Gallium anomaly

A neutrino can turn a gallium atom into germanium. Scientists place a powerful neutrino source beside gallium and count the germanium produced. Why is the count roughly one fifth below the usual reaction prediction?[1]

Why do known neutrino sources produce fewer germanium atoms in gallium than the adopted capture calculation predicts? The reaction νe + 71Ga → 71Ge + e− depends on the nuclear transition and the electron wave functions. Its calibration uses 71Ge electron capture, a decay in which the nucleus absorbs an electron and becomes gallium. The September 2026 proposal integrates spatially structured nuclear transition densities with the lepton wave functions, instead of treating their contributions as separate factors.[1] [2]

The shortage could mean the reaction was overestimated, the experiment missed something, or neutrinos changed into a form the detector cannot catch. Counting fewer atoms alone does not choose among those explanations.[1] [2] [3]

Its compact, nodal profiles are phenomenological surrogates, not computed shell-model transition densities. Excited-state normalization also needs reassessment. A resolution must propagate these uncertainties together with extraction/counting systematics and test any oscillation model against reactor, solar and beta-decay constraints.[1] [2] [3]

This is a calibration puzzle with a known source, rather than an uncertain astronomical neutrino supply. Getting the reaction right is essential before treating a deficit as evidence for a new particle.[1] [3]

A deficit attributed to a new neutrino state would change the account of which particles carry the flux. A smaller nuclear capture probability would instead change how the detector converts that flux into a count. An independently constrained reaction prediction, rather than a density profile fitted to the deficit, is needed to tell whether unexplained neutrino disappearance remains.[2] [3]