The Four Basic Ways of Creating Dark Matter Through a Portal

Abstract: We consider the possibility that along the thermal history of the Universe, dark matter (DM) would have been created from Standard Model particles, either through a kinetic mixing portal to an extra U(1) gauge field, or through the Higgs portal. Depending solely on the DM particle mass, on the portal and on the DM hidden sector interaction, we show how the observed DM relic density can be obtained. There are four possible freeze-in/reannihilation/freeze-out regimes, which together result in a simple characteristic relic density phase diagram, with the shape of a "Mesa". In the case of the kinetic mixing portal, we show that, unlike other freeze-in scenarios discussed in the literature, the freeze-in regime can be probed by forthcoming DM direct detection experiments. These results are well representative {of} any scenario where a DM hidden sector would be created out of the Standard Model {sector}.

• The paper presents four regimes—freeze-in, reannihilation, freeze-out, and connector freeze-out—that detail distinct mechanisms for dark matter production through standard model interactions.
• It demonstrates that even minimal coupling in the freeze-in regime can be probed by future experiments, such as Xenon1T, enhancing dark matter detection strategies.
• The study bridges theoretical models with experimental prospects by linking thermal histories and phase behaviors to observable collider and astrophysical phenomena.

The Four Basic Ways of Creating Dark Matter Through a Portal

The research paper investigates potential mechanisms by which dark matter (DM) could be produced within the universe through interaction with standard model (SM) particles, focusing on two major portal frameworks: kinetic mixing and the Higgs portal. This paper provides a detailed examination of the four distinct regimes—freeze-in, reannihilation, freeze-out, and connector freeze-out—through which DM relic densities can arise, showcasing their influence via a characteristic 'Mesa' phase diagram. Each mechanism offers nuanced contributions to the understanding of DM formation and its interactions with visible matter.

Mechanisms of Dark Matter Production

1. Kinetic Mixing Portal:
   • This involves an additional $U(1)'$ gauge in the hidden sector, corresponding to a massless mediator photon ($\gamma'$). Energy transfer happens via kinetic mixing with the SM's photon or Z boson. The kinetic mixing is characterized by the parameter $\kappa$, defined as the product of gauge coupling constant and kinetic mixing term.
   • The paper details how direct detection experiments could probe this DM production mechanism; interestingly, even the freeze-in regime, characterized by a minute coupling magnitude, might be discernible through future, highly sensitive experiments like Xenon1T.
2. Higgs Portal:
   • This mechanism involves the interaction between a scalar DM candidate and the SM Higgs boson. Here, the mediator (the Higgs boson) may be heavier than the DM. This situation results in unique production pathways, such as significant DM creation via the decay of the Higgs.
   • The phase diagram reveals variations between massive and massless mediators; for instance, when the DM mass is below half of the Higgs mass, decay processes dominate production.

Phase Diagrams and Regimes of Production

The Mesa phase diagram with four observable regimes—freeze-in, reannihilation, invisible sector freeze-out, and connector freeze-out—transcends specific models and remains consistent across interaction portals:

• Freeze-In: A non-equilibrium process driven by minute initial interactions, primarily producing DM without annihilation.
• Reannihilation: Characterized by hidden sector thermalization where DM formation is succeeded by hidden sector reannihilation, dictated by the balance between connection and hidden sector interactions.
• Freeze-Out and Connector Freeze-Out: These describe scenarios where DM eventually decouples, either through visible-interaction persistence or as SM and hidden sector temperatures equalize, determining mutual thermalization or independent sectorial freeze-outs.

Implications and Future Directions

The theoretical exploration and experimental relevance suggest significant implications for cosmological and particle physics, notably in interpreting thermal histories and relic densities. The study emphasizes testing prospects of proposed DM models via observable collider phenomena or astrophysical measurements, particularly for low coupling visibility—suggesting robust connectivity between DM model predictions and future empirical endeavors.

Moreover, this investigation broadens the scope of DM creation mechanisms within non-equilibrium cosmological frameworks. It facilitates a refined understanding of existing and future experimental constraints, thus guiding innovative theoretical advancements and experimental designs essential for unraveling fundamental cosmic compositions and behaviors.

Speculatively, new developments may also arise in formulating more complex, multi-component DM frameworks, integrating aspects such as asymmetric DM or multifaceted hidden sectors, potentially harmonizing these distinct creation avenues under more unified theoretical models. Such steps are pivotal to ensuring that a comprehensive architecture aligns observed universe characteristics with elaborate subatomic fabric models, extending the current understanding of dark matter's cosmological narrative.