UltraViolet Freeze-in (1410.6157v3)

Abstract: If dark matter is thermally decoupled from the visible sector, the observed relic density can potentially be obtained via freeze-in production of dark matter. Typically in such models it is assumed that the dark matter is connected to the thermal bath through feeble renormalisable interactions. Here, rather, we consider the case in which the hidden and visible sectors are coupled only via non-renormalisable operators. This is arguably a more generic realisation of the dark matter freeze-in scenario, as it does not require the introduction of diminutive renormalisable couplings. We examine general aspects of freeze-in via non-renormalisable operators in a number of toy models and present several motivated implementations in the context of Beyond the Standard Model physics. Specifically, we study models related to the Peccei-Quinn mechanism and Z' portals.

Non-renormalisable Operators in UltraViolet Freeze-in Models of Dark Matter

The paper "UltraViolet Freeze-in" authored by Fatemeh Elahi, Christopher Kolda, and James Unwin investigates a distinctive approach to the production of dark matter through freeze-in processes. This paper draws attention to scenarios where the hidden sector, which includes dark matter (DM), is only connected to the visible sector via non-renormalisable operators. Traditionally, freeze-in mechanisms presume feeble renormalisable interactions. However, this work posits that linking via non-renormalisable operators might represent a more universal depiction of hidden sector interactions owing to the avoidance of introducing negligible coupling constants.

Summary and Novel Contributions

The authors provide a comprehensive analysis of UltraViolet (UV) freeze-in, exploring several illustrative models to highlight the implications of such processes for dark matter abundance. They delve deeply into models linked with the Peccei-Quinn mechanism and $Z'$ portals, presenting novel theoretical frameworks within the domain of Beyond the Standard Model (BSM) physics.

• Exploration of UV freeze-in with non-renormalisable operators: Unlike InfraRed (IR) freeze-in that involves renormalisable operators, UV freeze-in links the hidden sector and visible sector through effective operators, leading to a relic density that significantly hinges on the reheat temperature. The analytical exploration of toy models with varying mass dimensions illustrates the sensitivity of DM production to the initial thermal conditions in the universe.
• Phase Space and VEV Insertion: The paper extends classical formulations to include operators of escalating mass dimensions, elucidating how DM production transitions via many-body final states. Further, models exhibiting operators that include vacuum expectation values (VEVs) are analyzed. These models present intriguing scenarios where IR freeze-in contributions might or might not dominate the DM yield.

Implications and Constraints

The vital aspect of their consideration lies in maintaining the thermal decoupling between visible and hidden sectors to prevent equilibrium and ensure the freeze-in process dictates DM density. This necessitates exceedingly suppressed interaction strengths or high-dimension operators that inherently evade equilibrium conditions.

• Constraints from Sector Equilibration: The authors provide a detailed assessment of equilibrium constraints, identifying an essential lower limit on DM mass that ensures it remains out of equilibrium across cosmological timescales. This aspect emphasizes the nuanced equilibrium evaluations necessary in freeze-in dynamics.

Future Directions

This paper serves as a foundational paper propelling new lines of inquiry within particle cosmology. Several implications emerge:

• BSM Physics Compatibility: The introduction of UV freeze-in within BSM frameworks, like the $Z'$ portal and axion models, intertwines dark matter production with plausible high-energy physics phenomena, potentially observable through future experimentations or indirect astrophysical evidence.
• Reheat Temperature Role: Identifying specific reheat temperatures consistent with inflationary physics or other cosmic events could offer experimental routes to validate or challenge this scenario. The interplay between cosmological conditions and DM yield could define future experimental strategies.

Conclusion

Elahi, Kolda, and Unwin's research elaborates on an intricate aspect of dark matter formation via UV freeze-in processes and presents substantial theoretical frameworks that enrich our understanding of how non-renormalisable operators might play a pivotal role in the hidden-visible sector interaction. Their paper paves the way for extensive explorations within the field of non-thermal dark matter and high-energy physics.