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.
Highlights
The existence of new, electromagnetically neutral particles is often a prediction of new-physics scenarios aimed at addressing some of the outstanding contemporary questions in particle physics, including the dark-matter and neutrinomass puzzles
The kinematics of three-body decays of Majorana fermions (MF) and Dirac fermions (DF) are in many cases, qualitatively different, as we explored in great detail in Ref. [60]
II, we review the kinematical properties of twobody and three-body decays of MF and DF
Summary
The existence of new, electromagnetically neutral particles is often a prediction of new-physics scenarios aimed at addressing some of the outstanding contemporary questions in particle physics, including the dark-matter and neutrinomass puzzles. Since HNLs participate in charged-current interactions, if they are DF, they can be assigned the same lepton number as the standard-model neutrinos Instead, if they are MF they will mediate lepton-number violating processes. We assume the experimental setup depicted in Fig. 1: an intense beam of ∼few GeV protons strikes a thick target, producing a large flux of mesons These are either captured or stopped in the target and decay at rest into charged-leptons and new fermions. The new fermions find their way to a gaseous argon time projection chamber and decay in such a way that the properties of their daughters, except for light neutrinos, are measured precisely It is either very difficult or outright impossible to establish, on an event-by-event basis, whether the new particle mediates lepton-number violating processes, for a couple of reasons. VI and detail the statistical methods we use in the Appendix
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