Two antiprotons (bottom of the photograph) entering the 72-inch hydrogen bubble chamber at the University of California's Lawrence Radiation Laboratory, Berkeley. The two four-pronged 'stars' are formed when the antiprotons, generated by the Bevatron, come close to protons, the nuclei of ordinary hydrogen atoms, in the chamber. The process is called annihilation, and is visualized by the profusion of tracks forming the stars. The tracks forming the 'stars' are made of pions, particles of lower mass into which the primary particles disintegrate. Not all of the particles are seen - the neutral ones (those with no electrical charge) make no tracks. It had been theoretically postulated that the Omega meson is composed of three pi mesons (pions), and is neutrally charged.