Emulsion photograph showing the classic pion-muon- electron (pi-mu-e) decay chain of a charged cosmic ray pion. The pion enters the picture at top left; it soon decays to a muon, which travels down the picture, & a neutrino, which is neutral & so leaves no track. At the bottom of the picture, the muon decays to an electron - the faint track heading to the right - & another neutrino. The muon's track, which is about 0.6 mm long, becomes denser as the muon slows down before decaying. The picture was taken in 1948 by Cecil Powell, the English physicist who pioneered the use of photo- graphic emulsions to record particle tracks.