Results from deep underground experiments confirm anomaly: possible new basic physics

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

Sterile neutrino, basic 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 neutrino, or indicate the need for a new interpretation of an aspect of standard model physics, such as the neutrino cross section, first measured for 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 review 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 Physics Division. “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 indicate that basic nuclear physics 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 completed two-zone gallium target, on the left, contains an inner and outer tank of gallium, which is irradiated with an electron neutrino source. Credit: AA Shikhin

More than a kilometer underground at the Baksan Neutrino Observatory in Russia’s Caucasus Mountains, BEST 26 used irradiated disks of chromium 51, a synthetic radioisotope of chromium and the 3.4 megacurie source of electron neutrinos, to irradiate an inner and outer soft gallium tank, a soft gallium tank. , silver-colored metal also used in previous experiments, but earlier in a one-tank setup. The reaction between the electron neutrinos from chromium 51 and gallium produces the isotope germanium 71.

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

BEST is based on a solar energy neutrino experiment, the Soviet-American gallium experiment (SAGE), of which the Los Alamos National Laboratory was a major contributor from 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 became known as “gallium anomaly.” An interpretation of the deficit may be evidence of fluctuations between electron neutrino and sterile neutrino states.

Chromium disks

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

The same anomaly was repeated 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 potential form of matter that is believed to constitute the vast majority of the physical universe. However, this interpretation may need further testing, because the measurement for each tank was 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 the electron neutrino has never been measured by 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 experiment’s methodology was thoroughly reviewed to ensure that no errors were made in aspects of the research, such as the location of radiation sources or counting system operations. Future iterations of the experiment, if performed, may include another radiation source 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 conditions in the BEST experiment” by VV Barinov et al., June 9, 2022, Physical review C.
DOI: 10.1103 / PhysRevC.105.065502

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