Semileptonic $B_c o η_c, J/ψ$ transitions

Using a systematic, symmetry-preserving continuum approach to the Standard Model strong-interaction bound-state problem, we deliver parameter-free predictions for all semileptonic $B_c o eta_c, J/psi$ transition form factors on the complete domains of empirically accessible momentum transfers. Working with branching fractions calculated therefrom, the following values of the ratios for $ au$ over $mu$ final states are obtained: $R_{eta_c}=0.313(22)$ and $R_{J/psi}=0.242(47)$. Combined with other recent results, our analysis confirms a $2sigma$ discrepancy between the Standard Model prediction for $R_{J/psi}$ and the single available experimental result.