Bubble chamber event. An antiproton (pale blue) strikes a proton in a bubble chamber at Berkeley Lab. In the resulting annihilation, the energy released rematerializes as four positive pions (red) and four negative pions (green). In the bubble chamber's magnetic field, the negative pions and the negative antiproton curve in a clockwise rotation, the positive particles anti-clockwise. The two lower pions have less energy than the others, and they therefore curve more and leave thicker tracks. the one on the left travels only a short distance and stops when it is captured by a proton. The one on the right ends by decaying into a muon (yellow) and an invisible neutrino. Tracks not involved in the interaction, including the characteristic curlicues of low-energy electrons knocked from atoms, are colored dark blue. A bubble chamber is a vessel filled with a superheated transparent liquid (most often liquid hydrogen) used to detect electrically charged particles moving through it.