This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: Glimpses of the "gallium anomaly" were first seen in the mid-1990s, but it was first clearly discovered and stated in 2007 by neutrino physicist Carlo Giunti of Italy. The problem is simple to state but potentially very important: about 20% fewer electron neutrinos were seen to be captured by gallium nuclei than expected.
But now the anomaly looks to be explained with a new calculation by a collaboration of physicists at Italian universities and research laboratories—and the calculation agrees with the observed number of electron neutrinos captured. Their work is published in the journal Physical Review Letters. When an electron neutrino (one of the three types of neutrinos) collides with a neutron in the nucleus of gallium-71 (⁷¹Ga), an electron is produced and the neutron transforms to a proton as a down quark converts to an up quark.
The gallium nucleus then becomes a germanium-71 (⁷¹Ge) nucleus. This is a weak force interaction with a W+ particle exchange, a so-called charged-current weak interaction, and was historically important to the solar neutrino problem recognized in the 1960s. The 20% puzzle persisted for years.
One interpretation took it as evidence for oscillations between electron neutrinos and so-called sterile neutrinos; the latter were hypothesized to interact only by their meager gravitational force, not by any of the other three forces in the Standard Model. The gallium anomaly was experimentally reaffirmed as recently as 2022 by the Baksan Experiment on Sterile Transitions with an international group of collaborators. "The gallium anomaly has been something of a personal obsession of mine for a very long time," lead author Matteo Cadeddu of the National Institute for Nuclear Physics in Italy told Phys.org.
He worked on it during his master's thesis under the supervision of Giunti. "I was never completely convinced that sterile neutrinos were the only possible explanation." Indeed, the sterile neutrino explanation for the gallium anomaly conflicted with other neutrino results such as reactor antineutrino experiments, a bound found for solar neutrinos, neutrino mass measurements and more. After leaving the problem for some time, Cadeddu started again about three years ago, "essentially from zero ... reconsidering the problem as broadly as possible: the theoretical prediction, the nuclear and atomic physics entering it, the experimental procedure, and any possible systematic effect that could mimic or contribute to the anomaly." Eventually he involved some master's students—current and former—in the project, many of whom are now Ph.D. students and postdoctoral physicists working with him on the topic.
Theoretical physics can be a long and lonely road that one follows only in their mind. "We explored many possibilities that led nowhere," Cadeddu said. "At some point you find yourself questioning things that are so standard that you normally no longer notice that they are assumptions." Although the theoretical calculation of the reaction had been honed and refined over the years, one issue that remained was a simplification that could be improved: the behavior of the gallium nucleus had been separated from that of the neutrino and electron.
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