Gallium Anomaly

A tally that refuses to balance: about 20% fewer gallium captures than predicted.

The Physics

Put a known electron-neutrino source inside gallium and count the germanium-71 atoms it makes. GALLEX and SAGE measured a combined measured/predicted ratio of 0.87 ± 0.05. BEST measured 0.79 ± 0.05 in its inner volume and 0.77 ± 0.05 in its outer volume, with no evidence of distance dependence.

⁷¹Ga + ν_e → ⁷¹Ge + e⁻; R = 0.80 ± 0.05 — about 4σ below 1.

• ⁷¹Ga and ⁷¹Ge: gallium-71 and germanium-71 nuclei.
• ν_e: electron neutrino; e⁻: electron.
• R: measured germanium-71 production rate divided by the predicted rate (dimensionless).
• σ here: standard deviations, expressing statistical significance; not the capture cross-section.
• Capture threshold: 232.4 keV (approximately 3.723 × 10⁻¹⁴ J).
• Germanium-71 half-life: 987 552 s (11.43 days).
• keV: kiloelectronvolt; MCi: megacurie, with 1 MCi = 3.7 × 10¹⁶ Bq (decays per second).

Honest caveat: These are fewer captures, not a count of “missing neutrinos”. The printed 0.80 ± 0.05 and about 4σ belong together; ratios and significance depend on the capture cross-section used. Sterile neutrinos? Tension with reactor, solar and KATRIN data. A wrong capture cross-section? Still debated. Cadeddu and co-authors’ 2026 nuclear-physics proposal is not independently confirmed. OPEN QUESTION.

Sources

• Barinov et al. (BEST), Phys. Rev. Lett. 128, 232501 (2022), doi:10.1103/PhysRevLett.128.232501.
• Barinov et al., Phys. Rev. C 105, 065502 (2022), doi:10.1103/PhysRevC.105.065502.
• Elliott, Gavrin & Haxton, “The gallium anomaly,” Prog. Part. Nucl. Phys. 134, 104082 (2024), doi:10.1016/j.ppnp.2023.104082.
• Cadeddu et al., Phys. Rev. Lett. 137, 131805 (2026), doi:10.1103/5pvf-mcr1 (proposed resolution).
• CERN Courier, “Two strikes for the light sterile neutrino” (14 January 2026).

On the shirt

Front:

“MISSING: 20%”

“THE GALLIUM ANOMALY · UNSOLVED”

The missing fraction refers to expected captures. Back:

“THE GALLIUM ANOMALY”

“⁷¹Ga + ν_e → ⁷¹Ge + e⁻ (threshold 232.4 keV; ⁷¹Ge half-life 11.43 days)”

“Put a known neutrino source inside a tank of gallium. Count the germanium atoms it makes.”

“GALLEX and SAGE (1990s, ⁵¹Cr and ³⁷Ar sources): measured/predicted = 0.87 ± 0.05”

“BEST (Baksan, 2019, 3.414 MCi of ⁵¹Cr, two nested gallium volumes):”

“ inner R = 0.79 ± 0.05 outer R = 0.77 ± 0.05”

“All gallium source experiments combined: 0.80 ± 0.05 - about 4σ below 1.”

“About 20% of the expected captures are missing, near and far alike.”

“Sterile neutrinos? Clash with reactor, solar and KATRIN data. A wrong ⁷¹Ga capture cross-section? Still debated.”

“OPEN QUESTION.”

“Barinov et al. (BEST), Phys. Rev. Lett. 128, 232501 (2022); Phys. Rev. C 105, 065502 (2022).”

Printed front and back. Physics. Personified.