A PQ-Symmetric High-Scale SUSY Interpretation of the LZ High-Energy Recoil

Deep in the quiet of the universe, a fundamental mystery persists: what is the invisible substance that holds galaxies together? Scientists call this "dark matter," and while we know it exists because of its gravitational pull, we have never directly seen a single particle of it. For decades, physicists have proposed that the universe is governed by a hidden symmetry called supersymmetry, a framework suggesting that every known particle has a heavier, unseen partner. If this theory is true, the lightest of these new partners could be the dark matter we are searching for. The challenge, however, is that these particles might be so heavy or so rare that our most sensitive detectors on Earth have missed them, leaving us with only a faint, puzzling signal to guide our theories.

Recently, the Large Underground Xenon experiment, known as LZ, reported a single, unusual event deep beneath the Earth's surface. In a region designed to be free of background noise, the detector registered a tiny burst of energy consistent with a nucleus being knocked backward by a collision. The energy of this recoil was measured at 248 kiloelectronvolts. While this is just one data point and not yet a confirmed discovery, it offers a tantalizing clue. A researcher at Tokyo Metropolitan University has taken this specific signal and asked a bold question: could this be the first glimpse of a very specific type of dark matter particle, one that is much heavier and behaves differently than previously thought?

The paper proposes that this event is not a random glitch, but the result of a near-perfect collision between a heavy dark matter particle and an atom in the detector. The author suggests the culprit is a "Higgsino," a theoretical particle that is a partner to the Higgs boson, the particle responsible for giving other particles mass. In this scenario, the dark matter is not a lightweight wanderer but a massive object weighing 1.08 teraelectronvolts, roughly a thousand times heavier than a proton. Because this particle is so heavy, it moves slowly through the galaxy, and when it bumps into a xenon atom in the detector, it transfers just enough energy to create the specific 248-kiloelectronvolt signal observed.

For this collision to produce the exact energy seen by the LZ experiment, the theory requires a very specific internal structure for the dark matter particle. The Higgsino must exist in two slightly different states with a tiny energy gap between them, a difference of about 0.3 to 0.35 million electronvolts. This small split is crucial because it allows the particle to "up-scatter," meaning it gains a tiny bit of energy from the collision rather than just bouncing off. This mechanism naturally explains why the signal appeared at that specific energy level. To make this work, the theory also implies that other heavy particles, known as gauginos, must exist at an incredibly high energy scale of roughly 10 million gigaelectronvolts, far beyond the reach of any current particle accelerator.

This high-energy setup fits into a broader picture of the universe known as high-scale supersymmetry. In this view, the heavy particles are so massive that they solve several long-standing problems in physics, such as why the universe has more matter than antimatter and why certain forces behave the way they do. The model also incorporates a mechanism called the Peccei-Quinn symmetry, which introduces a new particle called the axion. This axion helps solve a different puzzle regarding how particles interact with the strong nuclear force. Interestingly, in this specific arrangement, the axion's interaction with light is almost completely canceled out, making it extremely difficult to detect with current instruments, which adds a layer of stealth to the proposed dark matter mix.

The author checks this idea against what we know about the universe's history and structure. The calculations show that if these heavy particles existed, they would have decayed early enough in the universe's life to avoid disrupting the formation of light elements, a process known as nucleosynthesis. Furthermore, the model predicts that the Higgs boson, which was discovered at the Large Hadron Collider, should have a mass of about 125 gigaelectronvolts, which matches the value measured by scientists. The theory also suggests that the forces of nature might unify at a very high energy level, though this unification point is lower than what some older theories predicted. This lower unification scale means that protons, the building blocks of atoms, might eventually decay, but at a rate so slow that it has not yet been observed, keeping the theory consistent with current limits.

While the model is mathematically consistent and fits the single event reported by LZ, the author is careful to note that this is a working hypothesis based on one data point. The theory suggests that if we look for dark matter in the sky using gamma-ray telescopes, we might see a faint signal from the center of our galaxy, but current limits are not yet strict enough to rule it out. The presence of the axion and the Higgsino together could mean that dark matter is a mixture of two different types of particles, which would change how we expect them to behave in the galaxy. The paper concludes that this specific combination of a heavy Higgsino and a hidden axion offers a coherent explanation for the LZ event and the broader landscape of particle physics, waiting for future experiments to either confirm the signal or find the heavy particles that make this story possible.