Large Top Mass and Non-Linear Representation of Flavour Symmetry

TL;DR

This paper develops a non-linear effective theory framework for flavour-violating processes that incorporates a large top-quark Yukawa coupling, explaining flavour structures in rare decays and extending to 2-Higgs-doublet models with large tan(beta).

Contribution

It introduces a non-linear representation of flavour symmetry breaking with Goldstone modes and spurions, accounting for a large top Yukawa and providing a new perspective on flavour processes.

Findings

Large top Yukawa leads to significant flavour symmetry breaking at the UV scale.

Non-linear EFT captures dominant flavour structures in rare decays.

Framework generalizes to 2-Higgs-doublet models with large tan(beta).

Abstract

We consider an effective theory (ET) approach to flavour-violating processes beyond the Standard Model (SM), where the breaking of flavour symmetry is described by spurion fields whose low-energy vacuum expectation values are identified with the SM Yukawa couplings. Insisting on canonical mass dimensions for the spurion fields, the large top-quark Yukawa coupling also implies a large expectation value for the associated spurion, which breaks part of the flavour symmetry already at the UV scale Lambda of the ET. Below that scale, flavour symmetry in the ET is represented in a non-linear way by introducing Goldstone modes for the partly broken flavour symmetry and spurion fields transforming under the residual symmetry. As a result, the dominance of certain flavour structures in rare quark decays can be understood in terms of the 1/Lambda expansion in the ET. We also discuss the generalization to 2-Higgs-doublet models with large tan(beta).