Pion-proton collision. Particle tracks showing positive pions (parallel lines) passing from top to bottom through a bubble chamber full of hydrogen liquid. Several collision events are shown here, but the most obvious is the large anticlockwise spiral track (centre right). This was produced by an anti-electron (positron) formed as one of the positive pions hit a hydrogen nucleus (proton). The spiral decreases in size as the positron radiates and loses energy in the form of photons (not detected by the bubble chamber). This photograph was taken in the CERN 2-metre hydrogen bubble chamber. The region shown here is about 30 centimetres tall.