Detecting B-mesons
When a B0 and anti-B0 meson are created in an electron-positron collision they travel at relativistic speeds until they decay. One of them (the anti-B0 meson in this example) decays to a "tagging" final state, while the other decays to the CP final state, J/ψ-K-short. The J/ψ immediately decays to a muon-antimuon pair, µ+µ-, while the K-short decays to a pair of pions, µ+µ-. The distance between the decay positions of the B0 and the anti-B0 meson allows the time between the decays to be determined.