Emergent Dark Matter, Baryon, and Lepton Numbers

TL;DR

The paper investigates a mechanism where an early-universe mass mixing transfers a pre-existing lepton or baryon asymmetry to a dark matter asymmetry, potentially linking the two sectors without higher-dimensional transfer operators. It analyzes two broad realizations: a two-stage electroweak phase transition in a two-Higgs-doublet sector that temporarily activates X–L mixing, and Planck-suppressed mixing induced by moduli, flat directions, or background energy that decays as the universe cools. Depending on the mechanism and cosmological timing, the final dark matter mass can range from a few GeV to around 100 TeV, with the asymmetry ratio set by either mixing angles or thermal suppression, and with the observed relation $Ω_{DM} \approx 5 Ω_B$ emerging naturally in many scenarios. The framework expands the asymmetric DM landscape, offering distinctive collider and direct-detection phenomenology tied to the dynamically induced mixing and the presence of new scalars, while typically predicting suppressed indirect signals due to the dominance of the asymmetric component.

Abstract

We present a new mechanism for transferring a pre-existing lepton or baryon asymmetry to a dark matter asymmetry that relies on mass mixing which is dynamically induced in the early universe. Such mixing can succeed with only generic scales and operators and can give rise to distinctive relationships between the asymmetries in the two sectors. The mixing eliminates the need for the type of additional higher-dimensional operators that are inherent to many current asymmetric dark matter models. We consider several implementations of this idea. In one model, mass mixing is temporarily induced during a two-stage electroweak phase transition in a two Higgs doublet model. In the other class of models, mass mixing is induced by large field vacuum expectation values at high temperatures - either moduli fields or even more generic kinetic terms. Mass mixing models of this type can readily accommodate asymmetric dark matter masses ranging from 1 GeV to 100 TeV and expand the scope of possible relationships between the dark and visible sectors in such models.

Figures (2)

• Figure 1: Unshaded regions in the $k_2-k_3$ plane give rise to a two stage phase transition and a viable dark matter asymmetry through mass mixing for light dark matter. Each parameter point is consistent with dark matter of mass $3.3$ GeV. The other parameters are held fixed at $\mu_2=54$ GeV for both plots, $y_X=0.5$ (left) and $y_X=1.7$ (right). In the left shaded region (red), the barrier between vacua is too large for rapid bubble nucleation and the system remains trapped in the false vacuum. In the right shaded region (also red), the $\phi\neq0$ direction is not a stable vacuum. In the bottom shaded region (green), the scalar mass $m_\phi$ is excluded by LEP.
• Figure 2: Relationship between the temperature of the first-order phase transition $T_{\rm N}$ and the $\phi$ quartic coupling $k_2$. The other parameters are held fixed at $k_3=1.5$, $y_X=1.7$ and $\mu_2=54$ GeV. No first-order phase transition takes place in the shaded regions.