Lepton Flavour Universality in Semileptonic $\bar B_{(s)} \to D_{(s)}^{(\ast)}$ Transitions

Abstract

Using continuum Schwinger function methods, we deliver parameter-free pointwise predictions for the complete set of twelve Standard Model (SM) $\bar B_{(s)} \to D_{(s)}^{(\ast)}$, i.e., $b\to c$, semileptonic transition form factors on the entire kinematically accessible domain. Therewith, we calculate SM theory values for the decay-rate ratios of $τ$ and $μ$ final states: $\{ R(D) \, , \, R(D)^\ast \} = \{ 0.290(11) \, , \, 0.267(5) \}$, $\{ R(D_{s}) \, , \, R(D_{s}^{\ast}) \} = \{ 0.283(11) \, , \, 0.251(5) \}$. The former is consistent with an existing average of SM estimates, but disagrees with the experimental average by $2.8σ$. Comparing transitions with light quark and strange quark spectators, we find a $2.3σ$ signal for violation of SU$(3)$ flavour symmetry, expressed at the 5% level in $D_{(s)}^\ast$ final states. It is induced by Higgs boson couplings into the strong interaction and modulated by phenomena deriving from emergent features of the strong interaction.