Observation of the B s 0  →  J / ψK S 0 K ± π ∓ decay

Decays of the form B ( s ) 0  →  J / ψK S 0 h + h (′) − ( h (′)  = K ,  π ) are searched for in proton-proton collision data corresponding to an integrated luminosity of 1.0 fb −1 recorded with the LHCb detector. The first observation of the B s 0  →  J / ψK S 0 K ± π ∓ decay is reported, with significance in excess of 10 standard deviations. The B 0  →  J / ψK S 0 K + K − decay is also observed for the first time. The branching fraction of B 0  →  J / ψK S 0 π + π − is determined, to significantly better precision than previous measurements, using B 0  →  J / ψK S 0 as a normalisation channel. Branching fractions and upper limits of the other B ( s ) 0  →  J / ψK S 0 h + h (′) − modes are determined relative to that of the B 0  →  J / ψK S 0 π + π − decay.