Semileptonic $B_c\to {\eta }_c,J/\psi$ transitions

Abstract

Using a systematic, symmetry-preserving continuum approach to the Standard Model strong-interaction bound-state problem, we deliver parameter-free predictions for all semileptonic $B_c\to {\eta }_c,J/\psi$ 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 $\tau$ 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 $2\sigma$ discrepancy between the Standard Model prediction for $R_{J/\psi }$ and the single available experimental result.

Comments: 8 pages, 4 figures, 5 tables