Observation of the $B^{+}\to J\mathrm{/}\psi {\eta }^{\mathrm{\prime }}{K}^{+}$ decay

The $B^{+}\to J\mathrm{/}\psi {\eta }^{\mathrm{\prime }}{K}^{+}$ decay is observed for the first time using proton-proton collision data collected by the LHCb experiment at centre-of-mass energies of 7, 8, and 13TeV, corresponding to a total integrated luminosity of 9fb${}^{-1}$. The branching fraction of this decay is measured relative to the known branching fraction of the $B^{+}\to \psi (2S)K^{+}$ decays and found to be $$\frac{\mathcal{B}(B^{+}\to J\mathrm{/}\psi {\eta }^{\mathrm{\prime }}{K}^{+})}{\mathcal{B}(B^{+}\to \psi (2S)K^{+})}=(4.91\pm 0.47\pm 0.29\pm 0.07)\times {10}^{-2},$$ where the first uncertainty is statistical, the second is systematic and the third is related to external branching fractions. A first look at the $J\mathrm{/}\psi {\eta }^{\mathrm{\prime }}$ mass distribution is performed and no signal of intermediate resonances is observed.