When two lead nuclei collide at tremendous speed, they flatten and shatter into thousands of particles. The shape of this debris depends on how the nuclei were structured before the impact. Inside the nucleus, protons and neutrons are not distributed identically: neutrons stick out slightly beyond the edge: this is called the neutron skin. The thickness of this skin matters for understanding the densest objects in the universe, such as neutron stars.
Scientists simulated collisions of lead nuclei with different neutron skin thicknesses and compared the results with data from the ALICE detector. It turned out that the particle flow, measured as anisotropic flow, barely changes whether the skin is thin or medium. But if the skin is very thick, the calculated results no longer match the experiment. This means a very thick neutron skin is ruled out, but distinguishing between thin and medium is still not possible.
This 'geometric blindness' arises because the particle flow is mainly determined by the overall shape of the collision, not by fine details of the nuclear surface. To catch the difference, more precise measurements or other observables are needed. For now, we can say that the neutron skin of lead cannot be very thick, and that's already a step forward.
The thickness of the neutron skin of a lead-208 nucleus is estimated at just 0.1–0.2 femtometers — tens of thousands of times smaller than the diameter of an atom.
In Bruce Sterling's novel 'Schismatrix', neutron stars and their properties play a key role in interstellar politics — the thickness of the neutron skin could influence their behavior.