Kaon-proton collisions. Particle tracks showing negative kaons (parallel lines) passing from left to right through a bubble chamber full of hydrogen liquid. Several collisions with hydrogen nuclei (protons) are shown here, with the new particles produced by the collisions showing as slightly curving tracks. The tight clockwise-spiralling tracks are a side-show produced by electrons which lose their energy rapidly, causing them to spiral in smaller circles. This photograph was taken in the CERN 2-metre hydrogen bubble chamber. The region shown here is about 100 centimetres tall.