Sterile Neutrinos As Dark Matter PDF Download

We focus on two dark matter candidates: sterile neutrinos and primordial black holes (PBH). We explore the effects of non-standard pre-Big Bang Nucleosynthesis (pre-BBN) cosmologies, such scalar-tensor and kination cosmologies, on the abundance of sterile neutrinos over a large range of masses. In particular, sterile neutrinos of keV-scale mass represent a viable warm dark matter candidate whose decay can generate the putative 3.5 keV X-ray signal observed in galaxy and galaxy clusters. eV-scale sterile neutrinos can be the source of various accelerator/beam neutrino oscillation anomalies. Two production mechanisms are considered here, a collisional non-resonant Dodelson-Widrow (DW) mechanism and a resonant Shi-Fuller (SF) conversion (which requires a large lepton asymmetry). The DW mechanism is a freeze-in process, and the final abundance of sterile neutrinos using this production method is inversely proportional to the Hubble expansion rate. We find that in one of the scalar tensor models we consider, the sterile neutrino parameters necessary to generate the tentative 3.5 keV signal would be within reach of the TRISTAN upgrade to the ongoing KATRIN experiment as well as the planned upgrades to the HUNTER experiment, however the contribution to the dark matter density would be very small. In another scalar tensor model, sterile neutrinos could both generate the X-ray signal and comprise much of dark matter. In our study of resonant production, we find that the parameter space in which coherent and adiabatic resonant production can occur shifts with changing pre-BBN cosmology. We find that for a broad range of parameters (mass, mixing angle, lepton asymmetry), resonance can occur in the LSND/MiniBooNE and DANS/NEOSS experiments' preferred regions for at least one of the non-standard cosmologies we consider. With respect to PBH as dark matter candidates, we derive a new type of cosmology-independent bound. We consider the heating of the surrounding interstellar medium gas by dynamical friction and from the formation of accretion disks around intermediate mass $10-10^{5} M_\odot$ PBH. By estimating the cooling rate and assuming thermal equilibrium, we derive a new constraint. Light PBH with mass $10^{15}-10^{17}$ g emit significant Hawking radiation and are constrained by the same cooling argument. We extend this analysis to PBH with extreme spin, which results in stronger bounds compared to non-spinning PBH.