Learning about flavour structure from tau to ell gamma and mu to e gamma?

TL;DR

This paper investigates the flavor structure implications of observing tau to e or mu gamma decays, analyzing how new flavoured couplings and hierarchical structures affect experimental observability and underlying models.

Contribution

It demonstrates that in simple models, the dipole vertex function relates to flavoured matrices, and explores how hierarchical structures can limit observable decay modes.

Findings

Dipole vertex function proportional to flavoured matrix products

Hierarchical structure can restrict observable decay modes

Potential test of flavor hierarchy assumptions

Abstract

Current and upcoming experiments should improve the sensitivity to radiative lepton decays by an order of magnitude. This paper assumes that one of the tau --> (e, mu) gamma decays is observed, and explores the structure and consequences of the required new flavoured couplings. In simple models (a low-scale seesaw, leptoquarks) it is shown that the dipole vertex function is proportional to a product of flavoured matrices from the Lagrangian (a "Jarlskog-like" invariant), provided that the loop particles are weakly coupled to the Higgs. Secondly, if the dipole vertex function has a hierarchical structure, this can imply that only some of the tau --> (e, mu) gamma modes can be observed, due to the "approximate zero" implied by the bound on mu --> e gamma. The assumptions underlying this potential test of a hierarchical structure are discussed.