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The results of a deep underground experiment confirm the anomaly: a possible new fundamental physics

New results from the Bakan Experiment on Sterile Transitions (BEST) confirm an anomaly that suggests a new possibility in physics.

Sterile neutrinos, basics of physics among interpretations of anomalous results.

New scientific results confirm an anomaly seen in previous experiments, which may indicate a still unconfirmed new elementary particle, a sterile neutrino, or indicate the need for a new interpretation of one aspect of standard model physics, such as the neutrino cross. cross-section, first measured 60 years ago. The Los Alamos National Laboratory is a leading US institution collaborating on the Baku Experiment on Sterile Transitions (BEST), the results of which were recently published in journals Physical Review Letters i Physical examination C.

“The results are very exciting,” said Steve Eliot, a leading analyst at one of the data assessment teams and a member of the Los Alamos physics department. “This definitely confirms the anomaly we saw in previous experiments. But what this means is not obvious. There are now conflicting results on sterile neutrinos. If the results show that fundamental nuclear or atomic physics is misunderstood, it would also be very interesting. Other members of the Los Alamos team are Ralph Massarczik and Invook Kim.

BEST gallium target

Located deep underground at the Baksan Neutrino Observatory in the Caucasus Mountains of Russia, the completed dual-zone gallium target, on the left, contains an inner and outer gallium reservoir, which is irradiated with an electronic neutrino source. Credit: AA Shikhin

More than a mile underground at the Baksan Neutrino Observatory in Russia’s Caucasus Mountains, BEST used 26 irradiated chromium disks 51, a synthetic chromium radioisotope and a 3.4-megacurium electronic neutrino source to irradiate the soft inner and outer chromium tanks. , silver metal has also been used in previous experiments, but previously in a single-tank setup. The reaction between electron neutrinos from chromium 51 and gallium produces the isotope germanium 71.

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

BEST is based on the solar neutrino experiment, the Soviet-American Gallium Experiment (SAGE), in which the Los Alamos National Laboratory has made great contributions since the late 1980s. The experiment also used gallium and high-intensity neutrino sources. The results of that experiment and others showed a lack of electronic neutrinos – a discrepancy between the predicted and actual results that became known as the “gallium anomaly”. Interpretation of deficits can be evidence for oscillations between electronic neutrinos and sterile neutrinos.

Chromium drives

The set of 26 irradiated chromium disks 51 is a source of electronic neutrinos that react with gallium and produce germanium 71 at a speed that can be measured in relation to the predicted speeds. Credit: AA Shikhin

The same anomaly was repeated in the BEST experiment. Possible explanations again include oscillations in sterile neutrinos. A hypothetical particle can form an important part of dark matter, a potential form of matter thought to make up the vast majority of the physical universe. This interpretation may require additional testing, as the measurement for each tank was approximately the same, although lower than expected.

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

The methodology of the experiment was thoroughly reviewed to ensure that no errors were made in aspects of the research, such as the placement of the radiation source or the operation of the counting system. Future iterations of the experiment, if conducted, may include a different radiation source with higher energy, longer half-life, and sensitivity to shorter oscillation wavelengths.


“Results of the Bakan Experiment on Sterile Transitions (BEST)”, V. В. Barinov et al., June 9, 2022, Physical Review Letters.
DOI: 10.1103 / PhisRevLett.128.232501

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

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

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