Observation of Time-Reversal Violation in the B^{0} 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/ψKL0 or cc ̄KS0 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 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.