Endothermic dark matter with a light dark photon and the LUX--ZEPLIN high-energy nuclear-recoil candidate

Deep beneath the surface of the earth, in a tank of liquid xenon cooled to the temperature of deep space, scientists are listening for a whisper. This is the LUX-ZEPLIN experiment, a massive detector designed to catch the rarest of interactions: a collision between a particle of dark matter and an atom of ordinary matter. Dark matter is the invisible scaffolding of the universe, holding galaxies together, yet it refuses to shine, reflect, or absorb light. For decades, physicists have searched for it by waiting for a tiny, sudden jolt in their detectors, a signal that a ghostly particle has bumped into a heavy nucleus. Recently, the LZ team reported a single, intriguing event. It was a nuclear recoil, a kick to an atom, with a specific energy of 248 kiloelectronvolts. While this single flash could be a fluke or a background noise, it sits at a high energy level that standard theories of dark matter struggle to explain. It is a puzzle piece that does not quite fit the picture most scientists have been drawing.

A team of researchers has now proposed a new way to make that piece fit. They suggest that the dark matter particle responsible for this kick is not a simple, static object, but rather a two-faced entity with a hidden internal structure. Imagine a particle that exists in two states: a calm, low-energy ground state and a slightly heavier, excited state, separated by a small but crucial gap in energy. In this scenario, for the dark matter to bump into an atom in the detector, it must first absorb energy to jump up to that heavier state. This process, known as endothermic scattering, acts like a high-speed filter. It means that only the fastest, most energetic dark matter particles in our galaxy have enough speed to pay the energy cost of the jump. Slower particles simply pass by without interacting. This mechanism naturally explains why the detector saw a high-energy event while ignoring the thousands of slower, more common particles that should be passing through it every second.

The researchers built a detailed model around this idea, connecting the dark matter to a new, invisible force carrier called a dark photon. This particle acts as a messenger between the dark world and our own, allowing the dark matter to interact with the xenon atoms. By running extensive computer simulations that combined the rules of particle physics with the observed history of the universe, the team tested whether this specific setup could explain the single LZ event while also matching the total amount of dark matter we know exists in the cosmos. They found a narrow, precise region where everything aligns. In this scenario, the dark matter particles are heavy, weighing in at the scale of a few thousand protons, and the energy gap between their two states is a few hundred kiloelectronvolts. The messenger particle, the dark photon, is relatively light, existing at the scale of a few billion electronvolts.

The fit is remarkably tight. The model predicts that the dark matter particles responsible for the event must be moving at speeds near the very top limit of what is possible for particles in our galaxy's halo. Because the energy gap is so large, only the fastest few percent of the dark matter population can trigger the detector. This requirement forces the model into a specific corner of possibilities. The researchers calculated that if this model is correct, the single event seen by LZ is exactly what we should expect to see, with the right energy and the right frequency. Furthermore, the same physics that allows this high-speed collision also dictates how much dark matter was created in the early universe, and the model naturally produces the correct amount to match the cosmic inventory.

However, this elegant solution comes with a significant caveat that the researchers carefully highlight. Because the energy gap between the two states is so small, it is too small to allow the excited dark matter particle to decay into an electron and a positron, which are common particles. This blocks the fastest way for the excited state to disappear. Instead, the excited particle is forced to decay much more slowly, potentially lingering for billions of years. If a significant number of these excited particles survived from the birth of the universe until today, their eventual decay would inject energy into the cosmos in a way that contradicts what we see in the cosmic microwave background. The researchers note that this is a potential problem for the simplest version of their model. They suggest that a small addition to the theory, a specific type of interaction that allows the excited state to decay quickly into three photons, could solve this issue without ruining the explanation for the LZ event.

The beauty of this proposal is that it turns a mystery into a testable hypothesis. Unlike many dark matter theories that rely on invisible forces we cannot easily check, this model involves a light dark photon that interacts with our world in a way that future experiments can see. The specific mass and interaction strength required to explain the LZ event fall within a range that upcoming experiments at particle accelerators, such as those at Belle II and LHCb, are designed to probe. If the dark photon exists with the properties this model predicts, these machines should be able to create it and watch it decay. The single flash of light seen in the deep underground tank is no longer just a statistical anomaly; it is a potential beacon pointing toward a specific, testable structure of the dark sector. The researchers have not proven that this is the answer, but they have shown that it is a viable path forward, one that connects a single, strange event to the grand history of the universe and offers a clear roadmap for finding the truth.