R(J/ψ) and Bc−<…

We use our lattice QCD computation of the ${B}_{c}\ensuremath{\rightarrow}J/\ensuremath{\psi}$ form factors to determine the differential decay rate for the semitauonic decay channel and construct the ratio of branching fractions $R(J/\ensuremath{\psi})=\phantom{\rule{0ex}{0ex}}\mathcal{B}({B}_{c}^{\ensuremath{-}}\ensuremath{\rightarrow}J/\ensuremath{\psi}{\ensuremath{\tau}}^{\ensuremath{-}}{\overline{\ensuremath{\nu}}}_{\ensuremath{\tau}})/\mathcal{B}({B}_{c}^{\ensuremath{-}}\ensuremath{\rightarrow}J/\ensuremath{\psi}{\ensuremath{\mu}}^{\ensuremath{-}}{\overline{\ensuremath{\nu}}}_{\ensuremath{\mu}})$. We find $R(J/\ensuremath{\psi})=0.2582(38)$ and give an error budget. We also extend the relevant angular observables, which were recently suggested for the study of lepton flavor universality violating effects in $B\ensuremath{\rightarrow}{D}^{*}\ensuremath{\ell}\ensuremath{\nu}$, to ${B}_{c}\ensuremath{\rightarrow}J/\ensuremath{\psi}\ensuremath{\ell}\ensuremath{\nu}$ and make predictions for their values under different new physics scenarios.