Observation of time-reversal violation in the B0 meson system

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 flavor states (B0 or B̄0), and J/ψKL0 or cc̄KS0 final states (referred to as B+ or B-), allow comparisons between the probabilities of four pairs of T-conjugated transitions, for example, B ̄0→B- and B-→B ̄0, as a function of the time difference between the two B decays. Using 468×106 BB̄ pairs produced in Υ(4S) decays collected by the BABAR detector at SLAC, we measure T-violating parameters in the time evolution of neutral B mesons, yielding ΔST+=-1.37±0. 14(stat)±0.06(syst) and ΔST-=1.17±0.18(stat)±0. 11(syst). These nonzero 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. © 2012 American Physical Society.