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

Sterile neutrinos, Fundamentals of physics among explanations of anomalous results.

The new scientific findings confirm an anomaly seen in previous experiments, which may point to a new as-yet-confirmed elementary particle, the sterile neutrino, or point to the need for a new explanation for 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 lead US institution collaborating on the Baksan Experiment on Sterile Transformations (BEST), the results of which were recently published in journals. Physical Review Letters And the physical review c.

“The results are very exciting,” said Steve Elliott, a senior analyst on one of the teams evaluating the data and a member of the physics department at Los Alamos. “This certainly reconfirms the anomalies we saw in previous experiments. But what this means is not clear. There are now conflicting results about sterile neutrinos. If the results indicate a misunderstanding of basic nuclear or atomic physics, that would also be interesting.” Other members of the Los Alamos team include Ralph Masarczyk and Enuk Kim.

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

More than a mile underground at the Baksan Neutrino Observatory in the Russian Caucasus Mountains 26 radioactive disks of chromium 51, an artificial radioactive isotope of chromium and a 3.4 megapicurie source of electron neutrinos, are best used for gallium inner and outer tank radiation, soft material, silver metal Also in previous experiments, although previously it was used in a single tank. The reaction between the electron neutrinos of 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. This discrepancy is consistent with anomalies seen in previous experiments.

BEST is based on the solar neutrino experiment, the Soviet-American Gallium Experiment (SAGE), to which Los Alamos National Laboratory was a major contributor, beginning in the late 1980s. That experiment also used high-density gallium and neutrino sources. Results from that experiment and others indicated a deficit in electron neutrinos – a discrepancy between expected and actual results that has come to be known as the “gallium anomaly”. The explanation for the deficit could be evidence of oscillations between electron neutrinos and sterile neutrino states.

An array of 26 radioactive chromium-51 discs is the source of the electron neutrinos that interact with gallium and produce germanium-71 at rates that can be measured against expected rates. Credit: AA Shikhin

The same anomaly was repeated in the best experiment. Possible explanations again include oscillation in a sterile neutrino. A hypothetical particle may make up a significant part of dark matter, a possible form of matter believed to make up the vast majority of the physical universe. This interpretation may need further testing, because the measurement for each tank was nearly the same, albeit less than expected.

Other explanations for the anomaly include the possibility that there is a misunderstanding in the theoretical input to the experiment – that physics itself requires reformulation. Elliott points out that the cross-section of the electron neutrino has not previously been measured at these energies. For example, the theoretical entry for the measurement of the cross section, which is difficult to confirm, is the electron density in the atomic nucleus.

The experiment’s methodology was carefully reviewed to ensure that errors did not occur in aspects of the research, such as radiation source placement or counting system operations. 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.

References:

“Results of the Baksan Experiment on Sterile Transformations (Better)” By V.V. Barinov et al., 9 Jun 2022, Available here. Physical Review Letters.
DOI: 10.1103/ PhysRevLett.128.232501

“Searching for electron-neutrino transitions to sterile states in the best experiment” By V. V. Barinov et al., 9 Jun 2022, Available here. physical review c.
DOI: 10.1103/ PhysRevC.105.065502

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