First Observation of anti-B0 → J/ψ K+ K− and Search for anti-B0 → J/ψ ϕ Decays

The first observation of the anti-B0 → J/ψ K+ 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(anti-B0 → J/ψ K+ K−) = (2.53 ± 0.31 ± 0.19) x 10^−6, where the first uncertainty is statistical and the second is systematic. An amplitude analysis of the final state in the anti-B0 → J/ψ K+ K− decay is performed to separate resonant and nonresonant contributions in the K+ K− spectrum. Evidence of the a0(980) resonance is reported with statistical significance of 3.9 standard deviations. The corresponding product branching fraction is measured to be B(anti-B0 → J/ψ a0(980), a0(980) → K+ K−) = (4.70 ± 3.31 ± 0.72) x 10^−7, yielding an upper limit of B(anti-B0 → J/ψ a0(980), a0(980) → K+ K−) < 9.0 x 10^−7 at 90% confidence level. No evidence of the resonant decay anti-B0 → J/ψ ϕ is found, and an upper limit on its branching fraction is set to be B(anti-B0 → J/ψ ϕ) < 1.9 x 10^−7 at 90% confidence level.