Supermassive charged gravitinos and the Lux-Zeplin event
This paper proposes that the unusual event reported by the Lux-Zeplin collaboration, previously interpreted as evidence for WIMPs, is more likely caused by fractionally charged supermassive gravitinos, a hypothesis distinguished by large electromagnetic cross sections, low flux rates, and a predicted signature of sequential nuclear recoils within microseconds.
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
Dark matter is the invisible scaffolding of the universe, a mysterious substance that holds galaxies together but refuses to reveal itself. For decades, the leading theory has been that this substance consists of Weakly Interacting Massive Particles, or WIMPs. These are hypothetical particles that are heavy but barely interact with ordinary matter, passing through the Earth like ghosts. To find them, scientists have built massive, ultra-sensitive detectors deep underground, waiting for a rare, faint bump from a WIMP hitting an atomic nucleus. Recently, the Lux-Zeplin collaboration, operating a detector filled with liquid xenon, reported a single, unusual event that looked like a potential WIMP hit. However, two physicists, Krzysztof A. Meissner and Hermann Nicolai, have proposed a radically different explanation for this same event. They suggest that the particle responsible was not a ghostly WIMP, but a supermassive, electrically charged gravitino—a heavy particle predicted by theories of gravity and quantum mechanics that interacts strongly with matter, leaving a very different kind of trail.
The authors of this new study argue that the standard WIMP interpretation relies on the idea that dark matter particles interact so weakly that they almost never hit anything. In contrast, their alternative theory posits that dark matter could be made of gravitinos that carry an electric charge. Because they are charged, these particles would interact electromagnetically with the atoms in the detector, much like a heavy bullet striking a target, rather than slipping through unnoticed. While a WIMP would be expected to produce a single, isolated signal, a charged gravitino would be heavy enough to punch through the detector, knocking into multiple atoms along a straight path in rapid succession. The researchers point out that the single event recorded by the Lux-Zeplin detector, with a recoil energy of about 248 keV, fits the predictions for a gravitino just as well as it fits a WIMP, but the two theories make very different predictions about what else should have happened during that moment.
The core of the new proposal rests on the idea that these gravitinos are incredibly massive, with a mass close to the Planck mass, which is the scale at which gravity and quantum mechanics meet. Because they are so heavy, they move relatively slowly, yet they carry enough momentum to cause significant damage when they collide with an atom. Unlike WIMPs, which are theorized to interact only through the weak nuclear force, these charged gravitinos interact through electromagnetism. This means that as a gravitino travels through the liquid xenon in the detector, it does not just hit one nucleus and stop; it is likely to hit several nuclei in a row. The authors calculate that if a gravitino enters the detector, there is a high probability it will cause a second, and even a third, nuclear recoil within a few microseconds of the first. These secondary hits would be smaller in energy than the first but would occur in a perfectly straight line, tracing the path of the incoming particle.
The paper emphasizes that the single event observed by the Lux-Zeplin collaboration cannot definitively prove what kind of particle was seen, as the data alone does not identify the particle. However, the authors argue that the WIMP hypothesis requires the particle to have a tiny cross-section, meaning it rarely interacts, while the gravitino hypothesis requires a large cross-section, meaning it interacts frequently. In the gravitino scenario, the reason we see so few events is not because the particles are shy, but because they are so incredibly rare in number. The researchers estimate that while the flux of WIMPs might be high, the flux of these supermassive gravitinos would be vanishingly small, perhaps only a few particles per square meter per year. This scarcity explains why only one event was seen, even though the particle itself is a "loud" interacter.
To distinguish between these two possibilities, the authors suggest looking for a specific signature that a WIMP could never produce: a sequence of multiple recoils along a straight line. If the Lux-Zeplin detector were to record two or more nuclear hits occurring within a few microseconds of each other, and if those hits lined up perfectly, it would be a smoking gun for a charged, supermassive particle. The paper notes that background noise, such as neutrons from natural radioactivity, could theoretically cause multiple hits, but the energy and speed required for a neutron to mimic this pattern are so extreme that they are statistically unlikely. Furthermore, neutrons would move much faster than the slow-moving gravitinos, making the timing of the events a key differentiator. The authors also mention that a charged gravitino would interact with electrons in the detector, potentially creating a faint glow of light along its path, though this specific signal has not yet been calculated for xenon.
Ultimately, this paper does not claim to have solved the mystery of dark matter, but rather offers a compelling alternative to the standard model that fits the existing data. It challenges the assumption that dark matter must be weakly interacting, suggesting instead that it could be composed of heavy, charged particles that are simply too rare to be seen often. If future observations by the Lux-Zeplin detector reveal the predicted pattern of sequential, straight-line recoils, it would not only confirm the existence of these supermassive gravitinos but also provide the first direct observational evidence of physics at the Planck scale, linking the smallest particles to the force of gravity in a way that has never been seen before. Until such a pattern is found, the single event remains an open question, waiting for the next piece of the puzzle to fall into place.
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