An Etymological Dictionary of Astronomy and Astrophysics

A burst of secondary charged and neutral particles arising when → primary cosmic rays collide with the atmospheric oxygen or nitrogen nuclei in the upper atmosphere. The
collision produces mostly → pions (π), along with some → kaons (K), → antiprotons, and → antineutrons. Neutral pions very quickly decay, usually into two → gamma rays. Charged pions also decay but after a longer time. Therefore, some of the pions may collide with yet another nucleus of the air before decaying, which would be into a → muon and a → neutrino. The fragments of the incoming nucleus also interact again, also producing new particles.

See also: → secondary; → cosmic; → ray.