First observation of $\overline{B}^0 \rightarrow J/\psi K^{+}K^{-}$ and search for $\overline{B}^0 \rightarrow J/\psi \phi$ decays

The first observation of the $\overline{B}^0 \rightarrow J/\psi K^{+}K^{-}$ decay is presented with a data sample corresponding to an integrated luminosity of 1.0 $\rm fb^{-1}$ of $pp$ collisions at a center-of-mass energy of 7 TeV collected with the LHCb detector. The branching fraction is measured to be $\mathcal{B}(\overline{B}^0 \rightarrow J/\psi K^{+}K^{-}) = (2.53\pm 0.31 \pm 0.19)\times 10^{-6}$, where the first uncertainty is statistical and the second is systematic. An amplitude analysis of the final state in the $\overline{B}^0 \rightarrow J/\psi K^{+}K^{-}$ 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 $\mathcal{B}(\overline{B}^0 \rightarrow J/\psi a_0(980),~a_0(980)\rightarrow K^{+}K^{-})=(4.70\pm3.31\pm0.72)\times10^{-7}$, yielding an upper limit of $\mathcal{B}(\overline{B}^0 \rightarrow J/\psi a_0(980),~a_0(980)\rightarrow K^{+}K^{-})<9.0\times 10^{-7}$ at 90 % confidence level. No evidence of the resonant decay $\overline{B}^0 \rightarrow J/\psi \phi$ is found, and an upper limit on its branching fraction is set to be $\mathcal{B}(\overline{B}^0 \rightarrow J/\psi \phi)< 1.9\times 10^{-7}$ at 90 % confidence level.