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 $\barB^0$), and $J/ψ K_S^0$ or $c\barc K_S^0$ final states (referred to as $B_+$ or $B_-$), allow comparisons between the probabilities of four pairs of T-conjugated transitions, for example, $\barB^0 \rightarrow B_-$ and $B_- \rightarrow \barB^0$, as a function of the time difference between the two B decays. Using 468 million $B\barB$ 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 $Δ S_T^+ = -1.37 ± 0.14 (stat.) ± 0.06 (syst.)$ and $Δ S_T^- = 1.17 ± 0.18 (stat.) ± 0.11 (syst.)$. 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.