This is the fourth part of the Anomalies and Mysteries series. We turn to two short-baseline disappearance anomalies that have plagued the field for decades and remain partially unresolved: the reactor antineutrino anomaly and the gallium anomaly.
The reactor antineutrino anomaly
Reactor antineutrino experiments measure the inverse-beta-decay rate at known distances from a nuclear reactor. The expected rate depends on:
- The reactor’s thermal power and isotopic composition.
- The antineutrino energy spectrum per fission per isotope (calculated from nuclear physics).
- The inverse-beta-decay cross section (known to ~1%).
- The detector efficiency.
Most of these are known to ~1% or better. The dominant uncertainty has historically been the antineutrino spectrum per fission.
In 2011, Mention, Lasserre, and others re-evaluated the spectrum based on updated nuclear-physics inputs. The new predictions were about 3% higher than the old ones. Combined with high-statistics short-baseline measurements, this revealed a ~6% deficit: measurements at 10-100 meter baselines were about 6% lower than the new predictions.
The interpretation: either the flux predictions were still wrong, or short-baseline antineutrinos were oscillating into a sterile state.
The implication: sterile oscillation at $\Delta m^2 \sim 1$ eV²
Standard three-flavor oscillation predicts essentially no electron-antineutrino disappearance at the 10-meter scale. The atmospheric oscillation is too long-baseline to act at meters; the solar oscillation needs kilometers.
A fourth (sterile) state at $\Delta m^2 \sim 1$ eV² would produce disappearance at exactly the right L/E. The fitted parameters would be similar to those required to explain LSND.
The story converged on a four-neutrino picture: $\Delta m^2_{14} \sim 1$ eV², $|U_{e4}|^2 \sim 0.02$.
The short-baseline reactor experiments
Several experiments were built specifically to test the reactor anomaly at very short baselines:
PROSPECT (USA, ~7-10 m from a research reactor at Oak Ridge).
STEREO (France, ~10 m from the high-flux research reactor at Grenoble).
DANSS (Russia, segmented detector at ~10-12 m baselines, with adjustable height).
Neutrino-4 (Russia, claimed at one point to see oscillation signal at marginal significance).
The combined results through ~2023 mostly disfavored the sterile-oscillation interpretation. The data preferred mundane flux-prediction corrections.
The flux predictions resolved
Meanwhile, work on the nuclear-physics inputs has continued. Improved measurements of the beta spectra of fission products — especially uranium-235 — have shown that the 2011 predictions were probably too high. The “anomaly” was, in large part, a flux-prediction error.
The Daya Bay collaboration measured rate ratios for different reactor fuel mixes, isolating the contributions from individual fissioning isotopes. Their measurements suggested that the uranium-235 prediction was specifically off by 7-8%, with plutonium predictions accurate. This was incorporated into refined flux models.
After these corrections, much of the original reactor anomaly disappears. Residual ~1-2% tensions remain in some short-baseline data but are statistically modest.
Status (2026): Largely resolved into nuclear-physics corrections. Sterile-oscillation interpretation strongly disfavored.
The gallium anomaly: a separate puzzle
Gallium-based solar neutrino detectors (GALLEX, SAGE) needed periodic calibration using radioactive sources of known activity. The setup: a $^{51}$Cr or $^{37}$Ar source placed inside the detector, producing a known flux of $\nu_e$ at $\sim 0.7$ MeV. The detector’s response is the ratio of measured-to-predicted rate.
The expected ratio is 1. The measured ratios from GALLEX (1994-1997) and SAGE (1996-2006) calibration runs averaged about 0.85 — a 15% deficit.
The deficit was puzzling but not alarming on its own. Cross-section uncertainties of ~10% could plausibly explain it.
BEST sharpens it
The Baksan Experiment on Sterile Transitions (BEST) was built specifically to test the gallium anomaly with higher statistics. BEST used a 3.4 MCi $^{51}$Cr source and a two-zone gallium detector, allowing both an “inner” and “outer” rate measurement with different effective baselines.
BEST’s first results were published in 2022: a measured-to-predicted ratio of about 0.79 ± 0.05 — a ~4σ deficit. The inner and outer measurements agreed.
The result was unwelcome. The deficit was now too large to easily explain by cross-section uncertainties alone. Sterile-neutrino oscillation at $\Delta m^2 \gtrsim 1$ eV² with $|U_{e4}|^2 \sim 0.1$ remained the most natural sterile interpretation, but the required parameters were in tension with reactor short-baseline limits and with cosmology.
