First observation of the decays Bˉ0(s)→D+sK−π+π− and Bˉ0s→Ds1(2536)+π−

The first observation of the decays Bˉ0s→D+sK−π+π− and Bˉ0→D+sK−π+π− are reported using an integrated luminosity of 1.0 fb−1 recorded by the LHCb experiment. The branching fractions, normalized with respect to Bˉ0s→D+sπ−π+π− and Bˉ0s→D+sK−π+π−, respectively, are measured to be {\br(\bar{B}^0_{s}\to D_s^+K^-\pi^+\pi^-)\over\br(\bar{B}^0_{s}\to D_s^+\pi^-\pi^+\pi^-)} &= (5.2\pm0.5\pm0.3)\times10^{-2}, {\br(\bar{B}^0\to D_s^+K^-\pi^+\pi^-)\over\br(\bar{B}^0_{s}\to D_s^+K^-^\pi^+\pi^-)} &= 0.54\pm0.07\pm0.07, where the first uncertainty is statistical and the second is systematic. The Bˉ0s→D+sK−π+π− decay is of particular interest as it can be used to measure the weak phase γ. First observation of the Bˉ0s→Ds1(2536)+π−,D+s1→D+sπ−π+ decay is also presented, and its branching fraction relative to Bˉ0s→D+sπ−π+π− is found to be {\br(\bar{B}^0_s\to D_{s1}(2536)^+\pi^-, D_{s1}^+\to D_s^+\pi^-\pi^+)\over\br(\bar{B}^0_{s}\to D_s^+\pi^-\pi^+\pi^-)} &= (4.0\pm1.0\pm0.4)\times10^{-3}.