Distinguishing Dirac and Majorana neutrinos by their decays via Nambu-Goldstone bosons in the gravitational-anomaly model of neutrino masses

Neutrinos may acquire small Dirac or Majorana masses by new low-energy physics in terms of the chiral gravitational anomaly, as proposed by Dvali and Funcke (2016). This model predicts fast neutrino decays, νᵢ→νⱼ+φ and νᵢ→νⱼ+φ, where the gravi-Majorons φ are pseudoscalar Nambu-Goldstone bosons. The final-state neutrino and antineutrino distributions differ depending on the Dirac or Majorana mass of the initial state. This opens a channel for distinguishing these cases, for example in the spectrum of high-energy astrophysical neutrinos. In particular, we put bounds on the neutrino lifetimes in the Majorana case, τ₂/m₂>1.1×10^-3(6.7×10^-4) s/eV and τ₃/m₃>2.2×10^-5(1.3×10^-4) s/eV at 90% CL for hierarchical (degenerate) masses, using data from experiments searching for antineutrino appearance from the Sun.