Explain the LZ High-Energy Recoil Event with Inelastic Sneutrino Dark Matter in Supersymmetry
This paper proposes inelastic sneutrino dark matter within an inverse-seesaw extended next-to-minimal supersymmetric standard model as a viable explanation for the high-energy recoil event observed by LZ, demonstrating that specific annihilation and coannihilation mechanisms can simultaneously satisfy thermal relic abundance, solar neutrino, gamma-ray, and flavor constraints while reproducing neutrino oscillation data.
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
The universe is filled with invisible matter that we cannot see, touch, or smell, yet we know it is there because its gravity holds galaxies together. Scientists call this substance dark matter, and for decades, they have been trying to catch a glimpse of it by waiting for a dark matter particle to bump into an atom in a detector deep underground. Usually, these detectors are designed to catch a simple, elastic bounce, where the dark matter particle hits an atom and bounces off like a billiard ball, transferring a tiny amount of energy. However, the rules of physics allow for a more complex interaction: inelastic scattering. In this scenario, the dark matter particle hits an atom and excites it, causing the particle to change into a slightly heavier version of itself. This transformation consumes some of the particle's speed, meaning it can only happen if the particle is moving very fast, and it leaves behind a distinct, higher-energy signature than a standard bounce would.
Recently, the LUX-ZEPLIN experiment, a massive detector filled with liquid xenon located deep in a mine in South Dakota, reported a single event that looked exactly like this rare, high-energy bounce. The event occurred at an energy of 248 kiloelectronvolts, a value that is too high to be explained by the usual, simple dark matter models. While this single event does not prove that dark matter has been found, it offers a tantalizing clue that the particle might be more complex than previously thought. It suggests that the dark matter particle might have a hidden internal structure, allowing it to shift into a heavier state upon impact, a process that would naturally filter out the slower particles and leave only the fastest ones to create a detectable signal.
A team of researchers has now explored whether a specific type of theoretical particle, known as a sneutrino, could explain this mysterious signal. In the framework of supersymmetry, a theory that proposes every known particle has a heavier partner, the sneutrino is the partner of the neutrino. The researchers investigated a version of this particle that exists in a state where it is almost identical to a slightly heavier partner, separated by a tiny energy gap. This gap is protected by a fundamental symmetry in nature related to the conservation of lepton number, a property that keeps the two states distinct yet nearly identical. The team found that if these sneutrinos make up the dark matter in our galaxy, they could indeed produce the high-energy recoil seen by the LUX-ZEPLIN detector, provided that a small fraction of them are mixed with a specific type of interaction that allows them to talk to the detector's atoms.
To make this idea work, the researchers had to solve a significant problem: if these particles are so abundant and interact enough to be seen on Earth, they should also be captured by the Sun's gravity and annihilate there, creating a flood of high-energy neutrinos that our telescopes should have already seen. The paper demonstrates that this conflict can be resolved through three different mechanisms. First, the particles could annihilate into new, invisible particles that decay into pairs of light particles called photons, effectively hiding the energy from neutrino detectors. Second, the particles could have a resonance, a specific energy state that makes them annihilate much more efficiently in the hot, fast environment of the early universe than they do in the cold, slow environment of the Sun today. Third, they could interact with other heavy particles in the supersymmetric family, allowing them to disappear quickly in the early universe without leaving a strong signal in the Sun today.
The researchers built eight specific scenarios, or benchmarks, to test these ideas. They calculated exactly how many events each scenario would produce in the LUX-ZEPLIN detector and compared these predictions against the single event that was actually observed. They also checked these scenarios against data from gamma-ray telescopes looking at dwarf galaxies and neutrino telescopes looking at the Sun. Four of their eight scenarios passed every test. These successful models predict that the dark matter particle has a mass between 300 and 1100 gigaelectronvolts, with a mass gap of roughly 300 kiloelectronvolts, and a small mixing probability that allows it to interact with the detector. The fact that these models can simultaneously explain the high-energy event on Earth, avoid the constraints from the Sun, and satisfy limits from gamma-ray observations suggests that the sneutrino interpretation is a viable path forward.
This work does not claim to have solved the mystery of dark matter, but it provides a concrete, testable framework for understanding a specific, puzzling signal. It shows that the high-energy event reported by LUX-ZEPLIN is not necessarily a fluke or a background noise, but could be the first glimpse of a dark matter particle that changes its identity when it collides with ordinary matter. By proposing a model where the particle's mass splitting is naturally protected by symmetry and its annihilation is tuned to avoid detection in the Sun, the researchers have offered a coherent story that fits the current data. The study highlights that the search for dark matter requires looking beyond simple bounces and considering more complex interactions, and it sets the stage for future experiments to either confirm these specific predictions or rule them out. The path to understanding the invisible universe may lie in these subtle, high-energy shifts that only the most sensitive detectors can hope to catch.
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