Observation of Time-Reversal Violation in theB0Meson 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 ($B^0$ or ${\overline{B}}^0$), and $J/\psi K_L^0$ or $c\overline{c}{K}_S^0$ final states (referred to as $B_{+}$ or $B_{-}$), allow comparisons between the probabilities of four pairs of $T$-conjugated transitions, for example, ${\overline{B}}^0\to B_{-}$ and $B_{-}\to {\overline{B}}^0$, as a function of the time difference between the two $B$ decays. Using $468\times {10}^6$ $B\overline{B}$ pairs produced in ${\rm Y}(4S)$ decays collected by the BABAR detector at SLAC, we measure $T$-violating parameters in the time evolution of neutral $B$ mesons, yielding $\Delta S_T^{+}=-1.37\pm 0.14(\mathrm{stat})\pm 0.06(\mathrm{syst})$ and $\Delta S_T^{-}=1.17\pm 0.18(\mathrm{stat})\pm 0.11(\mathrm{syst})$. 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.