Semileptonic $B_c \to \eta_c, J/\psi$ transitions

TL;DR

This paper provides parameter-free predictions for semileptonic Bc transitions to eta_c and J/psi, analyzing form factors and branching ratios, and confirms a discrepancy with experimental data for R_{J/psi}.

Contribution

It introduces a symmetry-preserving continuum approach to predict Bc transition form factors without free parameters, covering the full momentum transfer domain.

Findings

Predicted R_{eta_c} = 0.313(22)

Predicted R_{J/psi} = 0.242(47)

Confirms a 2σ discrepancy with experimental R_{J/psi}

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.