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 → 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.