The Bearable Compositeness of Leptons

TL;DR

This paper explores a partial compositeness model for leptons, analyzing neutrino masses, flavor violation, and CP violation, proposing a symmetry to suppress constraints, and addressing anomalies in B-meson decays.

Contribution

It provides the first systematic analysis of neutrino mass textures and introduces a $U(1)^3\times CP$ symmetry to suppress flavor constraints in lepton compositeness models.

Findings

The $U(1)^3\times CP$ symmetry effectively suppresses electron EDM and $\,\mu\to e\gamma$ constraints.

The model naturally generates the observed CKM phase and large PMNS phases.

It can explain B-meson decay anomalies with correlated lepton flavor predictions.

Abstract

Partial compositeness as a theory of flavor in the lepton sector is assessed. We begin presenting the first systematic analysis of neutrino mass generation in this context, and identifying the distinctive mass textures. We then update the bounds from charged lepton flavor and CP violating observables. We put forward a $U(1)^3\times CP$ symmetry of the composite sector, in order to allow the new physics to be not far above the TeV scale. This hypothesis effectively suppresses the new contributions to the electron EDM and $\mu\to e\gamma$, by far the most constraining observables, and results in a novel pattern of flavor violation and neutrino masses. The CP violation in the elementary-composite mixing is shown to induce a CKM phase of the correct size, as well as order-one phases in the PMNS matrix. We compare with the alternative possibility of introducing multiple scales of compositeness for leptons, that also allow to evade flavor and CP constraints. Finally, we examine violations of lepton flavor universality in $B$-meson semi-leptonic decays. The neutral-current anomalies can be accommodated, predicting strong correlations among different lepton flavors, with a few channels close to the experimental sensitivity.