Degenerate fermion dark matter from a broken U (1) B-L gauge symmetry

Abstract

The extension of the Standard Model by assuming U(1)B-L gauge symmetry is very well motivated since it naturally explains the presence of heavy right-handed neutrinos required to account for the small active neutrino masses via the seesaw mechanism and thermal leptogenesis. Traditionally, we introduce three right-handed neutrinos to cancel the [U(1)B-L]3 anomaly. However, it suffices to introduce two heavy right-handed neutrinos for these purposes and therefore we can replace one right-handed neutrino by new chiral fermions to cancel the U(1)B-L gauge anomaly. Then, one of the chiral fermions can naturally play a role of a dark matter candidate. In this paper, we demonstrate how this framework produces a dark matter candidate which can address the so-called "core-cusp problem". As one of the small-scale problems that the Λ cold dark matter paradigm encounters, it may imply an important clue for the nature of dark matter. One of resolutions among many is hypothesizing that sub-keV fermion dark matter halos in dwarf spheroidal galaxies are in a (quasi) degenerate configuration. We show how the degenerate sub-keV fermion dark matter candidate can be nonthermally originated in our model and thus can be consistent with the Lyman-α forest observation. Thereby, the small neutrino mass, baryon asymmetry, and the sub-keV dark matter become consequences of the broken B-L gauge symmetry.