Two nuclei with the same number of particles but different neutron skin shapes collide at speeds close to the speed of light. Ruthenium-96 and zirconium-96 are like twins, one with a slightly thicker coat. This difference changes how particles carry charge in the fireball of the collision.
Scientists used the UrQMD model to simulate collisions and compared with STAR detector data. They varied the proton surface diffuseness parameter for zirconium and found the best match for the net charge difference ΔQ. It turned out that the neutron skin noticeably changes the ratio of baryon charge transport to charge $R_B$/Q. This explains why in central collisions $R_B$/Q is greater than one, even without the baryon junction hypothesis.
as written by the authors arXiv:2608.13935v1
The initial nuclear geometry of colliding nuclei, manifested in proton and neutron density profiles, plays a key role in determining final-state effects in high-energy heavy-ion collisions. We study $${^{96}\text{Ru}}+{^{96}\text{Ru}}$$ and $${^{96}\text{Zr}}+{^{96}\text{Zr}}$$ collisions at $$\sqrt{s_{\text{NN}}}=200$$ GeV incorporating an isospin-dependent Woods-Saxon distribution to account for neutron-skin effects in the UrQMD model. By systematically scanning the proton surface diffuseness parameter $$a_{\text{p,Zr}}$$ of the $$^{96}\text{Zr}$$ nucleus and performing a $$\chi^2$$ analysis by comparing with STAR experimental data, we extract the relative neutron-skin thickness difference $$0.278 \pm 0.018$$ fm between the two isobaric systems using the net-charge yield difference $$\Delta Q$$. We further show that the neutron-skin effect evidently modifies the baryon-to-charge transport ratio $$R_{B/Q}=\langle B \rangle/\Delta Q \times \Delta Z/A$$, and substantially influences the observed $$R_{B/Q}>1$$ behavior in central collisions. This work indicates that nuclear structure effects contribute non-negligibly to baryon transport observables without a baryon junction hypothesis, and underscores the importance of precise initial-state geometry constraints for exploring novel QCD phenomena in relativistic heavy-ion collisions.
The neutron skin of zirconium-96 is thicker than that of ruthenium-96, even though they have the same number of neutrons and protons.