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 ($B^0$ or $bar{B}^0$), and $J/psi K_S^0$ or $cbar{c} K_S^0$ final states (referred to as $B_+$ or $B_-$), allow comparisons between the probabilities of four pairs of T-conjugated transitions, for example, $bar{B}^0 rightarrow B_-$ and $B_- rightarrow bar{B}^0$, as a function of the time difference between the two B decays. Using 468 million $Bbar{B}$ pairs produced in $Upsilon(4S)$ decays collected by the BABAR detector at SLAC, we measure T-violating parameters in the time evolution of neutral B mesons, yielding $Delta S_T^+ = -1.37 pm 0.14 (stat.) pm 0.06 (syst.)$ and $Delta S_T^- = 1.17 pm 0.18 (stat.) pm 0.11 (syst.)$. 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.