Flavour physics from an approximate U(2)^3 symmetry

TL;DR

This paper explores how an approximate U(2)^3 flavour symmetry can explain quark mixing and CP violation, while allowing for potential observable deviations, within supersymmetry, composite Higgs models, and lepton sector extensions.

Contribution

It analyzes the effects of U(2)^3 symmetry breaking in various models and provides bounds and implications for new physics searches in flavour physics.

Findings

U(2)^3 symmetry explains CKM success and CP violation

Current bounds constrain new physics effects

Potential for observable deviations in flavour sector phenomena

Abstract

The quark sector of the Standard Model exhibits an approximate U(2)^3 flavour symmetry. This symmetry, broken in specific directions dictated by minimality, can explain the success of the Cabibbo-Kobayashi-Maskawa picture of flavour mixing and CP violation, confirmed by the data so far, while allowing for observable deviations from it, as expected in most models of ElectroWeak Symmetry Breaking. Building on previous work in the specific context of supersymmetry, we analyze the expected effects and we quantify the current bounds in a general Effective Field Theory framework. As a further relevant example we then show how the U(2)^3 symmetry and its breaking can be implemented in a generic composite Higgs model and we make a first analysis of its peculiar consequences. We also discuss how some partial extension of U(2)^3 to the lepton sector can arise, both in general and in composite Higgs models. An optimistic though conceivable interpretation of the considerations developed in this paper gives reasons to think that new physics searches in the flavour sector may be about to explore an interesting realm of phenomena.