Search for Magnetic and Spin-Independent Inelastic Dark Matter with XENONnT
Using 2.1 tonne-years of data from the XENONnT experiment, this study searches for magnetic and spin-independent inelastic dark matter by identifying unique signatures of nuclear recoils followed by delayed de-excitation photons, ultimately finding results consistent with background expectations and setting 90% C.L. upper limits across the GeV/c² to TeV/c² mass range.
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
The universe is filled with matter that we cannot see. Astronomers know this invisible substance, called dark matter, exists because its gravity holds galaxies together and shapes the cosmos, yet it makes up about eighty-five percent of all matter in existence. For decades, scientists have tried to catch a glimpse of it directly, hoping that a dark matter particle might occasionally bump into an atom in a detector deep underground. The most successful experiments so far have used tanks of liquid xenon, a heavy metal that is liquid at very low temperatures, to watch for these rare collisions. The standard idea is that dark matter is a single type of particle that bounces off an atomic nucleus and then disappears, leaving behind a tiny flash of light and a small electrical signal. This is the simplest picture, but it is not the only one. Some theories suggest that dark matter might be more complex, existing in two different states with slightly different weights, much like a person who can be either standing or sitting. If a dark matter particle is in the heavier, excited state, it might crash into an atom, lose some energy, and then settle back down to its lighter state a moment later, releasing a burst of energy as it does so.
A team of researchers using the XENONnT experiment, a massive detector located deep beneath the Gran Sasso mountain in Italy, recently looked for evidence of this more complex behavior. They analyzed data collected over a period corresponding to 2.1 tonne-years of observation, a measure of how much detector mass was watching for how long. Their goal was to find a very specific signature: a dark matter particle hitting a xenon atom, causing the atom to recoil, and then, a split second later, the dark matter particle itself releasing a photon of light as it relaxes back to its ground state. This would create a unique pattern of two distinct events happening in the same place, separated by a tiny fraction of a second. The team also looked for a simpler version of this scenario where the dark matter particle hits the atom but the second event happens outside the detector, leaving only the initial hit. They focused on two specific theoretical models, one involving magnetic interactions and another involving a general force that does not depend on the spin of the particles.
To find these signals, the researchers had to be incredibly precise. The detector is a cylinder filled with 5.9 tonnes of liquid xenon, surrounded by water tanks to block out other types of radiation. When a particle interacts with the xenon, it produces two types of signals: a prompt flash of light called S1 and a delayed electrical signal called S2. The team developed new ways to look at these signals, specifically searching for pairs of events that matched the timing and direction expected from a dark matter particle traveling through the detector and then decaying. They had to filter out background noise, which includes random coincidences where two unrelated events happen to occur at the same time, and decays from radioactive krypton that can mimic the signal. By using the speed and direction of the particles to distinguish real signals from these background events, they created a very clean search area.
After applying all their strict selection rules, the researchers found zero events that matched the signature of the double-hit decay. They did find 805 single-hit events, but these were consistent with what they expected from known background sources and did not show any unusual excess that would point to dark matter. Because they saw no evidence of the predicted signal, they did not discover a new type of dark matter. Instead, they used the absence of these events to set new, stricter limits on how likely these specific models are to be true. They calculated that if this type of dark matter exists, its interaction with ordinary matter must be weaker than their results allow. For the magnetic model, they ruled out values for the magnetic dipole moment that were up to three orders of magnitude larger than previous limits, effectively excluding a large range of possibilities for particles with masses between 50 GeV/c² and 1 TeV/c². For the spin-independent model, they similarly tightened the constraints on how strongly these particles could interact with atomic nuclei.
The study confirms that the universe does not contain this specific kind of inelastic dark matter at the levels the researchers were able to test. While the search did not find the particles, it significantly narrowed the field of possibilities for physicists. By proving that these particles do not exist with the properties they were looking for, the team has guided future research toward other possibilities. The XENONnT detector, with its ultra-low background and massive size, continues to be one of the most sensitive tools available for peering into the dark sector, and this latest result stands as a definitive test of these particular theories. The work demonstrates that even when a search comes up empty, the knowledge gained about what is not there is a crucial step in understanding what the universe is made of.
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