Flavour constraints on multi-Higgs-doublet models: Yukawa alignment

TL;DR

This paper explores how Yukawa alignment in multi-Higgs-doublet models prevents tree-level flavour-changing neutral currents but introduces new CP violation sources, with quantum corrections leading to a minimal-flavour-violation structure.

Contribution

It provides a detailed analysis of the flavour structure and phenomenology of aligned multi-Higgs-doublet models, highlighting the hierarchy of FCNC effects and CP violation.

Findings

Yukawa alignment prevents tree-level FCNCs in multi-Higgs models.

Quantum corrections induce minimal-flavour-violation with flavour-blind phases.

FCNC effects are suppressed in light-quark systems but significant in heavy-quark transitions.

Abstract

In multi-Higgs-doublet models, the alignment in flavour space of all Yukawa matrices coupling to a given right-handed fermion guarantees the absence of tree-level flavour-changing neutral couplings, while introducing new sources of CP violation. With N Higgs doublets (and no right-handed neutrinos) the Yukawa Lagrangian is characterized by the fermion masses, the CKM quark mixing matrix and 3(N-1) complex couplings. Quantum corrections break the alignment, generating a minimal-flavour-violation structure with flavour-blind phases. The aligned multi-Higgs-doublet models lead to a rich and viable phenomenology with an interesting hierarchy of flavour-changing neutral current effects, suppressing them in light-quark systems while allowing potentially relevant signals in heavy-quark transitions.