A slot car (sometimes, slotcar) is a powered miniature auto or other vehicle that is guided by a groove or slot in the track on which it runs.
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Muon. The muon is a lepton which decays to form an electron or positron. The fact that the above decay is a three-particle decay is an example of the conservation of lepton number; there must be one electron neutrino and one muon neutrino or antineutrino in the decay. The lifetime of the muon is 2.20 microseconds.