Pion-proton collision. Particle tracks showing positive pions (parallel lines) passing from bottom to top through a bubble chamber full of hydrogen liquid. One of the pions has collided (centre) with a hydrogen nucleus (proton), producing charged particles, one of which, a positive pion, loops clockwise to lower left before stopping. It then decays into a muon, which travels a short distance before itself decaying into an anti-electron or positron, producing the clockwise spiral at lower left. The slanted line at upper left is is the proton that was originally struck by the pion. This photograph was taken in the CERN 2-metre hydrogen bubble chamber.