Deep underground experiment results confirm an anomaly: possible new fundamental physics

New results from the Baksan Barren Transitions Experiment (BEST) confirm an anomaly suggesting a new physical possibility.

Neutrino sterile, fundamentals of physics among the interpretations of abnormal results.

New scientific results confirm an anomaly found in previous experiments, which could point to a new yet unconfirmed elementary particle, the sterile neutrino, or indicate the need for a new interpretation of an aspect of Standard Model physics, like the neutrino crossing section, first measured 60 years ago. Los Alamos National Laboratory is the lead US institution collaborating on the Baksan Experiment on Sterile Transitions (BEST) experiment, the results of which were recently published in the journals Physical examination letters and Physical examination C.

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

More than a mile underground in the Baksan Neutrino Observatory in Russia’s Caucasus Mountains, BEST used 26 irradiated discs of Chromium 51, a synthetic radioisotope of Chromium and the source of 3.4 megacuries of neutrinos electronics, to irradiate an inner and outer tank of gallium, a silver metal also used in previous experiments, but previously in a single-tank configuration. The reaction between chromium-51 electron neutrinos and gallium produces the isotope germanium-71.

The measured rate of germanium-71 production was 20-24% lower than predicted based on theoretical modeling. 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 a major contributor, beginning in the late 1980s. also used gallium and high intensity neutrino sources. The results of this experiment and others indicated an electron neutrino deficit – a discrepancy between predicted and actual results that became known as the “gallium anomaly”. One interpretation of the deficit could be evidence of oscillations between electron neutrino and sterile neutrino states.

Chrome discs

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

The same anomaly recurred in the BEST experiment. Possible explanations again include oscillation in a sterile neutrino. The hypothetical particle may constitute a significant portion of dark matter, a prospective form of matter believed to make up the vast majority of the physical universe. This interpretation, however, may require further testing, as the measurement for each tank was approximately the same, although 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 needs reworking. 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 experiment methodology was carefully reviewed to ensure that no errors were made 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.


“Results of the Baksan experiment on sterile transitions (BEST)” by VV Barinov et al., June 9, 2022, Physical examination letters.
DOI: 10.1103/PhysRevLett.128.232501

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

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

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