Sub-GeV dark matter as pseudo-Goldstone from the seesaw scale
  Pseudo Nambu-Goldstone bosons (pNGBs) are naturally light spin-zero particles, which can be interesting dark matter (DM) candidates. We study the phenomenology of a pNGB theta associated with an approximate symmetry of the neutrino seesaw sector. A small coupling of theta to the Higgs boson is induced radiatively by the neutrino Yukawa couplings. By virtue of this Higgs portal interaction (i) the pNGB acquires a mass m_theta proportional to the electroweak scale, and (ii) the observed DM relic density can be generated by the freeze-in of theta-particles with mass m_theta ~ 3 MeV. Alternatively, the coupling of theta to heavy sterile neutrinos can account for the DM relic density, in the 1 keV-3 MeV window. The decays of theta into light fermions are suppressed by the seesaw scale, making such pNGB sufficiently stable to play the role of DM. 
Michele Frigerio, Thomas Hambye, Eduard Masso
hep-ph