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/ψK_L⁰ or cc̄K_S⁰ final states (referred to as B₊ or B₋), allow comparisons between the probabilities of four pairs of T-conjugated transitions, for example, B̅⁰ → B₋ and B₋ → B̅⁰, 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 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.