Addressing the Majorana vs. Dirac Question with Neutrino Decays

TL;DR

This paper proposes a method to distinguish Majorana from Dirac neutrinos by analyzing decay angular distributions of heavy neutrinos, exploiting their different behaviors when non-relativistic, which could be tested in future collider and neutrino experiments.

Contribution

It introduces a novel approach based on decay angular distributions to determine the Majorana or Dirac nature of neutrinos, leveraging their non-relativistic behavior and CPT invariance.

Findings

Angular distribution is isotropic for Majorana neutrinos in decay

Distribution is anisotropic for Dirac neutrinos, in general

Feasibility of experimental detection at colliders and neutrino facilities

Abstract

The Majorana versus Dirac nature of neutrinos remains an open question. This is due, in part, to the fact that virtually all the experimentally accessible neutrinos are ultra-relativistic. Noting that Majorana neutrinos can behave quite differently from Dirac ones when they are non-relativistic, we show that, at leading order, the angular distribution of the daughters in the decay of a heavy neutrino into a lighter one and a self-conjugate boson is isotropic in the parent's rest frame if the neutrinos are Majorana, independent of the parent's polarization. If the neutrinos are Dirac fermions, this is, in general, not the case. This result follows from CPT invariance and is independent of the details of the physics responsible for the decay. We explore the feasibility of using these angular distributions -- or, equivalently, the energy distributions of the daughters in the laboratory frame -- in order to address the Majorana versus Dirac nature of neutrinos if a fourth, heavier neutrino mass eigenstate reveals itself in the current or next-generation of high-energy colliders, intense meson facilities, or neutrino beam experiments.