Inside the atomic nucleus, protons and neutrons are packed so tightly that the forces between them resemble tightly coiled springs. Some springs compress and expand equally in all directions: these are the familiar central forces. Others, the tensor forces, behave more cunningly: their tension depends on how the particles are oriented relative to each other. These 'angular' springs have puzzled physicists for decades: their contribution is hard to isolate from other interactions.
The authors found an elegant way to measure the tension of these springs. They took pairs of mirror nuclei — for example, nickel-54 and iron-54, where the number of protons in one equals the number of neutrons in the other. Such nuclei are nearly identical, but a tiny difference in their charge radii (on the order of hundredths of a femtometer) reveals the contribution of tensor forces. It turned out that the radius difference is especially sensitive to the odd component of the tensor interaction — the one that changes sign when particles are exchanged. This allowed them to refine the parameters of the popular SLy5 and SGII models without resorting to complex nuclear excitation experiments.
Now, knowing the tension of the 'odd springs,' physicists can more accurately calculate the properties of nuclear matter — from the stability of heavy elements to processes in neutron stars. And the mirror-nucleus method becomes a universal tool: instead of complex excitations, one only needs to measure the static size of a twin nucleus.
Mirror nuclei are rare: fewer than three dozen are known in nature. But they, like perfect laboratory twins, allow us to isolate effects that are masked in ordinary nuclei.