Characterizing Heavy Neutral Fermions via their Decays

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.

The present and future status of heavy neutral leptons

“…Reference [602] demonstrated that for large parts of parameter space, DUNE ND-GAr (thanks to the magnetization of the detector) cannot only discover a new particle N but also determine, via its decay rates, whether it is a Dirac or Majorana fermion. If N decays in a partially-invisible channel, then decay kinematics can be used to perform this separation [125,[635][636][637].…”

Dark Sector Studies with Neutrino Beams

“…[175] demonstrated that for large parts of parameter space, DUNE can not only discover a new particle N but also determine, via its decay rates, whether it is a Dirac or Majorana fermion. If N decays in a partially-invisible channel, then decay kinematics can be used to perform this separation [244][245][246][247].…”

Neutrino Portals, Terrestrial Upscattering, and Atmospheric Neutrinos

“…where d α is the (flavor dependent) transition dipole moment between ν α and the singlet fermion N . In complete generality one could consider a linear combination of magnetic and electric transition dipole portals (see [55] for a recent discussion and [56,57] for related work in the context of angular distributions in HNL decays). It suffices, however, to consider only the magnetic dipole portal as a simplified model in the majority of parameter space, and we restrict our attention to this case here.…”