of 2 microsecond; decays to electron and neutrino and antineutrino