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Results of deep underground experiments confirm anomaly: possible new fundamental physics

New results from the Baksan Experiment on Sterile Transitions (BEST) experiment confirm anomaly suggesting a new physics possibility.

Sterile neutrino, physics fundamentals under interpretations of anomalous results.

New scientific results confirm an anomaly observed in previous experiments, which may indicate an as-yet unconfirmed new elementary particle, the sterile neutrino, or the need for a new interpretation of some aspect of standard model physics, such as the neutrino cross-section, 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 Assessment Letters and Physical assessment C

“The results are very exciting,” said Steve Elliott, principal analyst for one of the teams evaluating the data and a member of the Los Alamos physics division. “This definitely confirms the anomaly we’ve seen in previous experiments. But what this means is not clear. There are conflicting results now on: sterile neutrinos† If the results show that fundamental nuclear or atomic physics is misunderstood, that 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 Russia’s Caucasus Mountains, the completed two-zone gallium target, left, contains an inner and outer tank of gallium, which is irradiated by an electron neutrino source. Credit: AA Shikhin

More than a mile underground at the Baksan Neutrino Observatory in Russia’s Caucasus Mountains, BEST used 26 irradiated disks of chromium 51, a synthetic radioisotope of chromium and the 3.4 megacurie source of electron neutrinos, to build an inner and outer tank. of gallium, a soft, silvery metal that was also used in previous experiments, although previously in a single-tank setup. The reaction between the electron neutrinos of the chromium 51 and the gallium produces the isotope germanium 71.

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

BEST builds on a solar neutrino experiment, the Soviet-US Gallium Experiment (SAGE), to which the Los Alamos National Laboratory made major contributions, beginning in the late 1980s. That experiment also used high-intensity gallium and neutrino sources. The results of that experiment and others indicated a shortage of electron neutrinos — a discrepancy between the predicted and the actual results that came to be known as the “gallium anomaly.” An interpretation of the deficit could provide evidence for oscillations between electron neutrino and sterile neutrino states.

Chrome discs

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

The same anomaly came back in the BEST experiment. Possible explanations include oscillation again in a sterile neutrino. The hypothetical particle may be an important part of dark matter, a future form of matter thought to make up the vast majority of the physical universe. That interpretation may need further testing, as the reading for each tank was roughly the same, albeit lower than expected.

Other explanations for the anomaly include the possibility of a misunderstanding in the theoretical input to the experiment — that the physics itself needs to be reworked. Elliott points out that the electron neutrino cross section 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 thoroughly revised to ensure that no errors were made in aspects of the study, such as the placement of radiation sources or the operation of the counting system. Future iterations of the experiment, if performed, may include a different radiation source with higher energy, longer half-life, and sensitivity to shorter oscillation wavelengths.


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

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

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

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