Where things stand
The reactor antineutrino anomaly: largely absorbed into flux-prediction corrections. Residual tension is small.
The gallium anomaly: real, sharp, unresolved. Possible explanations:
- Cross-section uncertainty: Maybe the $\nu_e + {}^{71}\text{Ga}$ cross section is genuinely lower than calculated, due to nuclear-structure effects in the daughter $^{71}$Ge that haven’t been fully accounted for.
- Source-activity calibration: Maybe the radioactive source activities were systematically over-estimated. (Unlikely, since multiple independent calibrations were done.)
- Sterile oscillation: Most natural fit to the data shape, but in tension with other constraints.
The community is currently divided. Most physicists lean toward the cross-section explanation. A minority view holds that BEST’s evidence for sterile oscillation should be taken seriously.
What would close the question
Future experiments that could decisively close (or confirm) the gallium anomaly:
- More gallium calibration campaigns with different isotopes or geometries.
- Independent cross-section measurements of $\nu_e$-Ga via $\beta^+$ decay of $^{71}$Ge.
- The Short-Baseline Neutrino program at Fermilab — if any sterile signal at $\Delta m^2 \sim 1$ eV² exists, SBN will see it.
- JUNO short-baseline near-detectors could rule out the corresponding parameter space.
The next part of this series turns to the most famous example of a “false alarm” in neutrino physics: OPERA’s 2011 superluminal episode.
Frequently asked
What is the reactor antineutrino anomaly?
A ~6% deficit in measured reactor antineutrino rates at short baselines (10-100 m) compared to flux predictions, first highlighted by Mention et al. in 2011. The deficit could indicate sterile-neutrino oscillation at Δm² ~ 1 eV². However, refined flux predictions and short-baseline measurements have largely absorbed the original anomaly into nuclear-physics corrections.
What is the gallium anomaly?
A ~20% deficit in gallium-based detection of radioactive calibration sources (chromium-51 or argon-37), compared to nominal cross-section predictions. First seen in GALLEX/SAGE calibration runs in the 1990s; sharpened by BEST in 2022 to ~4σ. The deficit could indicate sterile-neutrino oscillation at similar Δm² to the reactor anomaly, but other explanations (cross-section uncertainty, source activity calibration) remain possible.
Are these the same anomaly as LSND/MiniBooNE?
All four are potentially explained by the same sterile-neutrino mass-squared splitting around 1 eV². But the experiments measure different mixing parameters (LSND/MiniBooNE: |U_μ4|, gallium/reactor: |U_e4|), so the combined parameter-space fit is non-trivial. Global fits show tension between the appearance-oriented anomalies (LSND, MiniBooNE) and the disappearance-oriented anomalies (reactor, gallium).
What's the current best guess?
The reactor anomaly is largely resolved by updated flux predictions and short-baseline disappearance experiments — most of the deficit is nuclear-physics, not new physics. The gallium anomaly remains genuinely puzzling. BEST's recent result keeps it alive at ~4σ. Most physicists currently lean toward mundane explanations (cross-section issues), but the gallium anomaly is not closed.
Cite this article 5 formats
APA
Neutrino Times Editorial Team. (2026, March 18). Anomalies and Mysteries — Part 4: The reactor and gallium anomalies. Neutrino Times. https://neutrino-times.com/articles/anomalies-part-4-reactor-gallium/
Chicago
Neutrino Times Editorial Team. "Anomalies and Mysteries — Part 4: The reactor and gallium anomalies." Neutrino Times, March 18, 2026. https://neutrino-times.com/articles/anomalies-part-4-reactor-gallium/.
MLA
Neutrino Times Editorial Team. "Anomalies and Mysteries — Part 4: The reactor and gallium anomalies." Neutrino Times, 18 Mar. 2026, https://neutrino-times.com/articles/anomalies-part-4-reactor-gallium/.
BibTeX
@misc{neutrino-times-anomalies-part-4-reactor-gallium,
author = {Neutrino Times Editorial Team},
title = {Anomalies and Mysteries — Part 4: The reactor and gallium anomalies},
howpublished = {Neutrino Times},
year = {2026},
month = {mar},
url = {https://neutrino-times.com/articles/anomalies-part-4-reactor-gallium/},
note = {Accessed: 2026-03-18}
} RIS
TY - GEN TI - Anomalies and Mysteries — Part 4: The reactor and gallium anomalies AU - Neutrino Times Editorial Team PY - 2026 DA - 2026-03-18 PB - Neutrino Times UR - https://neutrino-times.com/articles/anomalies-part-4-reactor-gallium/ ER -