Thermal Relics in Hidden Sectors (0808.2318v3)

Abstract: Dark matter may be hidden, with no standard model gauge interactions. At the same time, in WIMPless models with hidden matter masses proportional to hidden gauge couplings squared, the hidden dark matter's thermal relic density may naturally be in the right range, preserving the key quantitative virtue of WIMPs. We consider this possibility in detail. We first determine model-independent constraints on hidden sectors from Big Bang nucleosynthesis and the cosmic microwave background. Contrary to conventional wisdom, large hidden sectors are easily accommodated. A flavour-free version of the standard model is allowed if the hidden sector is just 30% colder than the observable sector after reheating. Alternatively, if the hidden sector contains a 1-generation version of the standard model with characteristic mass scale below 1 MeV, even identical reheating temperatures are allowed. We then analyze hidden sector freezeout in detail for a concrete model, solving the Boltzmann equation numerically and understanding the results from both observable and hidden sector points of view. We find that WIMPless dark matter indeed obtains the correct relic density for masses in the range keV < m_X < TeV. The upper bound results from the requirement of perturbativity, and the lower bound assumes that the observable and hidden sectors reheat to the same temperature and is raised to the MeV scale if the hidden sector is 10 times colder. WIMPless dark matter therefore generalizes the WIMP paradigm to the largest mass range possible for viable thermal relics and provides a unified framework for exploring dark matter signals across nine orders of magnitude in dark matter mass.

• The paper demonstrates that hidden sector dark matter in WIMPless models naturally achieves the correct relic density via the mX ~ gX^2 relationship.
• It employs analytical and numerical evaluations of the Boltzmann equation to show how distinct reheating temperatures allow hidden sectors to evade standard observational limits.
• The research broadens the dark matter candidate landscape from keV to TeV scales, challenging the conventional WIMP paradigm and suggesting new astrophysical probes.

Thermal Relics in Hidden Sectors: A Comprehensive Analysis

The paper "Thermal Relics in Hidden Sectors" by Feng, Tu, and Yu explores the intriguing domain of hidden sector dark matter, with a special focus on WIMPless models. These models propose that dark matter resides in hidden sectors without Standard Model (SM) gauge interactions, challenging conventional wisdom while maintaining the desired thermal relic density attributed to WIMPs.

Key Insights and Findings

The paper explores the broad conceptual territory that reveals hidden sectors might significantly diverge from our observable sector yet still adhere to cosmological constraints such as Big Bang Nucleosynthesis (BBN) and the Cosmic Microwave Background (CMB). The authors argue against the prevalent notion that large hidden sectors are problematic, showing instead that they can be naturally accommodated under certain thermal conditions. Notably, they explore the implications of diverse reheating temperatures between the observable and hidden sectors, suggesting that hidden sectors can remain colder and invisible to standard observational constraints.

In WIMPless models, the characteristic hidden matter particles exhibit a mass and coupling relationship expressed by $m_X \sim g_X^2$, rooted in models like gauge-mediated supersymmetry breaking (GMSB). This fundamental relation allows WIMPless dark matter to display the correct relic density across a vast range of masses from keV to TeV levels. This range significantly exceeds traditional WIMP boundaries, broadening potential dark matter candidates.

Detailed Model and Constraints

Feng et al. thoroughly dissect hidden sector interactions through analytical and numerical evaluations of the Boltzmann equation, considering how hidden relic densities evolve. Their model-testing includes considering sectors analogously structured to the MSSM but scaled with unique parameters. Constraints from BBN and CMB, substantiated by numerical simulations and parameter sweeps, convincingly illustrate that variations in models, such as adjusting reheating temperature ratios, gracefully satisfy observational bounds.

Importantly, the authors present that even extensive hidden sectors do not necessarily violate cosmological data if slightly colder, hence not assuming the prejudice of equivalent temperatures post-reheating. Their treatment of the expansion rates and annihilation cross-sections progresses through refined approximations to gauge the relic densities accurately, achieving concordance with empirical data.

Implications and Future Directions

The implications of this paper suggest a reexamination of potential dark matter mass scales, which previously might have been disregarded within the WIMP paradigm. WIMPless models expand the theoretical landscape, inviting more complex hidden sector dynamics to possibly explain anomalies at lower mass scales. Furthermore, the analysis paves the way for revisiting astrophysical signals with mechanisms influenced by these models.

For future inquiry, it would be beneficial to further scrutinize kinetic decoupling temperatures and the resulting structure formation impacts in the WIMPless context. Understanding these effects at smaller comoving scales could offer critical insights, especially in the light of emerging precision cosmological datasets from experiments like Planck.

In sum, the analytical and numerical rigor of this paper contributes significantly to the hidden sector narrative, underpinning broader climate conditions for thermal relics that extend beyond conventional paradigms. The exploratory nature of the paper sets a foundation for future research in leveraging hidden sectors as viable, expansive canvases for dark matter physics.