Theoretical Aspects of Flavour and CP Violation in the Lepton Sector

TL;DR

This paper reviews how flavour and CP violation measurements in the lepton sector can reveal new physics beyond the standard model, discussing experimental bounds, operator limits, and mechanisms to suppress violations at accessible energy scales.

Contribution

It provides a comprehensive analysis of flavour and CP violation constraints, highlighting the implications for new physics scales and mechanisms like partial compositeness.

Findings

Dimension-six operators suggest a high scale of new physics.

Special mechanisms are needed to suppress violations at TeV scales.

Partial compositeness can reconcile experimental bounds with LHC accessibility.

Abstract

We review flavour and CP violations in the lepton sector as probes of new physics beyond the standard model and its minimal extensions accommodating massive neutrinos. After recalling the main experimental bounds and the future perspectives, we summarize the limits on a set of dimension-six gauge-invariant operators, pointing to a scale of new physics much larger than the electroweak scale. If we insist on having new physics at the TeV scale, as demanded by most of the standard model extensions addressing the gauge hierarchy problem, we should rely on special mechanisms to deplete flavour and CP violations in the charged lepton sector. We comment on the capability of partial compositeness in reconciling the absence of signals with a scale of new physics accessible to the LHC.