US, WASHINGTON (ORDO NEWS) — Deep beneath the mountains of Gran Sasso in Italy, the “most sensitive” experiment in the search for dark matter in the world is being conducted. But instead of dark matter, the experimenters found significantly more particle interaction events than predicted by the standard physics model.
Instead of 232 ± 15 low-energy events expected in the data for the year, from February 2017 to February 2018, 285 were detected during the XENON1T experiment – 53 more than predicted, which significantly exceeds the error margin.
A large international team of physicists involved in cooperation does not know what causes the excess, despite the fact that they have been working on the results since 2018.
After careful consideration, they reduced their options to three possibilities: one rather mundane … and two others that would have a huge impact on our understanding of fundamental physics.
Researchers presented their findings at an online seminar on June 17 and prepared a document, which is currently being prepared for peer review.
“We are seeing an excess of three sigma and we don’t know what it is,” said physicist Evan Shockley of the University of Chicago.
XENON1T is a tank filled with 3.2 metric tons of ultra-pure liquid xenon and equipped with arrays of photomultipliers. It is completely sealed and completely dark to detect scintillation and electroluminescence, which occurs when two particles interact with each other, creating tiny flashes of light and a tiny stream of electrons emitted from the xenon atom – what is known as electron transfer.
Since most of these interactions come from known particles, estimating the number of background events that should occur is relatively simple. Thus, the number 232 was obtained for electron recoil events at low energies.
So, “where do the additional 53 events come from” is a big question.
The first and most common of the three scenarios that could cause additional particle interactions is a previously unaccounted source of background events caused by very small amounts of a rare radioactive hydrogen isotope called tritium.
Researchers noted that tritium could get into the detector as a result of cosmogenic activation of xenon and hydrogen in the detector materials themselves. This will give only a small amount of tritium – only a few atoms for every 1025 xenon atoms, too few to be detectable. Attempts to detect tritium in other ways were fruitless, so the tritium hypothesis can neither be confirmed nor excluded.
A second, more intriguing possibility is that the signal can be triggered by a neutrino. These particles are similar to electrons, but have almost no mass and charge, and they very rarely interact with other particles. This is also good, since neutrinos are the most common particle in the universe.
According to the team’s calculations, neutrinos could be responsible for the excess signal if they had a stronger magnetic moment – that is, magnetic force and orientation – than we thought. If these stronger neutrinos are responsible for the signal, we will probably need new physics to explain how they can exist.
The third scenario is a type of hypothetical particle called the solar axion. This is the best match for data with a confidence level of 3.5 sigma, i.e. 2 to 10,000 probability that the signal is a random fluctuation. (Two other scenarios have a confidence level of 3.2 sigma.)
Axions are a hypothetical type of particle that was put forward in the 1970s to address the question of why strong atomic forces follow the so-called symmetry of charge parity, when most models say that they don’t need it.
Axions of a certain mass are a strong candidate for dark matter. Solar axions, hypothetically flowing from the Sun, do not coincide with the candidate axions for dark matter, but would be a strong hint of their existence – if there were solar axions, other axions should also exist.
If the Sun can produce axions, then all stars must; and, again, the observed heat loss in very hot stars imposes severe restrictions on the axion interaction with subatomic particles.
So, we still have questions that can only be solved, you guessed it, by additional experiments. As XENON1T moves on to the next phase, we just need to wait.
“The signals discussed here can be further studied in next-generation detectors,” the researchers write in their article.
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