General Conditions for Lepton Flavour Violation at Tree- and 1-Loop Level

TL;DR

This paper establishes comprehensive conditions for lepton flavour conservation at tree and 1-loop levels, providing a framework to identify models that suppress lepton flavour violation effectively.

Contribution

It introduces necessary and sufficient criteria for lepton flavour conservation applicable to various beyond Standard Model theories, including GIM suppression mechanisms.

Findings

Derived minimal conditions for lepton flavour conservation.

Identified how GIM suppression reduces lepton flavour violation.

Applied criteria to models like MSSM and LR-symmetric models.

Abstract

In this work, we compile the necessary and sufficient conditions a theory has to fulfill in order to ensure general lepton flavour conservation, in the spirit of the Glashow-Weinberg criteria for the absence of flavour-changing neutral currents. At tree-level, interactions involving electrically neutral and doubly charged bosons are investigated. We also investigate flavour changes at 1-loop level. In all cases we find that the essential theoretical requirements can be reduced to a few basic conditions on the particle content and the coupling matrices. For 1-loop diagrams, we also investigate how exactly a GIM-suppression can occur that will strongly reduce the rates of lepton flavour violating effects even if they are in principle present in a certain theory. In all chapters, we apply our criteria to several models which can in general induce lepton flavour violation, e.g. LR-symmetric models or the MSSM. In the end we give a summarizing table of the obtained results, thereby demonstrating the applicability of our criteria to a large class of models beyond the Standard Model.