Interpreting the LZ 248 keV Event using Dark QCD

Deep beneath the Earth's surface, in a tank of liquid xenon shielded from the chaos of the outside world, scientists are listening for a whisper. They are searching for dark matter, the invisible substance that makes up most of the mass in the universe but refuses to interact with light or ordinary matter in any way we can easily see. For decades, detectors have waited for a dark matter particle to bump into an atomic nucleus, creating a tiny flash of light or a faint electrical signal. Recently, the LUX-ZEPLIN experiment, a massive detector located in a South Dakota mine, reported a single, puzzling event. It was a recoil, a kick to an atom, with an energy of about 248 thousand electron volts. This energy is unusually high for the kind of dark matter most physicists expect to find, which typically moves slowly and delivers only a gentle tap. The event did not prove that dark matter exists, as it could have been a fluke of background noise, but its high energy sparked a new line of thinking: perhaps the dark matter particle is not a simple, solid ball, but something more complex that requires a specific, energetic push to reveal itself.

In a new study, researchers Francesco Sannino and Jessica Turner propose a way to explain this high-energy kick using a model of dark matter that behaves like a composite object, similar to how a proton is made of smaller quarks. They suggest that the dark matter particle is a "dark baryon," a heavy particle formed by three even smaller "dark quarks" bound together by a force unique to the dark sector. In their theory, this dark baryon is not a single, unchanging state. Instead, it exists in two slightly different versions, like a coin that can be heads or tails, with a tiny energy difference between them. The heavier version is an excited state, and the lighter one is the ground state. For the dark matter to interact with the xenon in the detector, it must undergo a transition from the lighter state to the heavier one. This process is "endothermic," meaning it requires the dark matter particle to absorb energy to make the jump. Because the particle must supply this extra energy to change its state, it cannot interact with the detector unless it is moving very fast. This naturally filters out the slow-moving particles that would create low-energy signals, leaving only the rare, high-speed particles capable of delivering the strong kick seen in the LUX-ZEPLIN data.

To make this idea work, the researchers constructed a detailed theoretical framework that includes a new force carrier, a heavy particle that acts as a messenger between the dark world and our own. This messenger connects to the dark baryons and to the protons and neutrons in ordinary matter, but it ignores electrons, a feature that helps the model avoid detection in other types of experiments. The team calculated what would happen if a dark baryon with a mass of one trillion electron volts, or one TeV, encountered a xenon nucleus. They found that if the energy gap between the two dark states is about 300 keV, the interaction would produce a recoil energy very close to the 248 keV event observed by the experiment. Their calculations show that the strength of the interaction required to produce this single event fits comfortably within the range of possibilities allowed by the LUX-ZEPLIN data. Specifically, the probability of the dark matter hitting a proton is estimated to be around 6.5 times 10 to the power of minus 43 square centimeters, a number that sits well within the experiment's reported confidence interval.

The researchers did not stop at matching the energy of the event; they also checked if their idea could survive other tests. They examined whether this model would create too many high-energy neutrinos from the Sun, a common way to rule out dark matter theories. They found that because the dark particles in their model decay into other dark particles before they can produce the usual cascade of neutrinos, the model remains safe from current solar neutrino limits. They also looked at data from particle colliders, where scientists smash protons together to look for new particles. The messenger particle in their theory would appear as a "dijet" resonance, a pair of jets of particles flying out from a collision. The current limits from the Large Hadron Collider do not rule out their proposed messenger, which has a mass of about 1.44 TeV, though future runs of the collider could test this more stringently. The team also considered the fate of the excited dark state. They showed that if a light, invisible particle exists in their model, the excited dark matter could decay into the lighter state very quickly, ensuring that the dark matter we see today is almost entirely in the ground state, ready to be excited by a fast-moving collision.

This work offers a concrete, albeit speculative, explanation for a single, anomalous data point. It does not claim to have solved the mystery of dark matter, nor does it prove that the 248 keV event was definitely a dark matter interaction. Instead, it demonstrates that a specific type of composite dark matter, with a heavy mass and a small energy split between two states, is a viable candidate that fits the observed data without contradicting other known constraints. The model suggests that if dark matter is indeed this complex, it has been hiding in plain sight, waiting for a fast enough collision to reveal its true nature. The researchers emphasize that further data from the LUX-ZEPLIN experiment and future runs of the Large Hadron Collider will be crucial. If more events appear at high energies, or if the collider finds the heavy messenger particle, this picture of a composite, endothermic dark matter could move from a mathematical possibility to a physical reality. Until then, the single event remains a quiet, intriguing hint that the universe may contain a hidden sector far more intricate than we have imagined.