Is A Neutrino Anomaly Due To Bad Nuclear Physics Modeling?

It is often the case that a seemingly anomalous result is due to flawed model for making a theoretical prediction of the expected value. The "Gallium anomaly" in neutrino physics might be among them. But a new paper argues that this isn't the problem. This would open the door to beyond the Standard Model neutrino physics (something that I still very much doubt).

The gallium anomaly, a 20% deficit in the measured νe captures on 71Ga, remains unexplained. A recent work [Phys. Rev. Lett. 137, 131805 (2026), arXiv:2512.20560] has proposed a solution based on the breakdown of the factorization of lepton and nuclear wavefunctions that underlies the prediction of the cross section, combined with ad hoc Gamow-Teller nuclear transition densities. 

We test this proposed solution with transition densities calculated within the nuclear shell model (NSM), normalized to the 71Ge electron-capture half-life and convolved with Dirac-Hartree-Fock-Slater electron wavefunctions. We explore several NSM interactions, bare and effective Gamow-Teller operators, as well as the impact of the valence space. In all cases, we obtain remarkably consistent NSM transition densities. Our factorized and nonfactorized cross sections agree, remaining 20% higher than the measured value. In addition, we use NSM single-nucleon transition densities as ansatz and find that solving the anomaly requires fine-tuned densities, with percent-level cancellations and an overall scale orders of magnitude larger than the calculated NSM ones. We conclude that the gallium anomaly is not solved when using realistic nuclear transition densities.

D. Castillo, D. Frycz, J. Menéndez, "Possible Solution to the Gallium Anomaly? Nuclear Transition Densities" arXiv:2611.09788 (November 6, 2026).