Direct observation of time.reversal violation in B mesons at BABAR

Although CP violation in the B meson system has been well established by the B factories, there has been no direct observation of time-reversal violation. The decays of entangled neutral B mesons into definite flavour states (B0 or B-bar 0), and J/ψK0L or c c-bar K0s final states (referred to as B+ or B−), allow comparisons between the probabilities of four pairs of T-conjugated transitions, for example, B-bar 0 → B− and B− → B-bar 0, as a function of the time difference between the two B decays. Using 468 million B B-bar pairs produced in Υ(4S) decays collected by the BABAR detector at SLAC, T-violating parameters in the time evolution of neutral B mesons have been measured, yielding ΔS+T = −1.37 ± 0.14 (stat.) ± 0.06 (syst.) and ΔS−T = 1.17 ± 0.18 (stat.) ± 0.11 (syst.) [1]. These non-zero results represent the first direct observation of T violation through the exchange of initial and final states in transitions that can only be connected by a T-symmetry transformation