On lepton flavour universality in semileptonic $B_c \to \eta_c, J/\psi$ decays

Understand

We discuss $B_c \to \eta_c$ and $B_c \to J/\psi$ semileptonic decays within the Standard Model (SM) and beyond.

• The relevant transition form factors, being the main source of theoretical uncertainties, are calculated in the sum rule approach and are provided in a full $q^2$ range.
• We calculate the semileptonic branching fractions and find for the ratios, $R_{\eta_c}|_{\rm SM}= 0.32 \pm 0.02$, $R_{J/\psi}|_{\rm SM} = 0.23 \pm 0.01$.
• Both predictions are in agreement with other existing calculations and support the current tension in $R_{J/\psi}$ at 2$\sigma$ level with the experiment.