Collider Searches for Heavy Neutral Leptons: beyond simplified scenarios

TL;DR

This paper revisits LHC searches for heavy neutral leptons (HNLs), emphasizing the need to reinterpret bounds in models with multiple HNLs and complex flavor mixing, including interference effects.

Contribution

It provides a framework for recasting experimental bounds on HNLs considering multiple states and flavor mixing, beyond simplified single-HNL scenarios.

Findings

Recasting bounds for multiple HNLs with complex mixing patterns.

Inclusion of interference effects in the reinterpretation process.

Guidelines for applying bounds to generic models with several HNLs.

Abstract

With very few exceptions, the large amount of available experimental bounds on heavy neutral leptons - HNL - have been derived relying on the assumption of the existence of a single (usually Majorana) sterile fermion state that mixes with only one lepton flavour. However, most of the extensions of the Standard Model involving sterile fermions predict the existence of several HNLs, with complex mixing patterns to all flavours. Consequently, most of the experimental bounds for HNLs need to be recast before being applied to a generic scenario. In this work, we focus on LHC searches of heavy neutral leptons and discuss how to reinterpret the available bounds when it comes to consider mixings to all active flavours, not only in the case with a single HNL, but also in the case when more heavy neutral leptons are involved. In the latter case, we also consider the possibility of interference effects and show how the bounds on the parameter space should be recast.