The B factory detector, BaBar, will be built by an international collaboration. It will be built especially to create pairs of electrically neutral B and anti-B mesons. The neutral B contains an anti-b quark and a d quark while the neutral anti-B contains a b quark and an anti-d. Physicists will tune the two beams so that when they collide they will have exactly enough energy to create these two mesons. Because the electrons and positrons are circling at different energies, the B and anti-B mesons which are created will be moving very quickly in the same direction. They both will travel in the same direction that the faster moving electrons were. This will let physicists infer how long they last before they decay by looking at how far they get. If, as physicists expect, the B's and the anti-B's decay to certain sets of particles at different rates (unlike most particle-antiparticle pairs, which are identical in every respect but charge), this study could help answer lingering questions about the differences between matter and antimatter.
The experiment is called "BaBar" because it deals with B and anti-b mesons. Since the physics term for anti-b-meson is b-bar, physicists decided that "B B- Bar" wasn't as cute a name as "BaBar."