Study of $ B_{(s)}^0\to K_{\mathrm{S}}^0{h^{+}}{h^{{\prime -}}} $ decays with first observation of $ B_s^0\to K_{\mathrm{S}}^0{K^{\pm }}{\pi^{\mp }} $ and $ B_s^0\to K_{\mathrm{S}}^0{\pi^{+}}{\pi^{-}} $

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 1.0fb−1, collected at a centre-of-mass energy of 7TeV recorded by the LHCb experiment. Branching fractions of the B0(s)→K0Sh+h′− decay modes (h(′)=π,K), relative to the well measured B0→K0Sπ+π− decay, are obtained. First observation of the decay modes B0s→K0SK±π∓ and B0s→K0Sπ+π− and confirmation of the decay B0→K0SK±π∓ are reported.