Distinguishing between Dirac and Majorana neutrinos at FASER

Abstract

Some of the simplest models for the origin of neutrino mass involve right-handed neutrinos (RHNs), which could be either Dirac or Majorana particles - a distinction that has profound implications for lepton number conservation and the fundamental nature of neutrinos. We investigate the potential of the FASER experiment to distinguish between these two possibilities using signatures predicted by the Standard Model Neutrino Effective Field Theory (SMNEFT), where RHNs interact with Standard Model particles through higher-dimensional operators. We focus on RHNs produced via $B$, $D$, $K$, and $\pi$ meson decays at the Large Hadron Collider and their subsequent three-body decays within the FASER detector. The kinematic and angular distributions of the decay products in the RHN rest frame differ significantly for Dirac and Majorana RHNs, and these differences manifest as distinct spatial distributions of electron-positron pairs at FASER. Using Monte Carlo simulations and a $\chi^2$ analysis, we demonstrate that these spatial observables provide a robust experimental probe for determining the Dirac or Majorana nature of RHNs. For select production and decay operator combinations and RHN masses around 0.1 GeV, FASER can achieve discrimination at the $3\sigma$ level.