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A revised calculation of the neutrino-capture reaction in gallium may account for the long-standing gallium anomaly by lowering the predicted capture cross section. The proposal stays within known-particle physics and could reduce or remove the apparent need for a sterile neutrino, but it has not been independently confirmed. A separate 2026 calculation reaches the problem through a different treatment of electron wave functions.
What the gallium anomaly is
The gallium anomaly is a persistent mismatch between radioactive-source calibration measurements and the number of neutrino captures predicted for gallium-71 (71Ga). GALLEX, SAGE and BEST exposed gallium targets to intense, well-characterized electron-neutrino sources made from chromium-51 (51Cr) and argon-37 (37Ar). Both isotopes decay by electron capture, producing low-energy electron neutrinos whose source activity can be measured.
The experiments recorded fewer captures than calculations expected. A 2025 analysis describes the shortfall as approximately 20%. In the BEST experiment, the observed-to-expected ratios were 0.791 ± 0.05 and 0.766 ± 0.05 for its two very short source-to-detector baselines, according to a 2023 cross-section analysis.
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Those ratios are not direct measurements of a new particle. They compare the measured number of 71Ge atoms produced in the target with a theoretical expectation. If the expected capture cross section is too large, the ratio will look artificially low even when the neutrinos behave exactly as the Standard Model predicts.
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Why the predicted cross section is central
A capture cross section summarizes the probability that a neutrino will convert a gallium-71 nucleus into germanium-71. The predicted event count depends on the source intensity, geometry, target mass, radioactive decay data and that cross section. The anomaly ratio is effectively measured captures divided by the calculated captures.
That makes the nuclear and atomic calculation part of the experimental interpretation. Reducing the predicted cross section raises the expected-to-observed agreement without changing the measured number of events. It also changes how strongly the data favor oscillations into an unobserved sterile-neutrino state.
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What the 2025 calculation changes
Moving beyond factorized wave functions
The 2025 Physical Review Letters paper by M. Cadeddu, N. Cargioli, F. Dordei, L. Ferro, C. Giunti and M. Pitzalis challenges a conventional treatment in which the leptonic wave functions and nuclear transition information are factorized. That framework uses nuclear information connected to detailed balance.
The authors instead incorporate phenomenologically constrained Gamow–Teller transition densities and move beyond the standard factorization. Their result allows a substantially lower 71Ga capture cross section. In the authors’ wording, “the revised cross section can be significantly reduced, potentially resolving the gallium anomaly without invoking new physics.”
The mechanism is important: the calculation does not remove neutrinos, alter the source activity or add a new oscillation channel. It changes the denominator used to calculate the anomaly ratio. A sufficiently large, correctly justified reduction would make the observed capture counts less anomalous.
What the 2026 calculation adds
A full Dirac–Coulomb treatment of electrons
A related Physical Review D paper published in 2026 by Ferro, Giunti and collaborators takes a technically different route. Instead of relying on a leading-order approximation for electronic wave functions, it numerically solves the Dirac–Coulomb equation for both bound and continuum electron states.
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The calculation then reevaluates the gallium anomaly and the interpretation in terms of sterile-neutrino oscillations. Because it addresses the atomic-electron part of the reaction with a different approximation scheme, it provides an independent check on whether details of the capture calculation can account for part of the deficit.
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The two papers are related but are not the same calculation. One emphasizes nuclear Gamow–Teller transition densities and factorization; the other explicitly recalculates bound and continuum electron states. Agreement between independent approaches would be more persuasive than either result alone.
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How the leading explanations compare
| Explanation | Input being changed | Requires a new particle? | What would test it | Current status |
|---|---|---|---|---|
| Revised capture cross section | Nuclear transition densities, factorization assumptions and/or electron wave functions | No; it uses known nuclear and atomic physics | Independent gallium-capture measurements, improved nuclear calculations and checks using other transitions | Recent theoretical proposals requiring validation |
| Source or experimental issue | Source strength, geometry, detector response or another calibration input | No | Independent source assays, detector calibrations and replication with different setups | Not established as the general explanation |
| Sterile-neutrino oscillation | Neutrino propagation, with an additional mass state and mixing | Yes | Baseline and energy dependence, plus reactor- and solar-neutrino constraints | Common new-physics interpretation, but not a confirmed cause |
What has been ruled out—and what has not
One proposed explanation was that the gallium-71 decay product, germanium-71, had an unexpectedly long half-life, which would have altered the calibration. A 2024 half-life experiment measured 71Ge at 11.468 ± 0.008 days. Norman and collaborators concluded that this possibility is ruled out, writing that the result “leav[es] the anomaly’s origin as an open question.”
The measurement pattern itself is well established: multiple gallium experiments found below-expectation rates with radioactive sources. The cause remains unsettled. Review literature has found no simple explanation and has treated short-baseline sterile-neutrino oscillations as the most prominent new-physics option. U.S. Department of Energy educational material likewise presents the broad alternatives as an experimental or calculation artifact versus new physics.
Has the gallium anomaly been solved?
No. The 2025 and 2026 papers support a plausible calculation-based explanation under active evaluation, not a settled resolution. Their language is conditional: the 2025 paper says the revised cross section could “potentially” resolve the anomaly, while the 2026 paper describes a reassessment of its interpretation.
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What experiments and calculations should come next
Validate the reaction calculation
- Repeat or improve radioactive-source capture measurements with independently characterized 51Cr and 37Ar sources.
- Compare the revised Gamow–Teller transition densities with data from other nuclear transitions and charge-exchange measurements.
- Test whether the full Dirac–Coulomb electron treatment changes predictions consistently across related electron-capture reactions.
Test the oscillation alternative
- Look for the baseline or energy dependence expected if neutrinos oscillate into a sterile state.
- Combine gallium results with reactor-neutrino and solar-neutrino limits, which probe overlapping mixing parameters.
- Check whether one sterile-neutrino model can fit all data without conflicting with disappearance searches elsewhere.
Until those tests converge, the most accurate conclusion is narrower than either “a sterile neutrino has been discovered” or “the anomaly is over.” Gallium experiments have measured a real below-expectation pattern, and new calculations show that the predicted cross section may be lower than previously assumed. Determining whether that correction is sufficient is now an empirical and theoretical validation problem.
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