Extraction of Neutron-Skin Parameters in Zr+Zr and Ru+Ru Collisions at $\sqrt{s_{\text{NN}}}$ = 200 GeV

Abstract

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.