Constraining the neutron skin of $^{208}$Pb with anisotropic flow in Pb+Pb collisions at the LHC

Abstract

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.