SLAC control room. Wide angle view of physicists and technicians in the control room of the Stanford Linear Accelerator Center (SLAC), California, USA. In this room, beams of electrons and positrons circulating in opposite directions along the two rings of the PEP-II collider are brought together. Electron-positron collisions produce large quantities of B mesons and their antimatter equivalent, anti- B mesons. For this reason PEP-II is known as the B factory. Differences in the decay rates of B mesons and anti-B mesons may explain why there is more matter than antimatter in the universe. Photographed in 1999.