Measurement of the ratio of branching fractions $\mathcal{B}(B_c^{+}\to J\mathrm{/}\psi {\tau }^{+}{\nu }_{\tau })$/$\mathcal{B}(B_c^{+}\to J\mathrm{/}\psi {\mu }^{+}{\nu }_{\mu })$

A measurement is reported of the ratio of branching fractions $$\mathcal{R}$(J/\psi)=$\mathcal{B}$(B_c^+,\to,J/\psi\tau^+\nu_\tau)/$\mathcal{B}$(B_c^+,\to,J/\psi \mu^+\nu_\mu)$,wherethe$\tau^+$leptonisidentifiedinthedecaymode$\tau^+,\to,\mu^+\nu_\mu\overline{\nu}_\tau$. This analysis uses a sample of proton-proton collision data corresponding to 3.0${\mathrm{f}\mathrm{b}}^{-1}$ of integrated luminosity recorded with the LHCb experiment at center-of-mass energies $7\mathrm{T}\mathrm{e}\mathrm{V}$ and $8\mathrm{T}\mathrm{e}\mathrm{V}$. A signal is found for the decay $B_c^{+}\to J\mathrm{/}\psi {\tau }^{+}{\nu }_{\tau }$ at a significance of 3 standard deviations, corrected for systematic uncertainty, and the ratio of the branching fractions is measured to be $$\mathcal{R}$(J/\psi) = 0.71 \pm 0.17 $(\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{t})$ \pm 0.18$(\mathrm{s}\mathrm{y}\mathrm{s}\mathrm{t})$$. This result lies within 2 standard deviations above the range of existing predictions in the Standard Model.