Sterile Neutrinos from Dark Matter: A ν Nightmare?

We provide a comprehensive study of observable spectra from dark matter pair-
annihilation or decay into sterile (right-handed) neutrinos. This occurs, for instance, in
neutrino portal dark matter models, where a sterile neutrino acts as the portal between dark
matter and the Standard Model sector. The subsequent decays of right-handed neutrinos
produce detectable Standard Model particles, notably photons, positrons, and neutrinos.
We study the phenomenology of models where the right-handed neutrino masses are below
the GeV scale, as well as models where they are at, or significantly heavier than, the TeV
scale. In both instances, and for different reasons, the standard tools, including Monte Carlo
simulations, are both inadequate and inaccurate. We present the complete framework to
compute the relevant branching ratios for right-handed neutrino decays and the spectra of
secondary photons, positrons, and neutrinos for a broad range of dark matter and right-handed
neutrino masses. We discuss the general features of such signals, and compare the spectra to
standard signals from dark matter annihilation/decay into bottom quarks. Additionally, we
provide open source code1 that can be used to compute such spectra. The code is available at https://github.com/LoganAMorrison/blackthorn.