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Deep underground experimental results confirm anomaly: possible new basic physics

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New results from the Baksan Experiment on Sterile Transitions (BEST) experiment confirm anomalies that indicate a new physics possibility.

Sterile neutrino, the basics of physics among interpretations of abnormal results.

New scientific results confirm an anomaly seen in previous experiments, which may point to an as yet unconfirmed new elementary particle, the sterile neutrinone, or indicate the need for a new interpretation of an aspect of standard model physics, such as the neutrino cross section, which was first measured 60 years ago. Los Alamos National Laboratory is the leading US institution collaborating on the Baksan Experiment on Sterile Transitions (BEST) experiment, the results of which were recently published in the journals Physical review letters and Physical examination C.

“The results are very exciting,” said Steve Elliott, chief analyst for one of the teams evaluating the data and a member of the Los Alamos Department of Physics. “This definitely confirms the anomaly we have seen in previous experiments. But what this means is not obvious. There are now conflicting results about sterile neutrinos. If the results suggest that basic nuclear or atomic physics is misunderstood, it would also be very interesting. “Other members of the Los Alamos team include Ralph Massarczyk and Inwook Kim.

BEST Gallium Target

Located deep underground at the Baksan Neutrino Observatory in the Caucasus Mountains of Russia, the finished gallium target contains two zones, on the left, an inner and outer tank of gallium, which is irradiated by an electron neutrino source. Credit: AA Shikhin

More than a mile underground in the Baksan Neutrino Observatory in Russia’s Caucasus Mountains, BEST 26 used irradiated plates of chromium 51, a synthetic radioisotope of chromium and the source of 3.4 megacurie of electron neutrino, to irradiate an inner and outer soft gallium tank, a soft gallium tank. , silver-colored metal that has also been used in previous experiments, but previously in a single-tank arrangement. The reaction between the electron neutrins from chromium 51 and gallium produces the isotope germanium 71.

The measured rate of production of germanium 71 was 20-24% lower than expected based on theoretical modeling. That discrepancy is in line with the anomaly seen in previous experiments.

BEST is based on a solar neutrino experiment, the Soviet American gallium experiment (SAGE), where the Los Alamos National Laboratory was a major contributor, starting in the late 1980s. That experiment also used gallium and high-intensity neutrino sources. The results of that experiment and others indicated a deficit of electron neutrinos – a discrepancy between the predicted and the actual results that came to be known as “gallium anomaly.” An interpretation of the deficit may be evidence of oscillations between electron neutrinos and sterile neutrino states.

Chrome discs

A set of 26 irradiated sheets of chromium 51 is the source of electron neutrinos that react with gallium to produce germanium 71 at velocities that can be measured at predicted velocities. Credit: AA Shikhin

The same anomaly recurred in the BEST experiment. The possible explanations again include oscillation to a sterile neutrino. The hypothetical particle can form an important part of dark matter, a future form of matter that is believed to constitute the vast majority of the physical universe. However, that interpretation may need further testing, as the dimensions of each tank were approximately the same, but lower than expected.

Other explanations for the anomaly include the possibility of a misunderstanding in the theoretical inputs to the experiment – that the physics itself requires reworking. Elliott points out that the cross section of electron neutrinos has never been measured at these energies. For example, a theoretical input for measuring the cross section, which is difficult to confirm, is the electron density at the atomic nucleus.

The methodology of the experiment was carefully examined to ensure that no errors were made in aspects of the research, such as placement of radiation sources or operation of counting systems. Future iterations of the experiment, if performed, may include another source of radiation with higher energy, longer half-life, and sensitivity to shorter oscillation wavelengths.


“Results from the Baksan Experiment on Sterile Transitions (BEST)” by VV Barinov et al., June 9, 2022, Physical review letters.
DOI: 10.1103 / PhysRevLett.128.232501

“Search for electron-neutrino transitions to sterile states in the BEST experiment” by VV Barinov et al., June 9, 2022, Physical examination C.
DOI: 10.1103 / PhysRevC.105.065502

Funding: Department of Energy, Office of Science, Office of Nuclear Physics.

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