Two tin nuclei collide at about two-thirds the speed of light. For an instant, a hot, dense cloud forms, and protons fly out in all directions. If you catch pairs of protons and measure how often they fly off in the same direction, you can work out the size of that cloud. This method is called femtoscopy: it works like a microscope for nuclear scales.
In the experiment, two variants were compared: tin nuclei with a neutron excess and those with a neutron deficit. In the former, the proton cloud turned out to be noticeably larger: a radius of 2.22 femtometers versus 1.74 femtometers. A femtometer is one millionth of one billionth of a meter. The 24% difference looks modest, but the original nuclei themselves differed in size by only 3%. Something puffed up the cloud in the neutron-rich system.
Ordinary calculations that only account for the mean field of nuclear forces do not reproduce this enhancement. So we need to include pairwise interactions — what physicists call 'beyond the mean field.' This is an important step: we now have a tool to study short-range correlations in nuclear matter that are hard to catch by other means.
Collisions of radioactive beams with precise femtoscopy open a new path. Previously, such correlations were inferred from electron scattering on light nuclei; now we can probe them in the hot, dense environment created in the lab. This brings us closer to understanding how matter behaves in neutron stars, where neutrons are even more abundant.
Femtoscopy is like measuring the size of a firework by its flashes: if sparks often fly in pairs in the same direction, they must have been born close together, and from the frequency of such coincidences, you reconstruct the size of the birth region.
In Bruce Sterling's novel Schismatrix, nuclear reactions and isotopes play a key role in politics and technology: there, precise knowledge of nuclear properties shifts the balance of power.