Secondary cosmic rays
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.
Cosmic rays