The Science
Scientists have confirmed possible evidence of a new elementary particle, the sterile neutrino. These particles, if they exist, interact only via gravity, not any of the other forces in the Standard Model of Particle Physics. The results from the Baksan Experiment on Sterile Transitions (BEST) confirm an anomaly found in earlier solar neutrino source experiments. BEST irradiated a tank of gallium, a soft silvery metal that is liquid at room temperature, using an intense source of neutrinos from decays of radioactive chromium. The neutrinos react in the gallium to produce the isotope germanium 71. This isotope can be extracted from the gallium and counted. The researchers found significantly smaller amounts of germanium than expected based on known nuclear physics. Scientists had found a similar gallium anomaly in a precursor experiment.
The Impact
The experiment found that the germanium 71 yield was 20% to 24% lower than expected, based on the intensity of the neutrino source and on scientists' knowledge of how neutrinos are absorbed. These findings are counter to theoretical predictions. However, they are consistent with earlier results on what scientists call the gallium anomaly. The researchers divided the target into inner and outer volumes to search for an indicator of neutrino oscillations. This is a known phenomenon in which an electron neutrino changes into another "flavor," such as a muon neutrino, which results from neutrinos having mass. The researchers did not observe signs of these oscillations. The origin of the anomaly remains a mystery.
Summary
BEST is an experiment more than a mile underground in the Baksan Neutrino Observatory in Russia's Caucasus Mountains. It was designed to explore the deficit of electron neutrinos (ne) previously reported in the four calibration experiments performed by the SAGE and GALLEX solar neutrino collaborations. In this study, researchers used about 47 metric tons of liquid gallium (Ga) metal, divided into two concentric zones, as the target for the absorption of neutrinos via the reaction 71Ga (ne,e)71Ge. They placed the chromium-51 neutrino source at the center of the gallium target, irradiating both zones. As the neutrino path length in each zone is about a meter, BEST has a high sensitivity to oscillations occurring on that scale, corresponding to differences in the square of neutrino masses of about 1 eV2 (a very small amount in the nuclear physics world). The researchers measured the strength of the source by calorimetry and other methods to a precision of better than 1%. The neutrino absorption cross section has a minimum value set by the known electron-capture lifetime of 71-germanium.
The persistence of this anomaly is puzzling. It could indicate either some unidentified experimental artifact that has so far evaded discovery or new physics capable of accounting for an unexpectedly large deficit of neutrinos.
Funding
This work is supported by the Department of Energy Office of Science, Office of Nuclear Physics and by the Federal Agency for Scientific Organizations, Ministry of Education and Science of Russian Federation, State Atomic Energy Corporation Rosatom.