FCNC Effects in a Minimal Theory of Fermion Masses

TL;DR

This paper explores how extending the Standard Model with heavy vectorlike fermions and flavor-anarchical Yukawa couplings can lead to flavor-changing neutral current effects, providing a minimal effective framework for understanding fermion mass hierarchies.

Contribution

It introduces a minimal model with heavy vectorlike fermions that generate SM Yukawa hierarchies and derives general formulas for FCNC effects and bounds on heavy fermion masses.

Findings

FCNC effects arise from massive boson exchange with modified couplings.

Stringent bounds on heavy fermion masses are in the few TeV range.

The model serves as a reference for flavor hierarchy studies.

Abstract

As a minimal theory of fermion masses we extend the SM by heavy vectorlike fermions, with flavor-anarchical Yukawa couplings, that mix with chiral fermions such that small SM Yukawa couplings arise from small mixing angles. This model can be regarded as an effective description of the fermionic sector of a large class of existing flavor models and thus might serve as a useful reference frame for a further understanding of flavor hierarchies in the SM. Already such a minimal framework gives rise to FCNC effects through exchange of massive SM bosons whose couplings to the light fermions get modified by the mixing. We derive general formulae for these corrections and discuss the bounds on the heavy fermion masses. Particularly stringent bounds, in a few TeV range, come from the corrections to the Z couplings.