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

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.

Insightful Overview of Flavour Physics from an Approximate $U(2)^3$ Symmetry

The paper addresses the complex topic of flavour physics, focusing on the symmetry properties of the Standard Model (SM) and their implications for flavour physics, especially within the context of potential new physics scenarios. At the heart of this exploration is the $U(2)^3$ flavour symmetry observed within the quark sector, which the authors propose as a minimal formalism capable of explaining the observed CKM matrix structure while simultaneously accommodating deviations that new physics might introduce at energies pertinent to ElectroWeak Symmetry Breaking (EWSB).

Initially, the paper acknowledges the impressive accuracy with which the CKM picture of flavour mixing and CP violation portrays experimental data. However, the confined deviations also impose limits on theoretical models extending the SM at the TeV scale. The authors propose that an approximate $U(2)^3$ symmetry can comprehensibly account for this structure through minimal but specific symmetry breaking, mainly via small spurions aligned in particular directions.

The core of the paper is dedicated to examining the viability of the $U(2)^3$ hypothesis in explaining flavour physics within a general framework of Effective Field Theory (EFT). It establishes the predicted effects of such an approximation, examining how current experimental bounds constrain these effects and systematically laying out the theoretical framework's consistency with observations. Notably, the discussion provides insights into new experimental searches that might uncover phenomena associated with this flavour symmetry, reinforcing its practical significance.

A substantive portion of the work applies the $U(2)^3$ framework to composite Higgs models, a particular type of beyond-the-SM scenario. Here, the paper elucidates how the assumed symmetry and its patterned breaking can seamlessly integrate into models proposing a composite structure for the Higgs boson, predict specific flavour-changing effects, and direct experimental tests. In these models, distinctive behaviours emerge between right-handed and left-handed compositeness, outlining how the symmetry might manifest in observable flavour violating phenomena.

Furthermore, the paper ventures into the domain of lepton flavour violation (LFV), extending the $U(2)^3$ symmetry to leptonic sectors with the assumption of limited influence from neutrino masses and mixings. This exploration broadens the discussion of flavour physics to encompass additional dimensions of potential new physics, punctuating the $U(2)^3$ framework with electricity through its predictions for LFV processes.

Overall, the paper resists rampant speculation and instead takes an orderly approach by blending phenomenological scrutiny with symmetry principles. Its conclusions demonstrate how present and future experimental data can probe the regions of the parameter space relevant to the symmetry, especially in flavour physics's rich and diverse landscape.

The implications of this work extend theoretical predictions across multiple realms of particle physics, potentially guiding experimental efforts towards unveiling new physics through flavour phenomena and providing a coherent narrative connecting symmetry principles to the complex observed patterns of quark and lepton flavours.