Experimental Searches For Heavy Neutral Leptons

TL;DR

This paper reviews experimental efforts to search for Heavy Neutral Leptons (HNLs), which are hypothesized particles that could explain neutrino masses, dark matter, and matter-antimatter asymmetry, covering a wide mass range and various experimental approaches.

Contribution

It provides a comprehensive overview of experimental searches for HNLs across different experiments and discusses the implications for neutrino physics and beyond Standard Model theories.

Findings

Current experiments constrain HNL mixing parameters.

HNL mass range explored spans from eV to GUT scale.

Experimental results limit certain theoretical models.

Abstract

The highly successful Standard Model is not complete. It does not explain the baryonic asymmetry in the Universe, the existence of dark matter or the non-zero masses of the neutrinos. Extensions of the Standard Model that propose the existence of additional Heavy Neutral Leptons (HNLs) are well-motivated and can explain several of these phenomena. In addition, light sterile neutrinos of $\mathcal{O}(\text{eV}/c^{2})$ can explain experimentally observed oscillation anomalies. The Neutrino Minimal Standard Model proposes HNLs with masses $\mathcal{O}(\text{keV}/c^{2} - \text{GeV}/c^{2})$, while more exotic models predict very large masses, up to the GUT scale. Due to the multitude of models which hypothesize HNLs, the mass range to be explored by experiments is large. Experimental searches for HNLs can be conducted at existing neutrino, beam dump and collider-based experiments, and, depending on the signature, can constrain mixing between additional neutrinos and any of the three active neutrinos.