Non-relativistic neutrinos and the question of Dirac vs. Majorana neutrino nature

Abstract

Finding out if neutrinos are Dirac or Majorana particles is known to be extremely difficult. This is generally believed to be due to the smallness of the neutrino mass compared to typical neutrino energies, and to the fact that in the limit of vanishing mass all distinctions between Dirac and Majorana neutrinos disappear. This, however, does not necessarily mean that for non-relativistic neutrinos distinguishing between them is an easy task. We consider this problem in detail. The issues discussed include the possibilities of studying neutrino nature with non-relativistic neutrinos produced in $β$ decay in direct neutrino mass measurement experiments, with relic neutrinos, as well as with slow neutrinos in pair production processes and in scattering, both elastic and inelastic. We also discuss processes allowed for only one of the two neutrino types as a means of uncovering neutrino nature.