A Common Origin of Asymmetric Self-interacting Dark Matter and Dirac Leptogenesis

TL;DR

The paper presents a unified framework in which Dirac neutrinos, a baryon asymmetry, and self-interacting asymmetric dark matter share a common origin: CP-violating decays of a heavy scalar η generate equal and opposite $B-L$ asymmetries in the visible and dark sectors, with sphalerons converting part of the ν_L asymmetry into the observed BAU $η_B ≈ 6.2 imes10^{-10}$. The dark sector contains a vector-like doublet ψ and singlet χ, with a MeV-scale Z' mediator under U(1)_D that drives DM self-interactions and annihilations of the symmetric component, leaving a predominantly asymmetric relic with $M_χ o ext{GeV}$ scale. The Dirac mass of neutrinos is protected by gauged $U(1)_{B-L}$, and a soft breaking term μ^2 η† H generates the required Dirac masses; kinetic mixing ε between Z' and the SM Z allows direct detection, while the Z' is too feebly coupled to yield observable collider or indirect signals. A distinctive collider signature is a collider-stable dark doublet ψ, producing HSCP tracks and MET, together with astrophysical SIDM observations that can corroborate the MeV mediator scenario.

Abstract

Assuming dark matter to be asymmetric as well as self-interacting and neutrinos to be Dirac fermions, we propose a framework to address the observed baryon imbalance of the universe. We add three right-handed neutrinos $ν_{R_i},\,{i=1,2,3}$, one singlet fermion $χ$, a doublet fermion $ψ$, and heavy scalar doublets $η_i,\,{i=1,2}$ to the Standard Model. A global $B-L$ is imposed to protect the Dirac nature of neutrinos. Both $χ$ and $ψ$ are fermions with non-zero charge under an extended $U(1)_{B-L} \times U(1)_D$ symmetry. Additionally, a $\mathcal{Z}_2$ symmetry is imposed, where the singlets $χ$, $ν_R$, and $η$ are negative and the doublet $ψ$ is positive. The CP-violating out-of-equilibrium decay of heavy scalar $η$ generates an equal and opposite $B-L$ asymmetry among the left-handed ($ν_L$) and right-handed ($ν_R$) neutrinos. The $ν_L-ν_R$ equilibration process does not take place until below the Electroweak phase transition scale because of tiny Yukawa couplings. During this time, Sphaleron processes, which are active at temperatures higher than 100 GeV, transform a portion of the $B-L$ asymmetry stored in left-handed neutrinos into baryon asymmetry. MeV scale gauge boson $Z'$ of $U(1)_D$ sector mediates both annihilation of symmetric dark matter component and self-interaction among dark matter particles. Moreover, $Z'$ mixes with the Standard Model Z-boson and provides a portal for dark matter direct detection.

Figures (15)

• Figure 1: Schematic of the model: A heavy scalar $\eta$ decays into visible ($l\nu_R$) and dark sectors ($\psi \chi$), where the dark sector includes the particles charged under $U(1)_D$ symmetry. The lightest dark sector particle $\chi$ is the DM candidate.
• Figure 2: Feynman diagram responsible for generation of Dirac neutrino mass.
• Figure 3: The interference of Tree level and Self-energy correction diagrams that contribute to the CP violation.
• Figure 4: The lepton asymmetry from $\xi_1$ decay. The Blue line shows the evolution of lepton asymmetry for $\epsilon_{L} = 4.5 \times 10^{-7}$. The Red line shows the evolution of $\xi_1$ particles. The Green(dotted) line shows the equilibrium number density of $\xi_1$.
• Figure 5: Feynman diagram responsible for bringing DM into thermal equilibrium.