Exotic Sterile Neutrinos and Pseudo-Goldstone Phenomenology
  We study the phenomenology of a light (GeV scale) sterile neutrino sector and
the pseudo-Goldstone boson (not the majoron) associated with a global symmetry
in this sector that is broken at a high scale. Such scenarios can be motivated
from considerations of singlet fermions from a hidden sector coupling to active
neutrinos via heavy right-handed seesaw neutrinos, effectively giving rise to a
secondary, low-energy seesaw framework. Such scenarios allow for rich
phenomenology with observable implications for cosmology, dark matter, and
direct searches, involving novel sterile neutrino dark matter production
mechanisms from the pseudo-Goldstone-mediated scattering or decay,
modifications of BBN bounds on sterile neutrinos, suppression of canonical
sterile neutrino decay channels at direct search experiments, late injection of
an additional population of neutrinos in the Universe after neutrino
decoupling, and measurable dark radiation at BBN or CMB decoupling.
