Violation of lepton flavour universality in composite Higgs models

We investigate whether the 2.6σ   deviation from lepton flavour universality in B+→K+ℓ+ℓ−B+→K+ℓ+ℓ− 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.