Characterizing Heavy Neutral Fermions via their Decays

TL;DR

This paper explores how analyzing three-body decays of Heavy Neutral Leptons can reveal whether they are Majorana or Dirac fermions and distinguish their underlying physics models.

Contribution

It introduces a method to determine the nature of heavy neutral fermions through decay analysis and compares different experimental reconstruction approaches.

Findings

Three-body decay measurements can identify Majorana vs. Dirac nature.

Full and partial reconstruction methods have different sensitivities.

Proposed experimental analyses enhance fermion characterization capabilities.

Abstract

Many extensions of the Standard Model of particle physics contain new electrically-neutral fermions. Should one of these particles be discovered, questions will naturally arise regarding its nature. For instance: is it a self-conjugate particle (i.e., is it a Dirac or a Majorana fermion)?, does it interact via the Standard Model force carriers or something else? One set of well-motivated particles in this class are Heavy Neutral Leptons (HNLs), Standard Model gauge-singlet fermions that mix with the neutrinos and may be produced in meson decays. We demonstrate that measuring the three body decays of the HNL (or phenomenologically similar heavy fermions) can help determine whether they are Majorana or Dirac fermions. We also investigate the ability to distinguish among different models for the physics responsible for the HNL decay. We compare the reach assuming full and partial event reconstruction, and propose experimental analyses. Should a new fermion be discovered, studying its three body decays provides a powerful diagnostic tool of its nature.