Exp. method to measure sin(phi_1) in B -> J/psi Ks

These first studies involve rate differences that depend on when a neutral B (B0) or anti-B0 decays. The decay is so fast, though, that no timing device exists that could measure it; the average B0 decays in one-trillionth of a second. Instead, the B0 is made to move rapidly, and the distance between its birth and its decay is measured: time=distance/velocity. B and anti-B pairs are produced by colliding electrons with positrons (the antimatter partners of electrons) at specific energies such that they annihilate completely, produce a meson known as the Upsilon(4S) (which contains a b and a b-bar quark), which decays almost immediately to a B and an anti-B meson pair. If the electrons and positrons are at different energies, the Upsilon(4S) is moving, as are the B and anti-B. An interesting characteristic of using the Upsilon(4S) as a source of B mesons is that the B pairs are produced in a coherent quantum mechanical state (like Schrodinger's cat or single atoms in a trap). The B mesons begin their life in a superposition of quantum mechanical states. When one of the pair decays, its partner is forced into a definite state. That this is indeed the case is amply demonstrated in the current data, where both decays are timed.

-- BostjanGolob - 11 Feb 2009