Argon as the test of two interpretations of the LZ 248 keV recoil

Deep in the silence of the universe, a mysterious substance known as dark matter is thought to drift through everything, including our planet. Unlike ordinary matter, which makes up stars, planets, and people, dark matter does not emit light or interact with electricity, making it invisible to telescopes and cameras. Scientists believe it reveals itself only through gravity, pulling on visible objects. To find it, researchers build massive detectors deep underground, shielding them from cosmic rays and other noise, hoping to catch a rare, tiny collision between a dark matter particle and an atomic nucleus. When such a collision happens, it should leave a faint flash of light or a small amount of heat, a signal that tells us something about the nature of this invisible stuff. Recently, a detector called LZ, filled with liquid xenon, recorded a single, puzzling event: a nucleus recoiling with an energy of 248 kilo-electronvolts. This energy is high, but the real mystery is that no other events were seen at lower energies, creating a gap in the data that defies standard expectations.

This unusual signal has sparked two very different stories about what might be happening. One story suggests that the dark matter particle is like a heavy ball that gets hit by the fastest moving particles in the galaxy, gaining energy and bouncing off a nucleus. This is called an endothermic process, where the particle needs a boost to move. The other story proposes that the dark matter particle is like a heavy ball that is already moving fast and drops down to a lower energy state, releasing a burst of energy that hits the nucleus. This is called an exothermic process. The difference between these two ideas is not just a matter of theory; it leads to completely opposite predictions for what should happen in a detector filled with a different element, argon. If the first story is true, argon should see nothing. If the second story is true, argon should see a clear, bright signal.

A new analysis by physicists H. Baer and V. Barger uses the laws of motion and the specific properties of atoms to show that the answer lies in the argon data that scientists have already collected. The researchers explain that if the dark matter particle is dropping to a lower energy state, it releases a fixed amount of energy. When this happens in xenon, it creates a recoil at 248 keV. However, because argon atoms are lighter than xenon atoms, the same energy release would push an argon nucleus harder, creating a recoil at a higher energy, specifically around 336 keV for a typical dark matter mass. Crucially, this process does not require the dark matter to be moving at a specific minimum speed; any particle in the galaxy could do it. This means that if this interpretation is correct, an argon detector should see a distinct line of events at this higher energy, appearing about ten times more frequently than the single event seen in xenon.

In contrast, if the dark matter particle needs to absorb energy to scatter, it requires a minimum speed to work. The analysis shows that the speed needed to create the 248 keV signal in xenon is so high that it is already at the very limit of what the galaxy allows. When the same calculation is applied to argon, the required speed becomes even higher, exceeding the maximum speed any particle in our galaxy can possibly have. Therefore, if this "energy-absorbing" story were true, an argon detector would see absolutely no events at all. The two possibilities are mutually exclusive: either argon shows a strong signal, or it shows nothing. There is no middle ground.

The beauty of this test is that it does not depend on complex theories about what the dark matter is made of or how it interacts with other forces. It relies only on the basic physics of how two objects collide and the known speeds of particles in our solar neighborhood. The researchers point out that the data needed to settle this question is already sitting on computer disks. The DEAP-3600 experiment, located deep underground in a mine in Canada, has been running with liquid argon for several years. It has recorded enough exposure to expect between six and twenty-five events if the "energy-releasing" story is correct, depending on how much of the detector is used for the search. If the "energy-absorbing" story is correct, the data should show zero events in that specific energy range.

The background noise in the argon detector is well understood and manageable. While there are natural sources of radiation that can mimic these signals, the researchers estimate that the number of false alarms is very low, likely less than one event in the relevant energy range. This means that if the argon data shows a cluster of events around 336 keV, it would be a clear sign of the energy-releasing process. Furthermore, finding this line would do more than just confirm the type of event; it would allow scientists to calculate the mass of the dark matter particle with high precision by comparing the exact energy of the line in argon to the energy seen in xenon. If the data shows no such line, the energy-releasing explanation for the original xenon event would be ruled out, forcing scientists to reconsider what caused that single, lonely flash of light.

The decision rests on a reanalysis of existing records, not on building new machines. The DEAP-3600 team has already gathered the necessary data, and a fresh look at these records could provide a definitive answer within the current understanding of physics. If the argon line appears, it would confirm that dark matter has an internal structure that allows it to release energy, a discovery that would reshape our understanding of the universe's hidden mass. If it does not appear, it would close the door on that specific explanation for the xenon event, leaving the mystery of that single recoil unsolved. The answer is already there, waiting to be read, turning a theoretical debate into a matter of simple observation.