Positron-electron collision. Computer illustration of an electron (blue) and a positron (red) colliding. The collision produces extremely short- lived B meson and anti-B meson particles (not seen). These particles consist of a quark and an antiquark. Collisions of positrons and electrons take place in the BaBar particle detector of the Stanford Linear Accelerator Center (SLAC), California, USA. The electrons and positrons are circulated around SLAC's PEP-II collider before being forced to collide. Differences in the rates of decay of B mesons and anti-B mesons may explain why there is more matter than antimatter in the universe.