False-colour streamer chamber photo of a tritium nucleus produced at the Low Energy Antiproton Ring (LEAR) at CERN, the European particle physics laboratory outside Geneva. An antiproton enters at the bottom of the picture & annihilates with a proton in the helium-filled chamber to produce the tritium nucleus & 2 pions. The nucleus of tritium, or heavy hydrogen", has 1 proton & 2 neutrons; it produces the thick curving track at centre right. The pions, one negative, one positive, produce the less dense tracks that shoot off towards top right & top left. Reactions of this kind occurred in the early Universe, when lightweight nuclei such as helium were beginning to form."