Lepton Flavour Violation in Composite Higgs Models

TL;DR

This paper analyzes constraints on lepton flavor violation in composite Higgs models, formulates a symmetry-based framework for partial compositeness, and computes relevant operators, revealing bounds on the compositeness scale and implications for flavor processes.

Contribution

It introduces a symmetry and spurion-based formulation of partial compositeness, and explicitly computes dimension-six operators in a two-site model, linking theoretical structure to experimental bounds.

Findings

Most general scenarios require compositeness scale above 10 TeV.

Universal mass parameters and Yukawas can lower bounds significantly.

Key processes include charged lepton decays, muon conversion, and electron EDM.

Abstract

We discuss in detail the constraints on partial compositeness coming from flavour and CP violation in the leptonic sector. In a first part we present a formulation of partial compositeness in terms of a flavour symmetry group and a set of spurions, whose background values specify the symmetry breaking pattern. In such a framework we construct the complete set of dimension-six operators describing lepton flavour violation and CP violation. By exploiting the existing bounds, we derive limits on the compositeness scale in different scenarios, characterised by increasing restrictions on the spurion properties. We confirm that in the most general case the compositeness scale should lie well-above 10 TeV. However, if in the composite sector mass parameters and Yukawa couplings are universal, such a bound can be significantly lowered, without necessarily reproducing the case of minimal flavour violation. The most sensitive processes are decays of charged leptons either of radiative type or into three charged leptons, muon to electron conversion in nuclei and the electric dipole moment of the electron. In a second part we explicitly compute the Wilson coefficients of the relevant dimension-six operators in the so-called two-site model, embodying the symmetry breaking pattern discussed in our first part, and we compare the results with those of the general spurion analysis.