Observation of Time-Reversal Violation in the B0 Meson System

Abstract

Although C P 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 ¯¯¯ B 0 ), and J / ψ K 0 L or c ¯ c K 0 S 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 × 10 6 B ¯¯¯ B pairs produced in Υ ( 4 S ) 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.