First observation of the decays B̄(s)0→Ds+K ^-π⁺π^- and B̄s0→D <inf>s1</inf>(2536)⁺π^-

The first observation of the decays B̄s0→Ds+K ^-π ⁺π ^- and B ^̄0→ Ds+K ^-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 B̄s0→Ds+π ^-π ⁺π ^- and B̄s0→Ds+K ^-π ⁺π ^-, respectively, are measured to be B(B̄s0→Ds+K ^-π ⁺π ^-) B(B̄s0→Ds+π ^-π ⁺π ^-)=(5. 2±0.5±0.3)×10 ^-2 and B(B ^̄0→ Ds+K ^-π ⁺π ^-)B(B̄s0→Ds+K ^-π ⁺π ^-)=0.54±0.07±0.07, where the first uncertainty is statistical and the second is systematic. The B̄s0→Ds+K ^-π ⁺π ^- decay is of particular interest as it can be used to measure the weak phase γ. First observation of the B̄s0→D _s1(2536) ⁺π ^-, Ds1+→Ds+π ^-π ⁺ decay is also presented, and its branching fraction relative to B̄s0→Ds+π ^-π ⁺π ^- is found to be B(B̄s0→ D _s1(2536) ⁺π ^-,Ds1+→Ds+π ^-π ⁺)B(B̄s0→Ds+π ^-π ⁺π ^-)=(4.0±1.0±0.4) ×10 ^-3.