Right-handed Dirac and Majorana neutrinos at Belle II

TL;DR

This paper evaluates Belle II's potential to determine whether a right-handed neutrino is Dirac or Majorana in the MeV to GeV mass range, using simulations of decay distributions and effective field theory models.

Contribution

It demonstrates Belle II's capability to distinguish neutrino nature and identify underlying operators through detailed angular distribution analysis and decay channel separation.

Findings

Belle II can discover right-handed neutrinos if kinematically accessible.

It can distinguish Dirac from Majorana neutrinos at over 5σ confidence for most operators.

Production and decay operators can be identified from decay distributions and event separation.

Abstract

We assess the ability of the Belle II experiment to probe the Dirac or Majorana nature of a massive right-handed neutrino (RHN) $N$ in the MeV to GeV mass range. We consider the production and decay of RHNs to proceed via new interactions described by the standard model effective field theory (SMEFT) extended with right-handed neutrino fields (SMNEFT), and not via mass mixing with active neutrinos. We find that Belle II has the potential to discover $N$ if kinematically accessible. We perform detailed simulations of the angular distributions of lepton pairs from the decay of $N$ produced in two-body and three-body decays of $B$ mesons. We show that for $m_N$ above 100 MeV, Belle II can distinguish between Dirac and Majorana neutrinos at more than the 5$\sigma$ CL for most operators, and the combination of the production and decay operators can be identified from the subsequent decay of the heavy neutrino. Also, the production operators can be identified using three-body $B$ meson decay for any $m_N$ if the $B\to D\ell N$ and $B\to D^*\ell N$ events can be well separated.