Peeling away neutron skin in ultracentral collisions of relativistic nuclei

Abstract

In heavy nuclei the ratio between local densities of neutrons and protons increases towards the nuclear periphery. This phenomenon is known as the neutron skin (NS), and it is characterized by a subtle difference (< 0.5 fm) between the r.m.s radii of the distributions of neutrons and protons. We show that the existence of NS in 208 Pb translates into the excess of spectator neutrons in ultracentral 208 Pb-208 Pb collisions. The yields of spectator neutrons and protons in such collisions at the CERN SPS and LHC were calculated within a new version of Abrasion-Ablation Monte Carlo for Colliders model (AAMCC-MST) taking into account NS and pre-equilibrium clustering of spectator matter. The average numbers of spectator neutrons and protons in ultracentral collisions vary insignificantly in calculations with different parameterizations of neutron and proton densities. In contrast, the cross sections of emission of certain numbers of spectrator neutrons in events with 0,1,...5 spectator protons are changed by 50-250% depending on the thickness of NS and relations between the halfdensity radii of neutron and proton density distributions and diffuseness parameter for neutrons. We argue that the measurements of such cross sections make it possible to restrict the existing variety of neutron density parameterizations in 208 Pb. These cross sections can be proposed for measurements by the ALICE experiment at the LHC by means of ALICE Zero Degree Calorimeters.