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Sub-GeV Millicharge Dark Matter Production via the Freeze-in Mechanism in a U(1)X Hidden Sector


Core Concepts
This paper investigates the production of sub-GeV millicharge dark matter particles through the freeze-in mechanism within a U(1)X hidden sector, emphasizing the crucial role of hidden sector temperature evolution and clarifying common misunderstandings about millicharge generation.
Abstract
  • Bibliographic Information: Feng, W.-Z., Zhang, Z.-H., & Zhang, K.-Y. (2024, October 4). Sub-GeV millicharge dark matter from the U(1)X hidden sector. arXiv.org. https://arxiv.org/abs/2312.03837v3
  • Research Objective: This paper aims to comprehensively study the production of sub-GeV millicharge dark matter through the freeze-in mechanism within a U(1)X hidden sector. The authors aim to clarify the mechanisms of millicharge generation and accurately calculate the relic abundance of dark matter candidates by considering the evolution of the hidden sector temperature.
  • Methodology: The authors employ a renormalizable model that incorporates both kinetic and mass mixing between the U(1)X hidden sector and the Standard Model. They analyze the evolution of hidden sector particles and temperature by solving a set of coupled Boltzmann equations, considering various benchmark models with different mass ranges for dark fermions and dark photons.
  • Key Findings: The study clarifies that millicharge generation primarily arises from mass mixing between a massive U(1)X and the hypercharge gauge field, not from kinetic mixing. The evolution of the hidden sector temperature significantly impacts the final relic density of dark matter. The authors also find that while O(keV) U(1)X dark photons can be viable dark matter candidates, they contribute at most ~5% to the total observed dark matter relic density.
  • Main Conclusions: The freeze-in mechanism within a U(1)X hidden sector offers a viable pathway for producing sub-GeV millicharge dark matter. The research highlights the importance of considering the interplay between kinetic and mass mixing, hidden sector temperature evolution, and detailed particle interactions when studying dark matter candidates and their production mechanisms.
  • Significance: This research contributes significantly to the field of dark matter research by providing a detailed analysis of millicharge dark matter production in a U(1)X hidden sector. It clarifies misconceptions regarding millicharge generation and emphasizes the importance of considering hidden sector dynamics for accurate relic density calculations.
  • Limitations and Future Research: The study focuses on specific benchmark models within the U(1)X hidden sector framework. Exploring a wider range of models and parameter space could provide a more comprehensive understanding of millicharge dark matter. Further investigation into the phenomenological consequences of these models and their potential detection signatures would be valuable.
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Stats
The dark photon can contribute at most ~5% to the total observed dark matter relic density. The lifetime of the dark photon is roughly 10^18 seconds.
Quotes
"The connection between kinetic mixing and the millicharge can be traced back to [28,29]." "It has been shown that pure kinetic mixing between an extra U(1) and the hypercharge gauge field cannot generate a millicharge [31]." "The millicharge carried by the U(1)X dark fermions can be obtained from: (1) The dark particle carries a tiny amount of hypercharge as a prior. (2) A kinetic mixing between a massless U(1) and the hypercharge gauge field, and the generated millicharge is proportional to the kinetic mixing parameter. (3) The mass mixing between a massive U(1) with the hypercharge gauge field [31, 32]."

Key Insights Distilled From

by Wan-Zhe Feng... at arxiv.org 10-07-2024

https://arxiv.org/pdf/2312.03837.pdf
Sub-GeV millicharge dark matter from the $U(1)_X$ hidden sector

Deeper Inquiries

How might future advancements in dark matter detection experiments constrain or support the existence of sub-GeV millicharge dark matter and the U(1)X hidden sector?

