Fermionic Dark Matter Absorption and the High-Energy Event in LUX-ZEPLIN

For decades, scientists have been hunting for dark matter, the invisible substance that makes up most of the universe's mass but refuses to interact with light. The most popular theory for years has been that this matter consists of heavy, slow-moving particles that occasionally bump into normal atoms, leaving a tiny, detectable spark. Experiments deep underground, shielded from cosmic rays, have listened for these faint whispers. However, as these detectors have become incredibly sensitive, they have found nothing in the low-energy range where the heavy particles were expected to strike. This silence has forced researchers to look elsewhere, expanding their search to higher energies and considering stranger possibilities, such as dark matter that does not just bounce off atoms but is absorbed by them, vanishing completely while transferring its entire mass into a burst of energy.

A recent analysis of data from the LUX-ZEPLIN experiment, a massive tank of liquid xenon deep in a South Dakota mine, has brought a new and puzzling candidate to light. The experiment recorded a single event where an atom in the tank recoiled with an energy of about 248 kiloelectronvolts, a value significantly higher than the background noise usually seen. While the experimenters did not declare this a discovery, the event stood out as a statistical anomaly that begged for an explanation. Two researchers from Nanjing Normal University in China have proposed a specific mechanism to explain this single flash: the absorption of a type of dark matter particle by a xenon nucleus. They suggest that if dark matter consists of fermions with a mass of roughly 247 million electronvolts, it could be absorbed by the nucleus, converting its entire rest mass into a precise, single-energy kick to the atom. This process would create a sharp, distinct peak in the data, matching the energy of the observed event perfectly.

The researchers built a detailed model to test this idea, showing how such an absorption event would look inside the detector. They calculated that for a dark matter particle of this specific mass, the energy transferred to the xenon atom would land right in the window where the single event was found. This coherent absorption, where the dark matter hits the whole nucleus at once, would produce a clean, monoenergetic signal. However, the same physics that allows this clean hit also predicts a messy side effect. Because the dark matter particle is heavy enough to resolve individual protons and neutrons inside the nucleus, the absorption should also frequently knock out single nucleons. This incoherent process would generate a broad spectrum of much higher energy events, creating dozens of additional signals that should be visible in the detector if the model were correct.

The team found that their proposed model could indeed reproduce the single observed event while remaining consistent with the lack of other signals in the low-energy range. The incoherent, high-energy events predicted by their theory are expected to be filtered out by the experiment's standard safety cuts and veto systems, which are designed to reject high-energy noise. This creates a scenario where the detector sees one clean event but misses the dozens of messy ones that should accompany it. The researchers calculated that the interaction strength required to produce this single event corresponds to a specific scale of new physics, roughly 11.5 teraelectronvolts. This value is high enough to explain the event without contradicting the empty low-energy spectrum, making the theory appear viable at first glance.

However, the story takes a sharp turn when the researchers look beyond the xenon tank to other experiments. They turned their attention to data from KamLAND, a massive liquid scintillator detector in Japan that is filled with oil and designed to observe neutrinos. This detector is sensitive to a different reaction: if the same dark matter particles exist, they should also be absorbed by carbon atoms in the detector, knocking out a neutron. This process would create a distinct signal of a neutron being emitted, followed by a delayed flash of light as the neutron is captured. By re-analyzing the existing data from KamLAND, the researchers found that the detector should have seen a significant number of these neutron-emission events if the dark matter model explaining the LUX-ZEPLIN event were true.

The re-analysis revealed a stark contradiction. The KamLAND data shows no evidence of these neutron events, placing a strict upper limit on how often such interactions can occur. This limit is far below the interaction strength required to produce the single event seen in the LUX-ZEPLIN experiment. In other words, the model that perfectly explains the xenon event is explicitly ruled out by the carbon data. The researchers conclude that while the idea of fermionic dark matter absorption is a compelling way to explain a high-energy nuclear recoil, the specific parameters needed to match the LUX-ZEPLIN observation are incompatible with the constraints from KamLAND. The tension between these two large detectors suggests that the single event in the xenon tank is likely not caused by this specific type of dark matter absorption, leaving the origin of the anomaly unresolved and pointing toward the need for future, dedicated high-energy analyses to solve the mystery.