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Several experiments conducted in the 1990s to study neutrinos found something really strange: there were too many particles appearing on the detectors. In particle physics, even small deviations from the expected experimental results excite scientists. Now, a new experiment conducted deep underground, more than two kilometers below Russia’s Caucasus Mountains, has confirmed an anomaly previously observed, pointing to a still unconfirmed new elementary particle called a “sterile neutrino.” Either that or our physics is wrong, so these results are incredibly consistent, regardless of the result.
Sterile neutrinos deep underground
Neutrinos are the most common particles in nature, perhaps second only to photons, particles of light. You can’t see them, but they are everywhere. In fact, about a trillion neutrinos pass through your hand every second. Most of them originate from the sun, while others are generated in the upper atmosphere when gases are hit by cosmic rays from supernovae and other events in space.
There are three known types or tastes of neutrinos: electronic, muon and tau neutrinos. But many scientists believe that there is a fourth fragrance that lingers in the shadows, waiting for its rightful place with its family of particles. Called conditionally sterile neutrinos, if they exist, they could help solve some enduring mysteries in physics, such as why neutrinos have mass when, in theory, they should be as massless as photons. Sterile neutrinos – so named because they are supposed to interact with other particles only by gravity, while the other three flavors also do so by weak force – may also explain the nature of dark matter, invisible and elusive matter, which accounts for 85% of all matter in the universe, although we cannot measure it directly.
Located deep underground at the Baksan Neutrino Observatory in the Caucasus Mountains of Russia, the completed two-zone gallium target, on the left, contains an inner and outer gallium reservoir that is irradiated by a neutrino electron source. Credit: AA Shikhin
Researchers associated with the Baksan Experiment on Sterile Transitions (BEST), which includes U.S. researchers from the Los Alamos National Laboratory, used irradiated disks with chromium 51 (a synthetic radioisotope of chromium) and a powerful source of electronic neutrinos to irradiate the interior and exterior. tank made of gallium. As a result of this reaction, the experiment produced the isotope germanium 71.
This was entirely expected, but what was anomalous was that the production rate was 20-24% lower than the proposed theory. The methodology of the experiment is considered to be flawless and, moreover, the discrepancy is in the same principle as registered by other previous experiments.
“The results are very exciting,” said Steve Elliott, a lead analyst at one of the data assessment teams and a member of Los Alamos’s physics department. “This definitely confirms the anomaly we have seen in previous experiments. But what this means is not obvious. There are now conflicting results for sterile neutrinos. If the results show that fundamental nuclear or atomic physics are misunderstood, this would also be very interesting.
One of the earlier experiments with similar results was the predecessor of BEST, a solar neutrino experiment from the 1980s called the Soviet-American Gallium Experiment (SAGE), which also uses gallium and a source of high-intensity neutrinos. Both BEST and SAGE were performed thousands of meters below the entrance of a tunnel at the Baksan Neutrino Observatory, located in the Baksan River gorge in Russia’s Caucasus Mountains.
Neutrino detectors are usually buried deep underground to protect them from interference from cosmic rays and other radiation, which would cause chaos in the experiment if the detectors were exposed to the surface. A next-generation neutrino detector, called the Deep Underground Neutrino Experiment, or DUNE, is currently being built 48 kilometers (30 miles) underground at the National Fermi Accelerator Laboratory in Batavia, Illinois. When completed, it will be able to fire neutrino rays through the Earth’s mantle.
Have we missed dark matter because our understanding of physics is wrong?
There are many reasons why physicists love neutrinos. They provide a direct link between us and the solar core, allowing scientists to peek into the processes of nuclear fusion without having to place detectors in space. But perhaps the most intriguing thing about neutrinos is that they fluctuate between tastes, like a chameleon changing color in response to its surroundings. A particle that starts as an electron neutrino, for example, can become a tau or muon neutrino and vice versa.
The time gaps in these fluctuations recorded by the experiment in Russia and the like before it suggest that there is a fourth taste that we lack. This hypothetical particle can also be an important component of dark matter.
But this does not mean that the fourth type of elementary particles is the only explanation. The results of the experiment also raise the intriguing possibility that our current theoretical framework, which describes neutrinos, is wrong. That would not be bad news at all. Science is a continuous work in which the status quo is always supplemented by new, convincing evidence. In this process, the institution of science becomes stronger and more reliable, as well as better prepared to answer increasingly complex questions about nature.
The findings appear in Physical Review Letters.
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