Distinguishing models with μ → e observables

Abstract

Upcoming experiments will improve the reach for the lepton flavour violating (LFV) processes μ → eγ, μ → $$ e\overline{e}e $$ e e ¯ e and μA → eA by orders of magnitude. We investigate whether this upcoming data could rule out some popular TeV-scale LFV models (the type II seesaw, the inverse seesaw and a scalar leptoquark) using a bottom-up EFT approach. We take the data to be the twelve Wilson coefficients that experiments can constrain and in principle determine independently. In this 12-dimensional coefficient space, each model can only predict points in a specific subspace; for instance, flavour change involving singlet electrons is suppressed in the seesaw models, and the leptoquark induces negligible coefficients for 4-lepton scalar operators. Using the fact that none of these models can populate the whole region accessible to upcoming experiments, we show that μ → e experiments have the ability to rule them out.

Theory introduction to Lepton Flavour Violation

“…If we find parts of parameter space that models cannot reach, an observation in those regions would rule them out. This approach has been followed in [26,27]. These works demonstrate that upcoming 𝜇 → 𝑒 searches have the potential to distinguish among some popular BSM models.…”

One-loop matching of the type-III seesaw model onto the Standard Model Effective Field Theory

“…To fully answer this question, one must perform a global-fit analysis of all existing experimental measurements in the framework of the SEFTs and make a model comparison. In the previous literature [46,[70][71][72][73], numerous efforts have been made to distinguish among different types of seesaw models from the EFT perspective, utilizing low-energy observables in the leptonic sector. In this subsection, we just outline a preliminary strategy to look for the answer via non-leptonic observables.…”