Study of B0(s) → K0S h+ h′− Decays with First Observation of B0s → K0S K± π∓ and B0s → K0S π+ π−

A search for charmless three-body decays of B0 and B0s mesons with a K0S meson in the final state is performed using the pp collision data, corresponding to an integrated luminosity of 1.0 fb^−1, collected at a centre-of-mass energy of 7 TeV recorded by the LHCb experiment. Branching fractions of the B0(s) → K0S h+ h′− decay modes (h(′) = π, K), relative to the well measured B0 → K0S π+ π− decay, are obtained. First observation of the decay modes B0s → K0S K± π∓ and B0s → K0S π+ π− and confirmation of the decay B0 → K0S K± π∓ are reported. The following relative branching fraction measurements or limits are obtained B(B0 → K0S K± π∓) / B(B0 → K0S π+ π−) = 0.128 ± 0.017(stat.) ± 0.009(syst.), B(B0 → K0S K+ K−) / B(B0 → K0S π+ π−) = 0.385 ± 0.031(stat.) ± 0.023(syst.), B(B0s → K0S π+ π−) / B(B0 → K0S π+ π−) = 0.29 ± 0.06(stat.) ± 0.03(syst.) ± 0.02(fs/fd), B(B0s → K0S K± π∓) / B(B0 → K0S π+ π−) = 1.48 ± 0.12(stat.) ± 0.08(syst.) ± 0.12(fs/fd), B(B0s → K0S K+ K−) / B(B0 → K0S π+ π−) ∈ [0.004; 0.068] at 90% CL.