$R_{K^{(*)}}$ and the origin of Yukawa couplings

Understand

We explore the possibility that the semi-leptonic $B$ decay ratios $R_{K^{(*)}}$ which violate $\mu - e$ universality are related to the origin of the fermion Yukawa couplings in the Standard Model.

• Some time ago, a vector-like fourth family (without a $Z'$) was used to generate fermion mass hierarchies and mixing patterns without introducing any family symmetry.
• Recently the idea of inducing flavourful $Z'$ couplings via mixing with a vector-like fourth family which carries gauged $U(1)'$ charges has been proposed as a simple way of producing controlled flavour universality violation while elegantly cancelling anomalies.
• We show that the fusion of these two ideas provides a nice connection between $R_{K^{(*)}}$ and the origin of Yukawa couplings in the quark sector.