Probing the heavy neutrino hypothesis

TL;DR

This paper reviews the current experimental efforts and constraints on heavy sterile neutrinos above the GeV scale, highlighting collider results, flavor analyses, and the importance of various search channels.

Contribution

It provides a comprehensive overview of collider and precision constraints on heavy neutrinos, emphasizing the role of the intensity frontier in probing masses above the electroweak scale.

Findings

LHC surpasses LEP in constraining electron and muon mixings

Tau mixing remains difficult to probe at hadron colliders

Electroweak precision and flavor observables set strong constraints for heavy neutrinos above the electroweak scale

Abstract

There is a strong experimental program searching for massive sterile neutrinos. Here we focus on the heavy regime above the GeV scale, where their existence can be probed in either high-energy colliders or in high-precision facilities. We first review the current experimental status at colliders, showing that the LHC already improves LEP results for mixings to electrons and muons, while mixings to taus remain challenging at a hadron collider. We also discuss the importance of exploring both lepton number violating and conserving signals, as well as different flavor channels. Finally, we present the latest global analysis of electroweak precision and flavor observables, showing that the intensity frontier provides the strongest constraints for heavy neutrino masses above the electroweak scale.