The Present and Future Status of Heavy Neutral Leptons

TL;DR

This paper reviews the current experimental status of Heavy Neutral Leptons (HNLs), discusses their significance in understanding neutrino masses and fundamental physics, and outlines future experimental prospects and impacts.

Contribution

It provides a comprehensive overview of HNL research, summarizing recent experimental results and proposing future directions for searches and studies.

Findings

Current experiments have set bounds on HNL properties.

HNLs could explain neutrino masses and matter-antimatter asymmetry.

Future experiments may discover or further constrain HNLs.

Abstract

The existence of non-zero neutrino masses points to the likely existence of multiple SM neutral fermions. When such states are heavy enough that they cannot be produced in oscillations, they are referred to as Heavy Neutral Leptons (HNLs). In this white paper we discuss the present experimental status of HNLs including colliders, beta decay, accelerators, as well as astrophysical and cosmological impacts. We discuss the importance of continuing to search for HNLs, and its potential impact on our understanding on key fundamental questions, and additionally we outline the future prospects for next-generation future experiments or upcoming accelerator run scenarios.