Kaon-proton collision. Particle tracks showing negative kaons (parallel lines) passing from bottom to top through a bubble chamber full of hydrogen liquid. One of the kaons has collided (bottom) with a hydrogen nucleus (proton), producing charged particles, one of which, a positive pion, loops round anticlockwise to upper left before stopping. It then decays into a muon, which travels a short distance before itself decaying into an anti-electron or positron. This spirals a few times, reaching the cross at upper left. This photograph was taken in the CERN 2-metre hydrogen bubble chamber. The region shown here is about 50 centimetres tall.