Spectator nucleons in ultracentral $^{208}$Pb–$^{208}$Pb collisions as a probe of nuclear periphery

Abstract

We study the composition of spectator matter in ultracentral collisions of heavy relativistic nuclei with neutron-rich periphery known as a neutron skin (NS). The yields of spectator neutrons and protons in ultracentral 208 Pb-208 Pb collisions at the LHC were calculated within a new version of Abrasion-Ablation Monte Carlo for Colliders (AAMCC) model. Calculations demonstrate variations of the cross sections ( , ) to produce given numbers of spectator neutrons and protons depending on the parameters of NS in 208 Pb, in particular, for events with low . The relative difference between ( , ) obtained with different NS thickness reaches 150%. Furthermore, the values of ( , ) calculated with same difference between neutron and proton RMS-radii, but with different neutron half-density radius and diffuseness parameter, differ by 45%. The considered cross sections can be measured in the ALICE experiment at the LHC for 208 Pb-208 Pb to restrict the parameters of NS in colliding nuclei.