First observation of the decay B̄s⁰→D⁰K^×0 and a measurement of the ratio of branching fractions B(B̄s⁰→D⁰K^×0) B(B̄⁰→D⁰ρ⁰)

The first observation of the decay B̄ _s ⁰→D ⁰K ^×0 using pp data collected by the LHCb detector at a centre-of-mass energy of 7 TeV, corresponding to an integrated luminosity of 36 pb ^-1, is reported. A signal of 34.4±6.8 events is obtained and the absence of signal is rejected with a statistical significance of more than nine standard deviations. The B̄ _s ⁰→D ⁰K ^×0 branching fraction is measured relative to that of B̄0→D ⁰ ρ ⁰: B(B̄ _s ⁰→D ⁰K ^×0) B(B̄ ⁰→D ⁰ρ ⁰)=1.48±0 .34±0.15±0.12, where the first uncertainty is statistical, the second systematic and the third is due to the uncertainty on the ratio of the B ⁰ and Bs0 hadronisation fractions.