<P> Cosmic rays are particles traveling through space with high energies . Energy events as high as 7001480652946100000 ♠ 3.0 × 10 eV have been recorded . When these particles collide with nucleons in the Earth's atmosphere, a shower of particles is generated, including pions . More than half of the cosmic radiation observed from the Earth's surface consists of muons . The particle called a muon is a lepton produced in the upper atmosphere by the decay of a pion . </P> <Dl> <Dd> π → μ + ν </Dd> </Dl> <Dd> π → μ + ν </Dd> <P> A muon, in turn, can decay to form an electron or positron . </P>

A medium of free floating positive and negative charges is called a(n)