A drop of water stretches in a strong electric field. The deuterium nucleus, a bound pair of a proton and a neutron, behaves the same way, except here the field comes from a passing gamma ray, and instead of just stretching, the nucleus can simply fall apart. At the SLEGS laboratory in Shanghai, this process was studied for the first time over a dense grid of energies, from 2.33 to 19.65 MeV, recording how often the nucleus gives way.

From these records, the electric polarizability was extracted — a number that tells how easily the nucleus deforms under an electric field. Previously, it was obtained indirectly from particle scattering, and the result disagreed with theory. Now the Baldin sum rule was applied — an old but reliable method that links the breakup of the nucleus to its ability to stretch. This was done for the first time on a continuous set of experimental points, with no gaps or guesswork.

The number fits theoretical predictions perfectly, and the long-standing dispute is over. The magnetic part of the polarizability, which cannot be measured directly, was taken from effective field theory calculations. The resulting precision is now a benchmark for models describing how the proton and neutron stick together. And that means the mechanism at work inside all atomic nuclei is a little better understood.