Study of the CP asymmetry of B0→J/ψ K0S decays in ALEPH

Abstract The decay B 0 → J /ψ K 0 S is reconstructed with J/ ψ →e + e − or μ + μ − and K 0 S → π + π − . From the full ALEPH dataset at LEP1 of about 4 million hadronic Z decays, 23 candidates are selected with an estimated purity of 71%. They are used to measure the CP asymmetry of this decay, given by sin2 β in the Standard Model, with the result sin 2β=0.84 +0.82 −1.04 ±0.16 . This is combined with existing measurements from other experiments, and increases the confidence level that CP violation has been observed in this channel to 98%.