Flavour, Accidentally

TL;DR

The paper proposes a class of non-Abelian flavour models where the symmetry-breaking spurion transforms in non-minimal irreps, enabling accidental generation of SM Yukawa hierarchies from a few $O(1)$ UV inputs. By analyzing two explicit constructions, one with a $Y$ spurion in a higher irrep that yields Yukawas via multiple insertions and another where a $\mathbf{6} \times \overline{\mathbf{6}}$ spurion enhances UV hierarchies, it shows that the resulting SMEFT flavour structure can differ markedly from MFV and FN expectations. The work highlights novel correlations between Yukawa textures and dimension-6 flavour operators, including potential unsuppressed light-quark mixing, and extends the framework to leptons with plausible TeV-scale flavour dynamics. These results motivate a broad experimental programme to probe flavour violation across generations and suggest a richer UV landscape for flavour than traditionally envisaged.

Abstract

We propose a new class of flavour models in which the spurion which breaks Standard Model flavour symmetries transforms in a non-minimal representation. Hierarchies in fermion masses, which arise from multiple insertions of this spurion, may be generated in a technically natural, accidental manner, from a handful of untuned $\mathcal{O}(1)$ elements in the UV. This relies explicitly on the non-Abelian nature of the symmetry, distinguishing it from standard Froggatt-Nielsen-like scenarios. The pattern of flavour violating operators at dimension-6 can radically differ from previously considered scenarios, and emphasises the need for a broad flavour programme across all generations.