Most cosmic rays are very high energy protons and neutrons that are traveling through space.
Pions decay to produce muons and muon-type neutrinos . Research on cosmic rays Particles found in cosmic rays were among the first discoveries of the plethora of particles that can be produced by high energy collisions. Muons and pions were both first discovered from their tracks in emulsion exposed to cosmic rays. Even today, cosmic ray events offer an opportunity to study collisions at higher energies than those produced in any man-made accelerator. Experiments to detect the results of these collisions use arrays of detectors spread over a large area of open ground.