부산대학교 도서관

We discuss numerous mechanisms for production of sterile neutrinos, which canaccount for all or a fraction of dark matter, and which can range from warm toeffectively cold dark matter, depending on the cosmological scenario. Weinvestigate production by Higgs boson decay, $(B-L)$ gauge boson production athigh temperature, as well as production via resonant and nonresonant neutrinooscillations. We calculate the effects on structure formation in these models,some for the first time. If two populations of sterile neutrinos, one warm andone cold, were produced by different mechanisms, or if sterile neutrinosaccount for only a fraction of dark matter, while the remainder is some othercold dark matter particle, the resulting multi-component dark matter mayalleviate some problems in galaxy formation. We examine the X-ray constraintsand the candidate signal at 3.5 keV. Finally, we also show that the $\sigma_8$problem can be a signature of fractional dark matter in the form of sterileneutrinos in several mechanisms.