A new calculation could reduce the predicted probability that a gallium-71 nucleus captures a neutrino, bringing theory closer to the lower counts measured in radioactive-source tests. The 2025 Physical Review Letters paper says this may resolve the gallium anomaly without sterile neutrinos, while a related 2026 Physical Review D study recalculates the electron wave functions used in the analysis.
Neither paper establishes that the anomaly is solved. Both present recently proposed calculations that require independent validation against capture data and other nuclear transitions.
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What is the gallium anomaly?
The gallium anomaly is a persistent mismatch between the number of neutrino captures measured in gallium experiments and the number predicted before the measurements were made. GALLEX, SAGE and BEST exposed gallium to intense, well-characterized radioactive sources and counted the germanium-71 atoms produced when electron neutrinos were captured by gallium-71:
νe + 71Ga → 71Ge + e−
The sources were chromium-51 (51Cr) and argon-37 (37Ar). Their electron-capture decays produce low-energy electron neutrinos, allowing the source activity, neutrino energy and exposure time to be calculated independently of a reactor or the Sun.
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The observed rate is commonly expressed as an observed-to-expected ratio. A ratio below 1 means fewer captures were recorded than the model predicted. A 2023 cross-section analysis reported BEST ratios of 0.791 ± 0.05 and 0.766 ± 0.05 for its two very short source-to-detector baselines. A 2025 analysis described the combined shortfall as approximately 20%.
Why the denominator matters
The anomaly ratio is calculated by dividing measured captures by the predicted capture rate. That prediction depends on the neutrino-capture cross section—the probability that a neutrino will trigger the gallium reaction. If the cross section is overestimated, the expected count is too high and the apparent deficit becomes larger even when the measured count is unchanged.
What does the new calculation change?
The 2025 nuclear-transition treatment
The 2025 Physical Review Letters work by M. Cadeddu, N. Cargioli, F. Dordei, L. Ferro, C. Giunti and M. Pitzalis questions a conventional treatment used to calculate the gallium-71 capture cross section. That treatment factorizes the leptonic wave functions and uses nuclear-transition information in a way connected to detailed balance.
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The authors instead move beyond that factorization and use phenomenologically constrained Gamow–Teller transition densities. Their result permits a substantially lower capture cross section. Lowering the predicted cross section lowers the expected number of captures, which can reduce or remove the difference between prediction and observation without adding a new neutrino state.
The paper’s wording is deliberately conditional: the revised cross section can be “significantly reduced, potentially resolving the gallium anomaly without invoking new physics.” It is a possible explanation, not a measurement showing that the revised value is correct.
The 2026 electron-wave-function calculation
A related but technically distinct 2026 Physical Review D paper by Ferro, Giunti and collaborators revisits the electronic part of the problem. Instead of relying on a leading-order approximation for the electron wave functions, the authors numerically solve the Dirac–Coulomb equation for both bound and continuum electron states, then reevaluate the gallium anomaly and the sterile-neutrino interpretation.
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Because the 2025 and 2026 studies change different pieces of the calculation, they should not be treated as one identical correction. Their common implication is that atomic and nuclear inputs in the standard cross-section estimate may deserve more scrutiny before the deficit is attributed to new particles.
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An unexpectedly long germanium half-life
One proposed conventional explanation was that germanium-71 decayed more slowly than assumed, causing experiments to miscount the atoms created by neutrino capture. A 2024 half-life measurement by E. B. Norman and collaborators found 11.468 ± 0.008 days for 71Ge. The result ruled out an unexpectedly long half-life as the source of the gallium discrepancy.
That result narrows the list of explanations, but it does not choose between a cross-section error, an experimental or source-related issue, and new neutrino physics.
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How the main explanations differ
| Explanation | Input being changed | Requires a new particle? | Key test |
|---|---|---|---|
| Revised nuclear cross section | Gamow–Teller transition densities and the treatment of nuclear matrix elements | No; it uses known particles and nuclear theory | Independent measurements of gallium capture and related nuclear transitions |
| Revised electronic calculation | Bound and continuum electron wave functions, calculated with the Dirac–Coulomb equation | No | Independent atomic-physics calculations and capture-rate comparisons |
| Sterile-neutrino oscillation | The neutrino propagation model, adding disappearance into an unobserved state | Yes; a sterile-neutrino state is required | Baseline- or energy-dependent disappearance, checked against reactor and solar-neutrino constraints |
| Source or experimental artifact | Source activity, detector efficiency, chemical recovery or another calibration input | No | Independent source assays, detector calibrations and replicated source experiments |
Reviews have described short-baseline sterile-neutrino oscillations as the most commonly discussed new-physics explanation. The U.S. Department of Energy’s explainer likewise frames the broad choice as an experimental problem or physics beyond the standard model. A cross-section revision changes the expected rate instead of making neutrinos disappear during flight.
Has the gallium anomaly been solved?
No. The measurements form a real, repeatable below-expectation pattern, but the cause remains unsettled. The 2025 and 2026 papers provide plausible calculation-based resolutions under active evaluation; their titles and conclusions do not claim a definitive solution.
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For the calculation explanation to become established, researchers would need to show that the revised ingredients reproduce the gallium data while also remaining consistent with independent measurements of the relevant nuclear transitions and electron wave functions. A correction that fits gallium alone would not be enough.
What evidence would decide between the options?
Tests of the cross-section explanation
- Measure neutrino-capture rates with independently characterized sources and detectors.
- Check the same Gamow–Teller transition inputs in other nuclear reactions instead of fitting only the gallium data.
- Compare independent calculations of bound and continuum electron states, including the effects treated in the Dirac–Coulomb solution.
Tests of sterile-neutrino oscillations
- Look for a change in the event rate with source-to-detector baseline.
- Search for the predicted dependence on neutrino energy.
- Require compatibility with reactor- and solar-neutrino constraints, not just the gallium results.
Tests of experimental explanations
- Reassay the activity of chromium-51 and argon-37 sources.
- Repeat chemical extraction, counting and detector-efficiency checks with independent procedures.
- Compare results across different laboratories and source geometries.
What readers should take from the new papers
The important change is not a newly discovered neutrino. It is the possibility that the expected gallium capture rate has been calculated too high because approximations in the nuclear or electronic parts of the cross-section model were too restrictive. If independent work confirms a lower cross section, the approximately 20% deficit could be an accounting problem in the prediction rather than evidence for sterile-neutrino oscillations.
For now, the strongest accurate description is: the gallium anomaly is an established measurement pattern with an unresolved cause, and new 2025–2026 calculations offer a credible no-new-particles explanation that still needs independent confirmation.
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