Measurement of CP observables in B⁰ → DK^*0 with D → K⁺K⁻

The decay B⁰ → DK^*0 and the charge conjugate mode are studied using 1.0 fb⁻¹ of pp collision data collected by the LHCb experiment at √s = 7 TeV in 2011. The CP asymmetry between the B⁰ → DK^*0 and the B⁰ → DK^*0 decay rates, with the neutral D meson in the CP-even final state K⁺K⁻‚ is found to be A_d^KK = −0.45 ± 0.23 ± 0.02, where the first uncertainty is statistical and the second is systematic. In addition, favoured B⁰ → DK^*0 decays are reconstructed with the D meson in the non-CP eigenstate K⁺π⁻. The ratio of the B-flavour averaged decay rates in D decays to CP and non-CP eigenstates is measured to be R_d^KK = 1.36 ^+0.37_−0.32 ± 0.07, where the ratio of the branching fractions of D0 → K⁻π⁺ to D⁰ → K⁺K⁻ decays is included as multiplicative factor. The CP asymmetries measured with two control channels, the favoured B⁰ → DK^*0 decay with D → K⁺π⁻ and the B⁰_s → DK^*0 decay with D → K⁺K⁻, are also reported.