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.
as written by the authors arXiv:2603.07597
We study neutron-skin effects of $$^{208}$$Pb in Pb+Pb collisions at $$\sqrt{s_{\mathrm{NN}}}=5.02$$~TeV using the improved string-melting version of a multi-phase transport model by varying the neutron density distribution. A systematic response is observed in both the initial eccentricities and the anisotropic flow, indicating that neutron-skin effects survive the full transport evolution of heavy-ion collisions. A $$\chi^2$$ comparison with ALICE data favors small to moderate neutron skin, while large neutron skin is excluded. The similar descriptions provided by zero and moderate neutron skin point to a geometric degeneracy in the current anisotropic flow in Pb+Pb collisions, where anisotropic flow is primarily driven by the overall collision geometry and size, thus lacking extreme sensitivity to the fine details of the nuclear surface profile. This highlights both sensitivity and limitation of constraining neutron-skin properties with flow measurements in Pb+Pb collisions at the LHC.
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.