First Measurement of Time-Dependent CP Violation in B0s → K+ K− Decays

Direct and mixing-induced CP-violating asymmetries in B0s → K+ K− decays are measured for the first time using a data sample of pp collisions, corresponding to an integrated luminosity of 1.0 fb^−1, collected with the LHCb detector at a centre-of-mass energy of 7 TeV. The results are C_KK  = 0.14 ± 0.11 ± 0.03 and S_KK = 0.30 ± 0.12 ± 0.04, where the first uncertainties are statistical and the second systematic. The corresponding quantities are also determined for B0 → π+ π− decays to be C_ππ = −0.38 ± 0.15 ± 0.02 and S_ππ = −0.71 ± 0.13 ± 0.02, in good agreement with existing measurements.