Underground experiment points to sterile neutrino, a new kind of fundamental particle bound to dark matter

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Several experiments conducted since the 1990s on neutrinos have found something really strange: there were far too many particles appearing on the detectors. In particle physics, even small deviations from expected experimental results excite scientists. Now, a new experiment conducted deep underground, more than two kilometers below the Caucasus roller coaster, has confirmed the previously seen anomaly, pointing to a new yet unconfirmed elementary particle called the “sterile neutrino”. It’s either that or our physique is faulty, so these results are incredibly consistent, regardless of the outcome.

Sterile neutrinos deep underground

Neutrinos are the most abundant particles in nature, perhaps second only to photons, particles of light. You may not notice them, but they are everywhere. In fact, every second about a trillion neutrinos pass through your hand. Most of them originate from the sun, while others are generated in the upper atmosphere when gases are struck by cosmic rays from supernovae and other events in space.

There are three known types, or flavors, of neutrinos: electron, muon, and tau neutrinos. But many scientists believe that there is a fourth flavor that lingers in the shadows, waiting for its rightful place with its family of particles. Tentatively called sterile neutrinos, if they exist they could help solve many lingering mysteries in physics, such as why neutrinos have mass when in theory they should be massless like photons. Sterile neutrinos – so named because they are believed to interact with other particles only by gravity, whereas the other three flavors also do so by weak force – may also explain the nature of dark matter, the 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 in Russia, the completed two-zone gallium target, left, contains an inner and outer reservoir of gallium, which is irradiated by an electron neutrino source . Credit: AA Shikhin

Researchers affiliated with the Baksan Barren Transitions Experiment (BEST), which includes US researchers from Los Alamos National Laboratory, used irradiated disks of chromium-51 (a synthetic radioisotope of chromium) and a powerful source of electron neutrinos to irradiate the interior and exterior parts of a gallium tank. As a result of this reaction, the experiment produced the isotope germanium-71.

This was completely planned, but what was abnormal was that the production rate was 20-24% lower than the theory suggested. The methodology of the experiment is considered to be flawless and, moreover, the deviation is in the same stage as that recorded by other previous experiments.

“The results are very exciting,” said Steve Elliott, senior analyst for one of the teams evaluating the data and a member of the Los Alamos Physics Division. “This definitely reaffirms the anomaly we’ve seen in previous experiments. But it’s not clear what it means. There are now conflicting results on sterile neutrinos. If the results indicate that fundamental nuclear or atomic physics is misunderstood, that would also be very interesting.

One of the first experiments that had similar results was the precursor to BEST, a 1980s solar neutrino experiment called the Soviet-American Gallium Experiment (SAGE), which also used gallium and a source of high intensity neutrinos. BEST and SAGE were executed thousands of meters below the entrance to 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 that would wreak havoc on the experiment if the detectors were exposed on the surface. A next-generation neutrino detector called the Deep Underground Neutrino Experiment, or DUNE, is currently under construction 48 kilometers (30 miles) underground at the Fermi National Accelerator Laboratory in Batavia, Illinois. Once completed, it will be able to shoot neutrino beams through the Earth’s mantle.

Have we run out of dark matter because our understanding of physics is flawed?

Physicists love neutrinos for many reasons. They provide a direct link between us and the core of the sun, allowing scientists to peer into nuclear fusion processes without having to place detectors in space. But perhaps the most intriguing thing about neutrinos is that they oscillate between flavors, like a chameleon changing color in response to its surroundings. A particle that begins as an electron neutrino, for example, can turn into a tau or muon neutrino, and vice versa.

The gaps in the timing of these oscillations recorded by the experiment in Russia, and others like it before it, suggest that there is a fourth flavor we are missing. This hypothetical particle could also very well be an important constituent of dark matter.

But that does not mean that a fourth type of elementary particle is the only explanation. The results of the experiment also raise the intriguing possibility that our current theoretical framework that describes neutrinos is flawed. That wouldn’t be bad news at all. Science is a constant work in progress in which the status quo is always supplemented by compelling new evidence. In the process, the scientific institution becomes stronger and more credible, as well as better equipped to answer increasingly complex questions about nature.

The findings appeared in the Physical examination letters.

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