A B0 meson is one formed of a d quark and a bantiquark. The B 0 meson has an antiparticle written B 0 with a bar above. It is pronounced B-bar.
The SLAC accelerator has a detector called BaBar designed to study this type of meson. They are an excellent laboratory for testing CP symmetry violation, which is at the origin of the matter-antimatter asymmetry in our universe.
It is highly likely that studying them will lead to a proof of the supersymmetry theories.