Violation of lepton flavour universality in composite Higgs models

We investigate whether the 2.6 sigma deviation from lepton flavour universality in 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. (C) 2015 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license.