Measurements of the Branching Fractions of the Decays B0s → Ds∓ K± and B0s → Ds− π+

The decay mode B0s → Ds∓ K± allows for one of the theoretically cleanest measurements of the CKM angle γ through the study of time-dependent CP violation. This paper reports a measurement of its branching fraction relative to the Cabibbo-favoured mode B0s → Ds− π+ based on a data sample corresponding to 0.37 fb^−1 of proton-proton collisions at √s = 7 TeV collected in 2011 with the LHCb detector. In addition, the ratio of B meson production fractions fs/fd, determined from semileptonic decays, together with the known branching fraction of the control channel B0 → D− π+, is used to perform an absolute measurement of the branching fractions: B(B0s → Ds− π+) = (2.95 ± 0.05 ± 0.17 +0.18 -0.22) x 10^−3, B(B0s → Ds∓ K±) = (1.90 ± 0.12 ± 0.13 +0.12 -0.14) x 10^−4, where the first uncertainty is statistical, the second the experimental systematic uncertainty, and the third the uncertainty due to fs/fd.