Asymmetric self-interacting dark matter via Dirac leptogenesis

Abstract

The nature of neutrinos, whether Dirac or Majorana, is hitherto not known. Assuming that the neutrinos are Dirac, which needs B-L to be an exact symmetry, we make an attempt to explain the observed proportionality between the relic densities of dark matter (DM) and baryonic matter in the present Universe i.e., ΩDM≈5ΩB. We extend the Standard Model (SM) by introducing heavy scalar doublets Xi,i=1, 2 and η, two singlet scalars Φ and Φ′, a vectorlike Dirac fermion χ representing the DM and three right-handed neutrinos νRi,i =1, 2, 3. Assuming B-L is an exact symmetry of the early Universe, the CP-violating out-of-equilibrium decay of heavy scalar doublets; Xi,i=1, 2 to the SM lepton doublet Land the right-handed neutrino νR , generate equal and opposite B-L asymmetry among left (νL) and right (νR)-handed neutrinos. We ensure that νL-νR equilibration does not occur until below the electroweak (EW) phase transition during which a part of the lepton asymmetry gets converted to dark matter asymmetry through a dimension eight operator, which conserves B-L symmetry and remains in thermal equilibrium above sphaleron decoupling temperature. A part of the remaining B-L asymmetry then gets converted to a net B asymmetry through EW-sphalerons which are active at a temperature above 100 GeV. To alleviate the small-scale anomalies of ΛCDM, we assume the DM (χ) to be self-interacting via a light mediator Φ, which not only depletes the symmetric component of the DM, but also paves a way to detect the DM at terrestrial laboratories through Φ-H mixing, where H is the SM Higgs doublet.