A possible interpretation of the LUX-ZEPLIN recoil event in the 2HD+a scenario
This paper proposes that the recent high-energy nuclear recoil event observed by the LUX-ZEPLIN experiment could be interpreted as a signal of fermionic dark matter within a 2HDM+a model featuring a light pseudoscalar boson, demonstrating that specific parameter regions of this scenario can simultaneously explain the event, satisfy cosmological density requirements, and evade existing constraints from direct detection, colliders, and precision measurements.
Original paper dedicated to the public domain under CC0 1.0 (http://creativecommons.org/publicdomain/zero/1.0/). 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 most persistent mystery in the universe has been the nature of dark matter. We know it is there because its gravity holds galaxies together and bends the path of light, yet it refuses to interact with ordinary matter in any way we can easily detect. It does not emit light, it does not reflect it, and it seems to pass through the solid ground beneath our feet without a whisper. To find it, scientists have built massive, ultra-sensitive detectors deep underground, shielding them from cosmic rays and other background noise. These experiments wait for a rare, faint collision: a dark matter particle striking an atomic nucleus and causing it to recoil, much like a billiard ball being hit by another. The challenge is that these collisions are expected to be incredibly rare and usually happen with very low energy, making them hard to distinguish from the background noise of the universe.
Recently, the LUX-ZEPLIN experiment, a massive detector filled with liquid xenon, reported a single, unusual event. A nucleus in the tank recoiled with a surprisingly high amount of energy, around 248 kilo-electronvolts. This is significantly higher than the typical recoil energy expected from standard dark matter theories, which usually predict a flood of low-energy hits and very few high-energy ones. While a single event is not enough to claim a discovery, its high energy and the lack of similar events at lower energies make it a tantalizing clue. It suggests that if this is indeed dark matter, it might be interacting in a way that scientists have not fully accounted for, perhaps involving a force that depends heavily on the speed of the collision or the momentum transferred.
A team of physicists has now explored whether this specific high-energy hit could be explained by a specific theoretical model called the 2HD+a scenario. This model expands on the standard description of particles by adding new types of Higgs bosons and a new, invisible particle that could be dark matter. In this framework, the dark matter particle is a heavy, invisible fermion that interacts with ordinary matter through a very light, ghostly particle called a pseudoscalar. Unlike the standard interactions that dominate at low speeds, this specific interaction becomes much stronger as the momentum of the collision increases. This momentum dependence is the key feature that allows the model to produce a high-energy recoil like the one seen by LUX-ZEPLIN, while naturally suppressing the low-energy hits that would otherwise overwhelm the detector.
The researchers performed a detailed investigation to see if this model could explain the event without breaking any other known laws of physics. They had to ensure that the same particles and forces responsible for the high-energy hit did not also produce signals that have already been ruled out by other experiments. For instance, if the dark matter interacts too strongly, it would have been seen in previous, more sensitive searches for low-energy collisions. Furthermore, the new particles in the model must not cause the known Higgs boson to decay in ways that have never been observed, nor should they disrupt the delicate balance of particle decays measured in high-energy colliders. The team systematically checked the model against all these constraints, including the requirement that the dark matter particles must have been produced in the early universe in just the right amount to match the total mass of dark matter we see today.
Their analysis revealed that the model can indeed accommodate the high-energy event, but only within a very narrow and specific set of conditions. The dark matter particle must have a mass of a few hundred times that of a proton, and the light pseudoscalar mediator must be extremely light, with a mass between 1 and 10 times that of a proton. The interaction strength between the dark matter and ordinary matter must be tuned precisely. When the researchers combined the requirement to explain the LUX-ZEPLIN event with the requirement to match the cosmic abundance of dark matter, they found a slight tension. The coupling strength needed to produce the observed single event was somewhat higher than the strength preferred by the thermal history of the universe. However, this discrepancy is not fatal.
The study concludes that the model remains a viable explanation for the event, provided one accepts a degree of statistical uncertainty inherent in observing just a single hit. When the researchers accounted for the fact that a single event has a wide range of possible underlying probabilities, the parameter space that satisfies all conditions—explaining the high-energy hit, matching the cosmic dark matter density, and passing all collider and flavor constraints—was found to be compatible at a 95% confidence level. The most promising solutions point to a dark matter particle with a mass around 100 to 120 GeV and a very light mediator around 1 to 1.5 GeV. While the model does not perfectly reproduce the central value of the observed event without some fine-tuning, it offers a coherent physical picture where a momentum-dependent force allows dark matter to deliver a high-energy kick. This suggests that if the LUX-ZEPLIN event is real, it may be the first glimpse of dark matter interacting through a mechanism that depends on how hard it hits, rather than just how often it hits.
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