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

Plasma Particles Physics Art Concept

New results from the Baksan Experiment on Sterile Transitions (BEST) experiment confirm the anomaly, which points to new physical possibilities.

Sterile neutrino, physical principles and interpretations of anomalous results.

New scientific results confirm an anomaly observed in previous experiments that could point to an as yet unconfirmed new elementary particle, the sterile neutrino, or the need to reinterpret an aspect of Standard Model physics such as the neutrino cross section, first measured 60 years ago. Los Alamos National Laboratory is the leading American institution collaborating on the Baksan Experiment on Sterile Transitions (BEST), the results of which were recently published in the scientific journals Physical Verification Letters and Physical Check C.

“The results are very exciting,” said Steve Elliott, senior analyst on one of the teams analyzing the data and a member of the Los Alamos physics department. “This definitely confirms the anomaly we saw in previous experiments. But what that means is not obvious. There are now conflicting results about sterile neutrinos. If the results indicate that there is a misunderstanding of fundamental nuclear or atomic physics, that would be very interesting too.” 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 dual-zone gallium target (left) contains an inner and outer tank of gallium irradiated by an electron-neutrino source. Credit: AA Shikhin

More than a mile underground at the Baksan Neutrino Observatory in the Russian Caucasus, BEST used 26 irradiated discs of Chromium 51, a synthetic radioisotope of Chromium, and the 3.4 megacurie source of electron neutrinos to encircle an inner and outer tank gallium, a soft, silvery metal that was also used in previous experiments, but previously in a one-tank setup. The reaction between the electron neutrinos from 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. This discrepancy is consistent with the anomaly observed in previous experiments.

BEST builds on a solar neutrino experiment, the Soviet-American Gallium Experiment (SAGE), in which Los Alamos National Laboratory was instrumental from the late 1980s. Gallium and high-intensity neutrino sources were also used in this experiment. The results of this and other experiments pointed to an electron neutrino deficit – a discrepancy between the predicted and actual results that has become known as the “gallium anomaly”. An interpretation of the deficit could give hints for oscillations between electron-neutrino and sterile neutrino states.

chrome disks

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

The same anomaly reappeared in the BEST experiment. Possible explanations include oscillation into a sterile neutrino. The hypothetical particle could represent an important piece of dark matter, a future form of matter thought to make up most of the physical universe. However, this interpretation may need further testing, as the reading for each tank was about the same, albeit lower than expected.

Other explanations for the anomaly include the possibility of a misunderstanding in the theoretical inputs to the experiment – that the physics themselves need to be revised. Elliott points out that the electron-neutrino cross section has never been measured at these energies. A theoretical input for the cross-section measurement that is difficult to confirm is, for example, the electron density at the atomic nucleus.

The experiment’s methodology was thoroughly checked 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 involve a different radiation source with higher energy, longer half-life, and sensitivity to shorter oscillation wavelengths.

References:

“Results from the Baksan Experiment on Sterile Transitions (BEST)” by VV Barinov et al., June 9, 2022, Physical Verification 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 Check C.
DOI: 10.1103/PhysRevC.105.065502

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