New Dark Matter Candidate: Nonthermal Sterile Neutrinos

Abstract

We propose a new and unique dark matter candidate: $\ensuremath{\sim}100$ eV to $\ensuremath{\sim}10$ keV sterile neutrinos produced via lepton-number-driven resonant Mikheyev-Smirnov-Wolfenstein conversion of active neutrinos. The requisite lepton number asymmetries in any of the active neutrino flavors range from ${10}^{\ensuremath{-}3}$ to ${10}^{\ensuremath{-}1}$ of the photon number. The unique feature here is that the adiabaticity condition of the resonance strongly favors the production of lower energy sterile neutrinos. The resulting nonthermal (cold) energy spectrum can cause these sterile neutrinos to revert to nonrelativistic kinematics at an early epoch, so that free-streaming lengths at or below the dwarf galaxy scale are possible. Therefore, the main problem associated with light neutrino dark matter can be circumvented in our model.