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

Abstract

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, ${\ensuremath{\nu}}_{i}\ensuremath{\rightarrow}{\ensuremath{\nu}}_{j}+\ensuremath{\phi}$ and ${\ensuremath{\nu}}_{i}\ensuremath{\rightarrow}{\overline{\ensuremath{\nu}}}_{j}+\ensuremath{\phi}$, where the gravi-Majorons $\ensuremath{\phi}$ 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, ${\ensuremath{\tau}}_{2}/{m}_{2}>1.1\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}(6.7\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4})\text{ }\text{ }\mathrm{s}/\mathrm{eV}$ and ${\ensuremath{\tau}}_{3}/{m}_{3}>2.2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5}(1.3\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4})\text{ }\text{ }\mathrm{s}/\mathrm{eV}$ at 90% CL for hierarchical (degenerate) masses, using data from experiments searching for antineutrino appearance from the Sun.

Topics & Concepts