Measurement of CP violation in B 0 → J/ψK S 0 and B 0 → ψ(2S)K S 0 decays

measurement is presented of decay-time-dependent CP violation in the decays B0 → J/ψ K0 S and B0 → ψ(2S)K0 S, where the J/ψ is reconstructed from two electrons and the ψ(2S) from two muons. The analysis uses a sample of pp collision data recorded with the LHCb experiment at centre-of-mass energies of 7 and 8TeV, corresponding to an integrated luminosity of 3fb−1. The CP-violation observables are measured to be CB0 → J/ψ K0 S= 0.12 ± 0.07± 0.02,S B0 → J/ψ K0 S= 0.83 ± 0.08± 0.01,C B0 → ψ(2S)K0 S= −0.05 ± 0.10± 0.01,S B0 → ψ(2S)K0 S= 0.84 ± 0.10± 0.01,where C describes CP violation in the direct decay, and S describes CP violation in the interference between the amplitudes for the direct decay and for the decay after B0–B0 oscillation. The first uncertainties are statistical and the second are systematic. The two sets of results are compatible with the previous LHCb measurement using B0 → J/ψ K0 S decays, where the J/ψ meson was reconstructed from two muons. The averages of all three sets of LHCb results are C(B0 → [c¯ c]K0 S) = −0.017±0.029, S(B0 → [c¯ c]K0 S) = 0.760±0.034, under the assumption that higher-order contributions to the decay amplitudes are negligible. The uncertainties include statistical and systematic contributions.