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

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 $\tau$ and $\mu$ 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 $2.8\sigma$. Comparing transitions with light quark and strange quark spectators, we find a $2.3\sigma$ 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.