Answer: Future dark matter detection experiments will play a crucial role in constraining or supporting the existence of sub-GeV millicharge dark matter and the U(1)X hidden sector. Here's how: Constraints: Improved Sensitivity of Direct Detection Experiments: Experiments like SENSEI, SuperCDMS, and DAMIC are searching for low-mass dark matter using novel detection techniques. Increasing the sensitivity of these experiments, particularly in the sub-GeV mass range, will be crucial. If no signal is observed, it will place stringent bounds on the kinetic and mass mixing parameters (δ and ϵ), constraining the allowed parameter space for millicharge dark matter. Indirect Detection Limits: Experiments searching for indirect signals of dark matter annihilation or decay, such as Fermi-LAT (gamma rays) and AMS-02 (cosmic rays), can impose limits on the properties of millicharge dark matter. More precise measurements and extended energy ranges in these experiments will be vital in further constraining the interaction strengths and masses of particles within the U(1)X hidden sector. Beam Dump Experiments: Experiments like NA64 and LDMX are designed to search for dark matter produced in electron and proton beam dumps, respectively. These experiments are particularly sensitive to millicharged particles due to their electromagnetic interactions. Future beam dump experiments with higher intensity beams and improved detectors could probe unexplored regions of parameter space for sub-GeV millicharge dark matter. Support: Observation of a Signal Consistent with Millicharge Dark Matter: A definitive observation of a signal consistent with the properties of sub-GeV millicharge dark matter in any of these experiments would be a groundbreaking discovery. This would provide strong evidence for the existence of a U(1)X hidden sector and offer insights into the nature of dark matter. Corroborating Evidence from Multiple Experiments: Finding consistent hints of millicharge dark matter across multiple experiments with different detection techniques would significantly strengthen the case for this scenario. For example, observing an excess in a direct detection experiment that aligns with an anomaly in an indirect detection experiment would be compelling evidence.

Could the millicharge carried by dark matter particles have implications for the formation of large-scale structures in the universe, and if so, how?

Answer: Yes, the millicharge carried by dark matter particles could indeed have subtle but potentially observable implications for the formation of large-scale structures in the universe. Here's why: Modified Dark Matter Self-Interactions: While millicharges are small, they could still facilitate interactions between dark matter particles. These interactions, even if extremely weak, could influence the dynamics of dark matter in the early universe. Impact on the Cosmic Microwave Background (CMB): The presence of millicharged dark matter could leave imprints on the Cosmic Microwave Background (CMB). The small electromagnetic interactions could modify the temperature and polarization anisotropies of the CMB, potentially providing a way to probe the properties of millicharge dark matter. Suppression of Small-Scale Structure: If dark matter self-interactions due to millicharges are strong enough, they could lead to a suppression of small-scale structures in the universe. This is because interactions can smooth out density perturbations, preventing the formation of the smallest dark matter halos. Observations of the abundance and properties of dwarf galaxies and other small-scale structures can therefore be used to constrain the strength of dark matter self-interactions and, consequently, the magnitude of the millicharge.

If our universe is filled with hidden sectors, what does this imply about the fundamental nature of reality and our understanding of the cosmos?

Answer: The possibility of our universe being filled with hidden sectors has profound implications for our understanding of the cosmos and the fundamental nature of reality: Incompleteness of the Standard Model: The Standard Model of particle physics, while incredibly successful, is known to be incomplete. It fails to explain the existence of dark matter, dark energy, neutrino masses, and other fundamental phenomena. The existence of hidden sectors would imply that the Standard Model is just a part of a much larger and more complex picture. New Forces and Particles: Hidden sectors could harbor new forces and particles beyond our current understanding. These forces and particles might interact with the Standard Model particles only very weakly, which is why they have evaded detection so far. A Multiverse?: The concept of hidden sectors raises the intriguing possibility of a multiverse – a vast collection of universes, each with its own set of physical laws and properties. Our universe, with its particular set of particles and forces, might be just one of many. The Nature of Dark Matter: Hidden sectors provide a natural framework for explaining the nature of dark matter. Dark matter could be composed of particles that reside in a hidden sector and interact with ordinary matter only through gravity or other feeble forces. New Windows into the Early Universe: Studying hidden sectors could offer new windows into the very early universe. The interactions between hidden sector particles and their potential influence on cosmological observables like the CMB could provide valuable information about the conditions in the early universe. In essence, the existence of hidden sectors suggests a universe far richer and more complex than we currently perceive. It challenges our understanding of the fundamental constituents of reality and the laws that govern them. Unraveling the mysteries of hidden sectors could lead to a paradigm shift in our understanding of the cosmos and our place within it.
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