Scientists from the United States have announced a possible breakthrough in the search for the enigmatic dark matter that makes up most of the matter in the universe and has so far never been directly observed. They have identified the trace of a particle that does not match any currently known particles but fits some theoretical models of dark matter.
Read more The food sector seeks new ways to grow without giving up its origin, at Auténtica 2026
A possible nuclear recoil of unknown origin
On June 16, 2023, nearly a mile and a half underground beneath the Black Hills of South Dakota, hundreds of sensors recorded two brief flashes inside a large liquid xenon tank. The combination of both signals indicated that something had struck the nucleus of a xenon atom with unusual energy.
More than three years later, researchers still do not know what produced that signal. The international LUX-ZEPLIN (LZ) collaboration, made up of about 250 scientists and engineers, has studied possible sources of radiation, known particles, and instrument failures without finding a convincing explanation. However, the impact fits some dark matter hypotheses. The fundamental limitation is that it has only been detected once.
The result was presented at the TeV Particle Astrophysics conference, held in Tendo (Japan), and appears in a manuscript that the collaboration has submitted to the journal Physical Review Letters. For now, it is a and has not yet been approved by independent scientists. “We do not want to get ahead of ourselves. We do not claim to have detected dark matter, but we have observed something interesting that we want to share with the scientific community to gather their opinion,” warned Rick Gaitskell, professor at Brown University and spokesperson for LZ.
The invisible matter of the universe
Dark matter neither emits nor reflects light, but its gravity is evident in the movement of galaxies and the structure of the universe. It represents about 85% of all matter. “We can think of dark matter as the cosmic glue that allowed the formation of galaxies like our Milky Way,” explained Alvine Kamaha, astrophysicist at UCLA and co-author, during the conference.
One of the most studied hypotheses proposes that it is made up of WIMPs, an acronym for “weakly interacting massive particles.” These are heavy particles that barely interact with ordinary matter.
Most WIMPs would pass through the Earth without colliding with any atom, but very occasionally, a WIMP could collide with a nucleus and cause it to recoil a tiny distance. This movement, called nuclear recoil, is the signal sought by LZ and other underground experiments such as XENONnT in Italy and PandaX-4T in China.
LZ is installed at the Sanford Underground Research Laboratory, built in an old gold mine and shielded from cosmic rays by rock. It contains seven tons of active liquid xenon. Each interaction generates a first flash and releases electrons that produce another. The relationship between both signals allows approximately distinguishing whether the impact displaced the nucleus, as a WIMP might, or hit its electrons, as usually happens with radioactivity.
Read more This is the smallest theater in the world
What the detector cannot clarify yet
The finding comes from a new analysis of 220 days of data collected between March 2023 and April 2024, in which they searched for more energetic collisions and more complex interactions. The detected signal resembles nuclear recoil much more than an interaction with electrons. Moreover, it occurred in the inner area of the tank, did not trigger the outer detectors, and had energy compatible with some models of very heavy WIMPs.
When comparing the event with some theoretical dark matter models, the result reaches 3.4 sigmas. Physicists use this measure to indicate how far a data point deviates from what is expected by chance. The more sigmas, the harder it is to attribute it to coincidence, and conventionally five sigmas are required to announce a discovery. However, the team tested 616 combinations of masses and interaction types. By making so many comparisons, the probability increases that some match the data by chance. After accounting for this, the final significance dropped to 2.6 sigmas. This means that if there were no dark matter and all background phenomena were well described, a result like this would appear by chance between five and ten times out of every thousand.
Scientists have sought ordinary explanations. Neutrons and neutrinos can also hit nuclei, although they would hardly produce such an energetic collision without other signals. Accidentally associated pulses and collisions with charge loss were also studied. The team has not identified any as a probable explanation, but not all can be ruled out.
The manuscript itself acknowledges several limitations. The duration and shape of the first flash do not allow deciding on their own whether the nucleus of an atom or one of its electrons moved. Also, to prevent their expectations from influencing the analysis, the collaboration introduced artificial events among the real data without telling the analysts which they were. However, those signals did not cover well the area where the anomaly appeared, so this part of the analysis was not completely “blind.”
If it were dark matter, it would be a very heavy particle
If the collision was really caused by dark matter, the responsible particle would probably have more than 200 times the mass of a proton. Therefore, a confirmation would point to a class of very heavy dark matter with a form of interaction different from that which has focused most searches so far. One possible explanation is so-called inelastic dark matter. In these models, the particle would enter the detector in one state and, during the collision, transform into a slightly heavier version.
The decisive test may already be stored. The laboratory has collected at least twice as much data as used in this analysis and plans to continue operating until at least 2028. The appearance of new collisions with similar characteristics would strengthen the dark matter interpretation.
Read more Why Legos and HP cartridges keep washing up on Europe’s beaches
