Streamer chamber photo of an event at the Low Energy Antiproton Ring (LEAR) at CERN, the European particle physics laboratory outside Geneva. A 600 MeV antiproton enters at the bottom of the picture & annihilates with a proton in a nucleus of the helium gas that fills the chamber. The collision produces a proton, a positive & negative pion, & a neutral kaon. Being neutral, the kaon leaves no track, but betrays its presence when it decays to a positive & negative pion pair. Reactions of this kind occurred in the early universe, at a time when lightweight nuclei such as helium were beginning to form. The distance between the crosses is 28 cm.