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 60 years ago. Los Alamos National Laboratory is the lead American
institution collaborating on the Baksan Experiment on Sterile Transitions
(BEST) experiment, 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 of 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 what this means is not obvious. There are now conflicting
results about sterile neutrinos. If the results indicate fundamental nuclear
or atomic physics are misunderstood, that would be very interesting, too."
Other members of the Los Alamos team include Ralph Massarczyk and Inwook
Kim.
More than a mile underground in 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 inner and outer tank of gallium, a soft, silvery
metal also used in previous experiments, though previously in a one-tank
set-up. The reaction between the electron neutrinos from the chromium 51 and
the gallium produces the isotope germanium 71.
The measured rate of germanium 71 production was 20–24% lower than expected
based on theoretical modeling. That discrepancy is in line with the anomaly
seen 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, starting in 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 came to be known as the "gallium
anomaly." An interpretation of the deficit could be evidence for
oscillations between electron neutrino and sterile neutrino states.
The same anomaly recurred in the BEST experiment. The possible explanations
again include oscillation into a sterile neutrino. The hypothetical particle
may constitute an important part of dark matter, a prospective form of
matter thought to make up the vast majority of the physical universe. That
interpretation may need further testing, though, because the measurement for
each tank was roughly the same, though 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 at these energies. For
example, a theoretical input to 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 no errors
were made in aspects of the research, such as radiation source placement or
counting system operations. Future iterations of the experiment, if carried
out, may include a different radiation source with higher energy, longer
half life, and sensitivity to shorter oscillation wave lengths.
References:
V. V. Barinov et al, Results from the Baksan Experiment on Sterile
Transitions (BEST), Physical Review Letters (2022).
DOI: 10.1103/PhysRevLett.128.232501
V. V. Barinov et al, Search for electron-neutrino transitions to sterile
states in the BEST experiment, Physical Review C (2022).
DOI: 10.1103/PhysRevC.105.065502
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Physics