Results of underground experiments confirm anomaly: possible new fundamental physics

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

Sterile neutrino, fundamentals of physics between interpretations of anomalous results.

New scientific results confirm an anomaly seen in previous experiments, which may point to a new as-yet-unconfirmed elementary particle, the sterile neutrino, or indicate the need for a new interpretation of an aspect of standard model physics, such as the cross section of neutrinos. , measured for the first time 60 years ago. Los Alamos National Laboratory is the main American 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, lead analyst on one of the teams evaluating the data and a member of the Physics division at Los Alamos. “This definitely reaffirms the anomaly we’ve seen in previous experiments. But what that means is not obvious. There are now conflicting results about sterile neutrinos. If the results indicate that fundamental nuclear or atomic physics is poorly understood, that would also be very interesting.” Other members of the Los Alamos team include Ralph Massarczyk and Inwook Kim.

BEST gallium target

Located deep within the Baksan Neutrino Observatory in the Caucasus Mountains of Russia, the completed two-zone gallium target, left, contains an internal and external tank of gallium, which is radiated by an electron neutrino source. Credit: AA Shikhin

Over 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 irradiate an internal tank and external gallium, a silvery, metal also used in previous experiments, although previously in a one-tank configuration. The reaction between chromium 51 electron neutrinos 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 in agreement with the anomaly observed in previous experiments.

BEST is based on a solar neutrino experiment, the Soviet-American Gallium Experiment (SAGE), to which the Los Alamos National Laboratory was a major contributor, starting in the late 1980s. This experiment also used sources of gallium and high-intensity neutrinos. The results of this experiment and others indicated an electron neutrino deficit – a discrepancy between the predicted and actual results that came to be known as the “gallium anomaly.” One interpretation of the deficit may be evidence of oscillations between the 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 was repeated in the BEST experiment. Possible explanations again include the oscillation in a sterile neutrino. The hypothetical particle could make up an important part of dark matter, a prospective form of matter thought to make up the vast majority of the physical universe. This interpretation may need further testing, however, because the measurement for each tank was roughly the same, albeit smaller than expected.

Other explanations for the anomaly include the possibility of a misunderstanding in the theoretical inputs to the experiment – ​​that physics itself requires rework. Elliott points out that the electron neutrino cross section has never been measured at these energies. For example, a theoretical input for measuring cross-section, which is difficult to confirm, is the electron density in the atomic nucleus.

The methodology of the experiment was thoroughly 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.

References:

“Results of 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 states 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.

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