Multi-Lepton Collider Signatures of Heavy Dirac and Majorana Neutrinos

TL;DR

This paper explores how multi-lepton signals at the LHC can reveal the nature of heavy neutrinos, distinguishing between Dirac and Majorana types, and provides a method to probe seesaw mechanisms at TeV energies.

Contribution

It introduces a novel approach to identify heavy neutrino nature and seesaw type through specific multi-lepton signals with minimal background at the LHC.

Findings

Majorana neutrinos produce same-sign di-lepton signals.

Dirac neutrinos lead to tri-lepton signals conserving lepton number.

Signals can be detected with little background beyond 1 TeV.

Abstract

We discuss the possibility of observing multi-lepton signals at the Large Hadron Collider (LHC) from the production and decay of heavy Standard Model (SM) singlet neutrinos added in extensions of SM to explain the observed light neutrino masses by seesaw mechanism. In particular, we analyze two `smoking gun' signals depending on the Dirac or Majorana nature of the heavy neutrino: (i) for Majorana case, the same-sign di-lepton signal which can be used as a probe of lepton-number violation, and (ii) for Dirac case, the tri-lepton signal which conserves lepton number but may violate lepton flavor. Within a minimal Left-Right symmetric framework in which these additional neutrino states arise naturally, we find that in both cases, the signals can be identified with virtually no background beyond a TeV, and the heavy gauge boson W_R can be discovered in this process. This analysis also provides a direct way to probe the nature of seesaw physics involving the SM singlets at TeV scale, and in particular, to distinguish type-I seesaw with purely Majorana heavy neutrinos from inverse seesaw with pseudo-Dirac counterparts.