TeV Scale See-Saw Mechanisms of Neutrino Mass Generation, the Majorana Nature of the Heavy Singlet Neutrinos and $\betabeta$-Decay

TL;DR

This paper discusses the challenges in observing the Majorana nature of TeV-scale heavy neutrinos in current and future experiments, highlighting constraints on their couplings and implications for neutrino mass mechanisms.

Contribution

It analyzes the detectability of Majorana heavy neutrinos in TeV-scale see-saw models and explores conditions under which their Majorana nature could be observed or inferred.

Findings

Majorana heavy neutrinos at TeV scale are unlikely observable in current experiments.

If observed as Majorana, they likely have additional strong couplings or different mass generation mechanisms.

Heavy neutrinos behave as pseudo-Dirac particles in typical see-saw scenarios.

Abstract

It is shown that the Majorana nature of the heavy neutrinos $N_j$ having masses in the range of $M_j \sim (100 - 1000)$ GeV and present in the TeV scale type I and inverse see-saw scenarios of neutrino mass generation, is unlikely to be observable in the currently operating and future planned accelerator experiments (including LHC) due to the existence of very strong constraints on the parameters and couplings responsible for the corresponding $|\Delta L| = 2$ processes, $L$ being the total lepton charge. If the heavy Majorana neutrinos $N_j$ are observed and they are associated only with the type I or inverse see-saw mechanisms and no additional TeV scale "new physics", they will behave like Dirac fermions to a relatively high level of precision, being actually pseudo-Dirac particles. The observation of effects proving the Majorana nature of $N_j$ would imply that these heavy neutrinos have additional relatively strong couplings to the Standard Model particles (as, e.g. in the type III see-saw scenario), or that light neutrino masses compatible with the observations are generated by a mechanism other than see-saw (e.g., radiatively at one or two loop level) in which the heavy Majorana neutrinos $N_j$ are nevertheless involved.