Violation of lepton flavour universality in composite Higgs models

Abstract

We investigate whether the $2.6\sigma$ deviation from lepton flavour universality in $B^{+}\to K^{+}{\ell }^{+}{\ell }^{-}$ decays recently observed at the LHCb experiment can be explained in minimal composite Higgs models. We show that a visible departure from universality is indeed possible if left-handed muons have a sizable degree of compositeness. Constraints from $Z$-pole observables are avoided by a custodial protection of the muon coupling. The deficit in the invisible $Z$ width at LEP is explained in the same region of parameters.

5 pages, 2 figures. v4: discussion of invisible Z width added