Revised calculations may explain long-standing gallium neutrino anomaly

For more than 30 years, physicists have puzzled over a persistent shortfall in the number of neutrinos detected by gallium-based solar neutrino experiments such as SAGE and GALLEX. These detectors use large volumes of gallium-71, which captures electron neutrinos and transforms into radioactive germanium-71; the germanium atoms are then chemically extracted and counted. Consistently, the measured production rate of germanium-71 has fallen 20–25% below the rate predicted by standard solar models, a discrepancy known as the gallium anomaly.

One leading explanation has been the existence of a sterile neutrino, a hypothetical particle that does not interact via the weak force and would represent physics beyond the Standard Model. However, a study published in Physical Review Letters by a team led by Kostas Mavrokoridis of the University of Oxford and the Rutherford Appleton Laboratory proposes a more conventional solution. The researchers re-examined the nuclear physics inputs used to calculate the detection efficiency, focusing on the electron-capture decay rate of germanium-71 back to gallium-71.

The team found that the half-life of germanium-71 used in the original analyses was based on a 1980 measurement with a relatively large uncertainty. More recent precision measurements of the germanium-71 half-life, including a 2023 result from the Borexino collaboration, indicate a value about 1.5% shorter than the older figure. When this updated half-life is combined with refined calculations of the atomic electron wave functions at the nucleus — which affect the capture probability — the predicted germanium-71 production rate increases.

According to the new analysis, these revisions together account for most, if not all, of the observed deficit. The authors state that the gallium anomaly can be resolved within the framework of standard three-neutrino oscillations without invoking sterile neutrinos or other exotic physics. They emphasize that the remaining small uncertainty is comparable to the experimental and theoretical errors in the solar neutrino flux predictions themselves.

Independent experts note that while the revised nuclear data significantly reduce the tension, final confirmation will require the gallium experiments to repeat their calibrations using the updated constants. The BEST experiment at the Baksan Neutrino Observatory, which recently completed a new measurement campaign with an artificial neutrino source, is expected to provide a critical cross-check. If the new calculations hold, the gallium anomaly would join a list of past neutrino puzzles resolved by improved nuclear physics rather than new particles.

The finding illustrates how precision nuclear data can have outsized implications for fundamental physics searches. The gallium anomaly has been a primary motivation for several proposed sterile neutrino experiments; a conventional resolution would redirect those efforts. Researchers caution that sterile neutrinos are not ruled out by this work, but the specific anomaly that supported their existence in the gallium channel appears to have a standard explanation.

The study has been accepted for publication in Physical Review Letters and is available as a preprint. The collaboration plans further work to quantify the impact of atomic screening effects and to coordinate with experimental groups on updated calibration procedures.