Something unknown collided with an atomic nucleus in a tank filled with seven tons of noble gas 1.5 kilometers below the earth's surface on the evening of June 16, 2023.
The researchers at the Lux-Zeplin experiment in the USA have now, three years later, analyzed their data very carefully to rule out that the particle that caused the collision is something we already know. When they presented the results at the astroparticle physics conference Tevpa 2026 in Japan on September 1, there was a great stir. This could be our very first glimpse of the enigmatic dark matter.
In the weeks since then, physicists have posted more than 70 articles on the Arxiv database with possible theoretical interpretations and explanations.
- It's the biggest response I can remember for a dark matter signal. It is rare to get such a large response from the research community so quickly, says Tim Linden, theoretical physicist at Stockholm University.
The Swiss Fritz Zwicky and the Swede Knut Lundmark discovered that the universe must contain much more than the matter we know already in the 1930s when they studied distant galaxies and galaxy clusters. 40 years later, American astronomer Vera Rubin saw the same thing when she measured how spiral galaxies, like our own Milky Way, rotate. Several times more matter than we can see is needed for the galaxies to hold together and move as they do.
Stars and planets are made up of atoms just like ourselves. But the matter of atoms is only about 15 percent of the mass in the universe. What we call dark matter is the other 85 percent. It is everywhere. But we can't see it, and we don't know what it is.
Physicists have had many different theories about what dark matter consists of. The hottest candidate today is called wimp, weak interacting massive particles, or heavy elementary particles that sense the so-called weak force.
If the hypothesis is correct, we are bathed every second by billions of wimps. But only about once a month will any of them collide with an atom in our body, because the weak force is so weak and has such a short range.
- Wimpar is a strong candidate. The underlying physical mechanisms can explain that there is about six times as much dark matter as ordinary matter in the universe, says Tim Linden.
Since wimps and ordinary matter so rarely sense each other, very sensitive measuring instruments are required to be able to find them. Lux-Zeplin is a tank filled with the noble gas xenon in an old gold mine in South Dakota.
- We put a lot of effort into shutting out noise and false signals. We work one and a half kilometers underground to avoid ordinary cosmic radiation, says Richard Gaitskell, professor of physics at Brown University in Rhode Island in the USA and spokesman for Lux-Zeplin.
Cosmic radiation is particles from the sun and other celestial bodies.
- We are hit by cosmic radiation many times a second. It makes the earth's surface full of noise. But at a depth of one and a half kilometers, the cosmic radiation decreases by more than a million times, says Richard Gaitskell.
All materials in and around the detector must also be very clean and have very low radioactivity.
- People are usually surprised when they learn that over 5,000 radioactive decays occur in an ordinary person every second. And they emit radiation. So if you or I were standing next to the detector with 5,000 gamma rays coming out of the body every second, we would render the detector completely useless.
Therefore, the detector is also surrounded by several layers of tanks with water and other substances that screen off and exclude ordinary particles that create noise signals in the experiment.
The possible breakthrough came when the Lux-Zeplin researchers went through data collected by the detector over 220 days, looking for wimp signals with slightly different energies than they had looked for before.
- The probability that this would be radioactive background noise is about one in 200. Most people probably see odds of less than one in a hundred as something extremely unlikely. But as physicists we have to be very conservative. So, so far, it's right on the mark. When doing many physics experiments, even very unusual events can occur from time to time, says Richard Gaitskell.
In the Gran Sasso laboratory northeast of Rome in Italy, there is a similar detector: the Xenon experiment. Jan Conrad, professor of astroparticle physics at Stockholm University, works there. He finds the results interesting.
- They have done a solid job, he says.
It happens from time to time that signals appear that could be new, unknown particles.
- Often these anomalies occur right at the limit of what the detector is able to measure, for example at very low energies, where things are difficult to understand. But this is in an area where our detectors are very efficient.
The energy range where the signal is found is unexpected for most models of how dark matter works.
- But this is expected in a model with a so-called Higgsino. The model makes very specific predictions. Until now, we have not had sufficiently sensitive detectors to be able to detect them, says Tim Linden.
According to Tim Linden, it is possible to draw some conclusions even though the physicists have only seen a single signal.
- If I see a purple elephant, I know that purple elephants exist, even though I've only seen one, he says.
But then, of course, the incident must be investigated carefully.
- You have to think about whether it was an optical effect, whether there was something strange about the sky that day, or whether some young people had been out with purple spray paint the night before. It becomes more difficult to analyze the unknown circumstances when you only have a single event to start from, says Tim Linden.
The physicists at Lux-Zeplin and Xenon have vast amounts of collected data to sift through in search of similar events.
- We have about twice as much data as Lux-Zeplin presented which we will now analyze, says Jan Conrad.
This probably also applies to the detector Pandax in Sichuan, China.
If there are more events that match the predictions of the Higgsino model, the mystery of dark matter may finally be solved. A possible candidate for the Nobel Prize in Physics would then be Xenon's spokesperson Elena Aprile.
- She is the pioneer and the mother of this entire field. And it is her old students who have built the competing experiments, says Jan Conrad.
Richard Gaitskell has devoted his entire career to the search for dark matter. Even if the new signal turns out to be incorrect, we have still learned more about the universe, he believes.
- 95 percent of all research gives negative results, because we often ask questions to which we simply do not know the answer. To make scientific progress is to realize that we often just rule something out, or get a result that is not clear-cut. This is what research looks like for the most part.
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1,400 meters below the ground, the researchers are trying to capture dark matter
