An underground dark-matter experiment may have stumbled on the 'holy grail': a new particle that could upend the laws of physics

Experts are designing the top array of photomultiplier tubes that capture flashes of light from particle interactions.
The XENON experiment
An experiment with dark matter in an underground Italian laboratory may have discovered a new particle called the sun axion.
If this was actually found, it would be the first direct evidence of a particle that should not exist under the known laws of physics.
Alternatively, the data could also reveal new and surprising properties of mysterious particles called neutrinos.
Larger, more sensitive experiments next year will help scientists find out whether they have actually discovered a new particle.
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An underground tub of liquid xenon in Italy may have just discovered a new particle that was born in the heart of the sun.
If that actually happened, it could turn the laws of physics upside down, which have been around for about 50 years.
The researchers created the underground vat to search for dark matter, the elusive material that makes up 85% of all matter in the universe. Scientists know that dark matter exists because it can measure how its gravity affects distant galaxies, but they have never directly discovered it.
For this reason, an international research group carried out the experiment in the Italian Gran Sasso National Laboratory. The vat is filled with 3.2 tons of liquid xenon, and these atoms interact with tiny particles when they collide. Every interaction or "event" creates a flash of light and sheds electrons.
In theory, this experiment is sensitive enough to detect interactions with dark matter particles.
The XENON underground experiment. To the left is the water tank with a poster that shows what's inside. on the right is the three-story service building.
The xenon experiment
In the latest version of the experiment, the researchers expected the machine to recognize 232 events based on known particles within a year. Instead, 285 events were identified - 53 more than predicted.
In addition, the amount of energy released in these additional events corresponded to the predicted energies of an as yet undiscovered particle called the solar axion: a type of particle that physicists believed to exist but were never observed.
"The hypothetical particle that could possibly explain the XENON data is far too heavy to be dark matter, but could be created by the sun," said Sean Carroll, a physicist at the California Institute of Technology who is not using XENON connected is. Business Insider said. "If that were true, it would be hugely important - it would be a Nobel Prize result."
However, it is also possible that the interactions were anomalies that constantly occur in highly sensitive physical experiments like XENON.
A new particle that was forged in the heart of the sun
On September 14, 1999, a huge, handle-shaped projection erupted from the sun.
Particle physicists study the smallest and most basic components of the universe: elementary particles such as quarks and gluons, and forces such as gravity and electromagnetism.
"Particle physics is an important part of modern physics, but it has also been stuck for a long time," said Carroll. "The last really surprising discovery in particle physics was in the 1970s."
At this point, the so-called standard model was established - a set of all rules known to particle physics that describe all particles discovered by scientists and how they interact with each other.
"It essentially explains everything we see in a particle physics laboratory," said Aaron Manalaysay, a dark matter physicist at Lawrence Berkeley National Laboratory who is not affiliated with XENON, to Business Insider. "It is probably the most accurate scientific model in history. But we also have good reason to believe that it is not the most basic model of nature that there is."
Engineers who assemble the electrical field cage of the XENON experiment.
The XENON experiment
Physicists have evidence that the model does not fully understand the behavior of our universe - their indirect observations of dark matter are among these. However, you still have to directly recognize a particle that lies beyond the standard model.
So it would be a big deal if XENON really found a solar axion.
"That would be the first concrete discovery of anything beyond the standard model," said Manalaysay. "It's kind of a holy grail of particle physics right now."
Carroll agreed - but he added that the unprecedented nature of the possible discovery is "one of the reasons why we think it is probably not there".
In other words, no one celebrates without further evidence.
At the moment, several other theories could explain the additional events that XENON researchers have seen.
Bad behavior of neutrinos could indicate "new physics".
The top arrangement of photomultiplier tubes in the XENON experiment with the electrical cables.
The XENON experiment
Another possible explanation for XENON's 53 additional events is that neutrinos - a subatomic particle with no electrical charge - could have controlled the interactions.
However, this would also contradict the known laws of physics, since it would mean that neutrinos have a magnetic field that is much larger than predicted by the standard model.
"That could possibly indicate new physics beyond the standard model," said Manalaysay.
A cryostat that hangs on the support structure in the water tank of the XENON experiment.
The XENON experiment
It would not be the first time that neutrinos break the rules. According to the standard model, neutrinos shouldn't have mass - but they do. The discovery that they have a significant magnetic field would be another indication that something is missing in the model.
"Neutrinos are really strange animals and we don't really understand them," said Manalaysay.
Larger, more sensitive experiments with dark matter are pending
The XENON1T data acquisition system room in the Gran Sasso laboratory in Italy.
The XENON experiment
It is also possible that the additional events of XENON did not take place at all - although this is unlikely. The researchers calculated a probability of two out of 10,000 that the detected events were due to random fluctuations.
However, the signals could come from other secular particle interactions, which makes their explanation far less interesting than axions or neutrinos. The additional events could come from tiny amounts of tridium, a radioactive hydrogen isotope that disintegrates in the vat. Argon isotopes would produce a similar effect, according to Manalaysay.
"It wouldn't take long. It would only take a few atoms," he said, adding that a number of other things that are unknown to researchers could also be responsible for the excessive interactions.
"We've walked this path before where there's a little anomaly you don't expect ... and then it goes away," said Carroll. "So this is clearly a place where you need to do a better experiment, and they're planning exactly that."
The XENON1T time projection chamber after assembly.
The xenon experiment
A new generation of XENON-like experiments, currently underway in the United States and Europe, is designed to help researchers study these additional events and determine what particles are causing them. This is because the new experiments will be bigger and much more sensitive.
"If this is real, we will definitely see it in our next generation of experiments," said Manalaysay. He worked with such an effort, the Large Underground Xenon experiment with dark matter. "It's like going into an ever quieter room ... you hear new things that you couldn't hear in a noisier room."
While XENON caught 53 unexplained events, the successor of LUX - called LUX-ZEPLIN - was able to identify 800 according to Manalaysay. Despite the delays caused by the corona virus, new experiments will likely be conducted and results returned "within the next year".
"It's like a teaser," he said. "The season finale ends on a cliff hanger and you have to wait until the next season."
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