Can one ever prove that neutrinos are Dirac particles?

TL;DR

The paper discusses conditions under which the nature of neutrinos (Dirac or Majorana) can be inferred, proposing that a combination of null neutrinoless double beta decay and observed quadruple beta decay signals implies neutrinos are Dirac particles.

Contribution

It introduces a novel argument that the simultaneous null $0 u 2 eta$ decay and presence of $0 u 4 eta$ decay signals indicate neutrinos are Dirac particles, independent of decay mechanisms.

Findings

Null $0 u 2 eta$ decay does not confirm neutrinos are Majorana.

Detection of $0 u 4 eta$ decay alongside null $0 u 2 eta$ decay suggests neutrinos are Dirac.

The argument is process-independent, relying on decay signals.

Abstract

According to the "Black Box" theorem the experimental confirmation of neutrinoless double beta decay ($0 \nu 2 \beta$) would imply that at least one of the neutrinos is a Majorana particle. However, a null $0 \nu 2 \beta$ signal cannot decide the nature of neutrinos, as it can be suppressed even for Majorana neutrinos. In this letter we argue that if the null $0 \nu 2 \beta$ decay signal is accompanied by a $0 \nu 4 \beta$ quadruple beta decay signal, then at least one neutrino should be a Dirac particle. This argument holds irrespective of the underlying processes leading to such decays.