Violation of lepton flavour universality in composite Higgs models

TL;DR

This paper explores if minimal composite Higgs models can explain the observed lepton flavour universality violation in B decays, showing that muon compositeness and custodial protection can account for the deviation.

Contribution

It demonstrates that composite Higgs models with left-handed muon compositeness and custodial protection can explain recent lepton universality deviations in B decays.

Findings

Possible explanation of lepton universality violation in composite Higgs models.

Compatibility with Z-pole constraints and LEP Z width deficit.

Parameter space allowing sizable muon compositeness without conflicting with existing data.

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.