First observation of B^̄0→J/ψK⁺K ^- and search for B^̄0→J/ψφ decays

The first observation of the B ^̄0→J/ψK ⁺K ^- decay is presented with a data sample corresponding to an integrated luminosity of 1.0 fb ^-1 of pp collisions at a center-of-mass energy of 7 TeV collected with the LHCb detector. The branching fraction is measured to be B(B ^̄0→J/ψK ⁺K ^-)=(2.53±0.31±0.19)×10 ^-6, where the first uncertainty is statistical and the second is systematic. An amplitude analysis of the final state in the B ^̄0→J/ψK ⁺K ^- decay is performed to separate resonant and nonresonant contributions in the K ⁺K ^- spectrum. Evidence of the a ₀(980) resonance is reported with statistical significance of 3.9 standard deviations. The corresponding product branching fraction is measured to be B(B ^̄0→J/ψa ₀(980),a ₀(980)→K ⁺K ^-)=(4.70±3.31±0. 72)×10 ^-7, yielding an upper limit of B(B ^̄0→J/ψa ₀(980),a ₀(980)→K ⁺K ^-)<9.0×10 ^-7 at 90% confidence level. No evidence of the resonant decay B ^̄0→J/ψφ is found, and an upper limit on its branching fraction is set to be B(B ^̄0→J/ψφ)<1.9×10 ^-7 at 90% confidence level.