Self-interacting sterile neutrino dark matter: the heavy-mediator case
  For active-sterile mixing to be responsible for the full relic abundance of
dark matter additional new physics is needed beyond the keV-scale sterile
neutrino itself. The extra ingredient we consider here is the presence of
self-interactions among the sterile neutrinos. We examine whether
active-to-sterile conversion is amplified enough in this scenario that the
observed abundance of dark matter can be obtained with a subconstraint mixing
angle. This turns out never to be the case in the region we explore: either
self-interactions have too small an impact and cannot escape bounds on the mass
and mixing angle, or they have too great an impact and cause dark matter to be
overproduced. The sharp transition from marginal to excessive effectiveness
occurs because a resonance criterion is met in the effective in-medium mixing
angle. Once the system goes resonant the game is as good as over, as
nonlinearity in the Boltzmann equation leads to runaway production of sterile
neutrinos, beginning at a plasma temperature of a few hundred MeV and typically
ending at a few tens of MeV. The scenario is therefore ruled out largely by its
own dynamics. In this study we focus exclusively on mediators heavier than
$\sim 1$ GeV; future work will extend the analysis to lighter mediators,
allowing for contact to be made with the kinds of scenarios motivated by issues
of small-scale structure.
