Muon
A massive cousin of the electron. The muon shares the same properties, except for its mass, which is 207 times greater. It is not affected by the strong interaction, carries a negative electric charge equal to the elementary charge, and has a very short lifetime (2.2 μs at rest). Denoted μ–, the muon has an antiparticle, denoted μ+. On Earth, muons are produced by the decay of charged pions, which come from cosmic rays penetrating the upper atmosphere.