Measurement of the ratio of branching fractions ℬ B c + → J / ψ K + / ℬ B c + → J / ψ π + $$ \mathrm{\mathcal{B}}\left({\mathrm{B}}_{\mathrm{c}}^{+}\to \mathrm{J}/{\uppsi \mathrm{K}}^{+}\right)/\mathrm{\mathcal{B}}\left({\mathrm{B}}_{\mathrm{c}}^{+}\to \mathrm{J}/{\uppsi \uppi}^{+}\right) $$

The ratio of branching fractions $R_{K/\pi} \equiv \mathcal{B}(B_{c}^{+} \to J/\psi K^{+})/\mathcal{B}(B_{c}^{+} \to J/\psi\pi^{+})$ is measured with $pp$ collision data collected by the LHCb experiment at centre-of-mass energies of 7 TeV and 8 TeV, corresponding to an integrated luminosity of 3${\mbox{fb}^{-1}}$. It is found to be $ R_{K/\pi} = 0.079\pm0.007\pm0.003$, where the first uncertainty is statistical and the second is systematic. This measurement is consistent with the previous LHCb result, while the uncertainties are significantly reduced.