Shining light on the mass scale and nature of neutrinos with eγ→eνν¯

Abstract

The discovery of neutrino oscillations invites many fundamental physics questions that have yet to be answered. Two of these questions are simple, easy to state, and essential: What are the values of the neutrino masses? Are neutrinos Majorana fermions? The reason we don't know the answer to those questions is that it is difficult to measure neutrino properties outside of the ultrarelativistic regime. We discuss the physics of eγ → eνν near threshold, where one has access to nonrelativistic neutrinos and only nonrelativistic neutrinos. Near threshold, eγ → eνν is a rich phenomenon and its cross section is sensitive to the individual values of the neutrino masses and the nature of the neutrinos. We show that if one could scan the threshold region, it would be simple to identify the mass of the lightest neutrino, the neutrino mass ordering, and whether the neutrinos are Majorana fermions. In practice, however, event rates are tiny and backgrounds are huge; the observation of eγ → eνν in the sub-eV regime appears to be utterly inaccessible in the laboratory. Our results, nonetheless, effectively illustrate the discriminatory power of nonrelativistic neutrino observables.

Neutrinoless Double-Beta Decay: Status and Prospects

“…(19) is a recent summary. The observation of these modes typically require either non-relativistic neutrinos (65,66,67) or new interactions (68,69). For heavier neutrinos, the effects are observable only in certain mass ranges, such as in meson or W decays (70).…”

Dirac neutrinos and dark matter stability from lepton quarticity

“…Indeed, the 'Dirac-Majorana confusion theorem' [97][98][99] states that the difference between Dirac and Majorana neutrinos must vanish in the massless limit. Still, some works have tried to find alternative ways to distinguish the neutrino nature, see [100][101][102][103][104] and the references within. As of now the nature of neutrinos remains as mysterious as the mechanism responsible for generating their small masses.…”

Dirac neutrinos in the 2HDM with restrictive Abelian symmetries

“…The last relation simply indicates that 4 3 = P 13 4 3 P 23 in the limit a 3 → 0 and, thus, the two pairs would be equivalent in this case. 6 Obviously, a 3 = 0 would lead to a massless charged lepton, which is not acceptable. However, it turns out that a 3 is very small and, indeed, 4 3 P 13 4 3 P 23 , leading to a similar U for both pairs.…”

“…Unfortunately, this question cannot be addressed by neutrino oscillation experiments and the experimental data presently available is compatible with the existence of either Dirac or Majorana massive neutrinos. Establishing the nature of neutrinos has proved very challenging and, among the several proposals to solve this riddle [1][2][3][4][5][6][7], the most promising one seems to rely on neutrinoless double beta decays [8]. Meantime, in the absence of a solid evidence in favor (or against) the existence of Dirac and/or Majorana neutrinos, both scenarios should be equally considered.…”