$R(J/\psi)$ and $B_c^- \rightarrow J/\psi \ell^-\bar{\nu}_\ell$ Lepton Flavor Universality Violating Observables from Lattice QCD

Abstract

We use our lattice QCD computation of the $B_c\rightarrow J/\psi$ form factors to determine the differential decay rate for the semitauonic decay channel and construct the ratio of branching fractions $R(J/\psi) = \mathcal{B}(B_c^- \rightarrow J/\psi \tau^-\bar{\nu}_\tau)/\mathcal{B}(B_c^- \rightarrow J/\psi \mu^-\bar{\nu}_\mu)$. We find $R(J/\psi) = 0.2582(38)$ 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 $B\rightarrow D^*\ell\nu$, to $B_c \rightarrow J/\psi\ell\nu$ and make predictions for their values under different new physics scenarios.