Analysis of freeze-in scenario with a scalar Leptoquark and a scalar Dark Matter
This paper investigates the generation of Dark Matter relic density via the freeze-in mechanism in a model featuring a heavy scalar leptoquark that mediates renormalizable interactions between Dark Matter and Standard Model particles, exploring the resulting parameter space to identify values consistent with observational constraints.
Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer
For decades, the search for the invisible substance that holds the universe together has followed a specific path. Scientists have long suspected that the dark matter making up most of the cosmos consists of heavy particles that occasionally bump into ordinary matter, a theory known as the "freeze-out" paradigm. This idea suggests that in the hot, dense early universe, these particles were abundant and constantly interacting, only to freeze out and leave behind the small amount we see today. However, despite years of searching with massive detectors deep underground and in space, no one has found a single sign of these particles. This silence has forced physicists to consider a different possibility: what if dark matter never interacted strongly enough to be in balance with the rest of the universe? Instead of freezing out, it might have slowly trickled into existence, generated by rare, feeble interactions that never allowed it to reach a state of equilibrium. This alternative path, known as "freeze-in," proposes a universe where dark matter is a ghostly byproduct of ordinary particle collisions, so weakly connected to our world that it has remained undetected until now.
In a recent study, a researcher named Joydeep Roy explored this quiet, slow-creation scenario by introducing a specific, heavy particle into the mix: a scalar leptoquark. Leptoquarks are hypothetical particles that act as bridges between the two main families of matter in the Standard Model: quarks, which build protons and neutrons, and leptons, such as electrons. While these particles have never been observed, they are a popular idea in physics because they could explain certain strange behaviors seen in recent experiments. Roy's work asked a simple but profound question: if a heavy leptoquark existed in the early universe alongside a scalar dark matter particle, how would they interact to create the dark matter we see today? The study focused on a framework where these two particles communicate through very weak forces, ensuring that the dark matter remains hidden from our current experiments while still being produced in the correct amounts to match the universe's observed density.
The researcher built a detailed mathematical model to simulate the birth of the universe, tracking how these particles would behave as the cosmos expanded and cooled. The simulation revealed that the heavy leptoquark plays a crucial role as a factory for dark matter. Before the universe cooled enough for the Higgs field to give particles their mass, the leptoquarks and Higgs particles would collide and scatter, occasionally producing a pair of dark matter particles. Once the universe cooled further, the Higgs particle itself would begin to decay directly into dark matter. The study found that for this process to work without overproducing dark matter or creating particles that would have been detected by now, the connections between these particles must be incredibly weak. Specifically, the coupling between the dark matter and the thermal plasma must be of the order of 10⁻¹¹, a value so small that it renders the interactions virtually undetectable by current methods.
These tiny connection strengths are not a flaw but a feature of the scenario. Because the interactions are so feeble, the dark matter particles never build up enough to annihilate each other or to be caught by current detectors, which explains why we have not found them yet. The study also determined that for this specific setup to produce the exact amount of dark matter observed in the universe, the dark matter particle itself must be very light, weighing less than one millionth of a gram. This is significantly lighter than the heavy particles often sought in traditional experiments. Furthermore, the research showed that the presence of the heavy leptoquark, which must weigh at least 1.5 trillion electron volts to avoid detection, does not disrupt the process. Instead, it acts as a steady source, helping to generate the dark matter abundance over time.
The findings suggest that the universe could be filled with dark matter that is far lighter and more elusive than previously thought, born from a slow, steady drip of interactions rather than a violent freeze-out. The study confirms that a heavy leptoquark can comfortably exist in the early universe's thermal bath without violating any known physical laws or experimental limits. By mapping out the specific ranges of mass and interaction strength required, the research provides a clear roadmap for what to look for. If the dark matter in our universe is indeed this light and this weakly connected, it will likely remain invisible to the massive underground detectors currently in use, which are designed to catch heavier, more interactive particles. Instead, the evidence for this scenario might one day come from subtle signals in high-energy particle colliders or from the faintest whispers of radiation in the cosmos, pointing to a hidden sector of the universe that has been quietly shaping our reality all along.
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