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 B¯⁰), and J/ψK_L⁰ or cc¯K_S⁰ 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 BB¯ 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 ΔS_T⁺=-1.37±0.14(stat)±0.06(syst) and ΔS_T^-=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.