Semileptonic $B_c \to η_c, J/ψ$ 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 $B_c \to η_c, J/ψ$ 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 $τ$ over $μ$ final states are obtained: $R_{η_c}=0.313(22)$ and $R_{J/ψ}=0.242(47)$. Combined with other recent results, our analysis confirms a $2σ$ discrepancy between the Standard Model prediction for $R_{J/ψ}$ and the single available experimental result.