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

Abstract

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

Comments: 6 pages, 4 figures, Version accepted for publication in Phys. Rev. Lett