SLAC PEP-II collider. Simulated beams of electrons (blue) and positrons (pink) travelling along the PEP-II collider rings at the Stanford Linear Accelerator Center (SLAC), California, USA. Electrons and positrons circulate in opposite directions along the two rings before being forced to collide. These collisions produce large quantities of B mesons and anti-B mesons. For this reason PEP-II is known as the B factory. Differences in these particles' rates of decay may explain why there is more matter than antimatter in the universe. The copper chambers on the lower ring are microwave cavities which restore the energy lost by the electrons during each circuit.