Originally Asymmetric Dark Matter

TL;DR

This paper introduces a novel asymmetric dark matter scenario where a fermion transitions from a Dirac to a Majorana particle after relic density is set, with models predicting detectable signals in future experiments.

Contribution

It proposes two simple models for asymmetric dark matter that undergo a late-time phase transition to become Majorana fermions, with testable predictions for direct detection.

Findings

Potential for near-future detection via direct and indirect searches.

Models predict sizable elastic scattering cross sections.

Dark matter relic density linked to leptogenesis-like asymmetry.

Abstract

We propose a scenario with a fermion dark matter, where the dark matter particle used to be the Dirac fermion, but it takes the form of the Majorana fermion at a late time. The relic number density of the dark matter is determined by the dark matter asymmetry generated through the same mechanism as leptogenesis when the dark matter was the Dirac fermion. After efficient dark matter annihilation processes have frozen out, a phase transition of a scalar field takes place and generates Majorana mass terms to turn the dark matter particle into the Majorana fermion. In order to address this scenario in detail, we propose two simple models. The first one is based on the Standard Model (SM) gauge group and the dark matter originates the $SU(2)_L$ doublet Dirac fermion, analogous to the Higgsino-like neutralino in supersymmetric models. We estimate the spin-independent/dependent elastic scattering cross sections of this late-time Majorana dark matter with a proton and find the possibility to discover it by the direct and/or indirect dark matter search experiments in the near future. The second model is based on the $B-L$ gauged extension of the SM, where the dark matter is a SM singlet. Although this model is similar to the so-called Higgs portal dark matter scenario, the spin-independent elastic scattering cross section can be large enough to detect this dark matter in future experiments.