Observation of the decay Bbars0 → ψ (2 S)K+π-

The decay B 0 s → ψ(2S)K+π − is observed using a data set corresponding to an integrated luminosity of 3.0 fb−1 collected by the LHCb experiment in pp collisions at centre-of-mass energies of 7 and 8 TeV. The branching fraction relative to the B0 → ψ(2S)K+π − decay mode is measured to be B(B 0 s → ψ(2S)K+π −) B(B0 → ψ(2S)K+π−) = 5.38 ± 0.36 (stat) ± 0.22 (syst) ± 0.31 (fs/fd) %, where fs/fd indicates the uncertainty due to the ratio of probabilities for a b quark to hadronise into a B0 s or B0 meson. Using an amplitude analysis, the fraction of decays proceeding via an intermediate K∗ (892)0 meson is measured to be 0.645 ± 0.049 (stat) ± 0.049 (syst) and its longitudinal polarisation fraction is 0.524±0.056 (stat)±0.029 (syst). The relative branching fraction for this component is determined to be B(B 0 s → ψ(2S)K∗ (892)0 ) B(B0 → ψ(2S)K∗(892)0) = 5.58 ± 0.57 (stat) ± 0.40 (syst) ± 0.32 (fs/fd) %. In addition, the mass splitting between the B0 s and B0 mesons is measured as M(B 0 s ) − M(B 0 ) = 87.45 ± 0.44 (stat) ± 0.09 (syst) MeV/c2