The sterile neutrino, a new elementary particle that has not yet been confirmed, or the need for a new interpretation of a feature of the standard model of physics, such as the neutrino cross section, which was first measured 60 years ago, are both indicated by recent scientific findings that support an anomaly observed in earlier experiments.
The Baksan Experiment on Sterile Transitions (BEST) experiment is being led by Los Alamos National Laboratory in the United States, and its findings have just been 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 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.”
The Los Alamos group also includes Ralph Massarczyk and Inwook Kim. 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 setup, more than a mile underground in the Baksan Neutrino Observatory in Russia’s Caucasus Mountains.
The isotope germanium 71 is created when the electron neutrinos from chromium 51 and gallium interact.
According to theoretical modeling, the measured rate of germanium 71 production was 20–24% lower than anticipated. This mismatch fits the anomaly observed in earlier investigations.
BEST is based on the Soviet-American Gallium Experiment (SAGE), a solar neutrino experiment in which Los Alamos National Laboratory played a significant role beginning in the late 1980s. High intensity neutrino sources and gallium were also used in that experiment.
The findings of that and other experiments pointed to an electron neutrino deficiency, which became known as the “gallium anomaly” because of the disparity between expected and observed results.
The results are very exciting. 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.
Steve Elliott
The deficiency could be interpreted as evidence for oscillations between the sterile neutrino and electron neutrino states. The BEST experiment saw a recurrence of the same oddity. The oscillation into a sterile neutrino is one of the potential explanations once more.
The speculative particle might be a significant component of dark matter, a hypothetical type of substance that is assumed to make up the vast bulk of the universe’s physical matter. The measurement for each tank was nearly the same, though lower than anticipated, so that interpretation could need further testing.
Another reason for the anomaly is that the theoretical inputs to the experiment may have been misinterpreted to mean that the physics itself needs to be revised. Elliott notes that no measurements of the electron neutrino’s cross section have ever been made at these energies.
The electron density at the atomic nucleus, for instance, is a difficult to confirm theoretical input for measuring the cross section.
To guarantee that no mistakes were made in the experiment’s technique or in the positioning of the radiation sources or the way the counting system worked, for example. If the experiment is continued, a new radiation source with greater energy, a longer half-life, and sensitivity to shorter oscillation wave lengths might be used.
