Thermalizing Sterile Neutrino Dark Matter

Dark Population Transfer Mechanism for Sterile Neutrino Dark Matter

We present a mechanism for producing a cosmologically significant relic density of one or more sterile neutrinos. This scheme invokes two steps: First, a population of "heavy" sterile neutrinos is created by scattering-induced decoherence of active neutrinos. Second, this population is transferred, via sterile neutrino self-interaction-mediated scatterings and decays, to one or more lighter mass (∼10  keV to ∼1  GeV) sterile neutrinos that are far more weakly (or not at all) mixed with active species and could constitute dark matter. Dark matter produced this way can evade current electromagnetic and structure-based bounds, but may nevertheless be probed by future observations.

Dark Radiation from Neutrino Mixing after Big Bang Nucleosynthesis

Light dark fermions can mass mix with the standard model (SM) neutrinos. As a result, through oscillations and scattering, they can equilibrate in the early universe. Interactions of the dark fermion generically suppress such production at high temperatures but enhance it at later times. We find that for a wide range of mixing angles and interaction strengths equilibration with SM neutrinos occurs at temperatures near the dark fermion mass. For masses below an MeV, this naturally occurs after nucleosynthesis and opens the door to a variety of dark sector dynamics with observable imprints on the CMB and large scale structure, and with potential relevance to the tensions in H_{0} and S_{8}.

Intimate Relationship between Sterile Neutrino Dark Matter and ΔN_{eff}

The self-interacting neutrino hypothesis is well motivated for addressing the tension between the origin of sterile neutrino dark matter and indirect detection constraints. It can also result in a number of testable signals from the laboratories to the cosmos. We show that, in a broad class of models, where the sterile neutrino dark matter relic density is generated by a light neutrinophilic mediator, there must be a lower bound on the amount of extra radiation in early Universe, in particular, ΔN_{eff}>0.12 at the cosmic microwave background (CMB) epoch. This lower bound will be further strengthened with an improved x-ray search at the Athena observatory. Such an intimate relationship will be unambiguously tested by the upcoming CMB Stage 4 project.

Dodelson-Widrow Mechanism in the Presence of Self-Interacting Neutrinos

keV-scale gauge-singlet fermions, when allowed to mix with the active neutrinos, are elegant dark matter (DM) candidates. They are produced in the early Universe via the Dodelson-Widrow mechanism and can be detected as they decay very slowly, emitting x-rays. In the absence of new physics, this hypothesis is virtually ruled out by astrophysical observations. Here, we show that new interactions among the active neutrinos allow these sterile neutrinos to make up all the DM while safely evading all current experimental bounds. The existence of these new neutrino interactions may manifest itself in next-generation experiments, including DUNE.

Scalar singlet dark matter in non-standard cosmologies

We study production of dark matter (DM) in models with a non-standard expansion history. We consider both freeze-out and freeze-in mechanisms for producing the observed DM abundance in a model where the DM consists of scalar singlet particles coupled to the Standard Model sector via the Higgs portal. We show that a non-standard expansion phase can lead to a significant change in the DM abundance and therefore to observational ramifications. For example, for DM freeze-in the required portal coupling can be much larger, whereas for DM freeze-out much smaller values become allowed. We evaluate the relevant constraints and discuss prospects for direct detection of such DM.

X-Ray Lines from Dark Matter Annihilation at the keV Scale

In 2014, several groups reported hints for a yet unidentified line in astrophysical x-ray signals from galaxies and galaxy clusters at an energy of 3.5 keV. While it is not unlikely that this line is simply a reflection of imperfectly modeled atomic transitions, it has renewed the community's interest in models of keV-scale dark matter, whose decay would lead to such a line. The alternative possibility of dark matter annihilation into monochromatic photons is far less explored, a lapse that we strive to amend in this Letter. More precisely, we introduce a novel model of fermionic dark matter χ with O(keV) mass, annihilating to a scalar state ϕ which in turn decays to photons, for instance via loops of heavy vectorlike fermions. The resulting photon spectrum is box shaped, but if χ and ϕ are nearly degenerate in mass, it can also resemble a narrow line. We discuss dark matter production via two different mechanisms-misalignment and freeze-in-which both turn out to be viable in vast regions of parameter space. We constrain the model using astrophysical x-ray data, and we demonstrate that, thanks to the velocity dependence of the annihilation cross section, it has the potential to reconcile the various observations of the 3.5 keV line. We finally argue that the model can easily avoid structure formation constraints on keV-scale dark matter.