Image from a hydrogen bubble chamber showing the annihilation of an antiproton (vertical line entering at bottom) and a proton. The annihilation event has occurred at the intersection point just above the center. Only charged particles leave tracks in such chambers; the passage of neutral particles has to be inferred from these data. Spiral tracks are due to low-energy electrons and positrons. In modern, extremely high-energy particle experiments the bubble chamber has been supplanted by electronic detectors.