Probing the Neutron Skin with Extreme Collision Geometries in Heavy-Ion Collisions

Abstract

Understanding how protons and neutrons are located differently in an atomic nucleus can provide fundamental information on nuclear structure and have far-reaching implications for astrophysics. A precise determination of this important difference, often quantified by the so-called neutron skin thickness, is challenging both theoretically and experimentally. Here we show how one can use a new category of observables in heavy ion collisions to probe the neutron skin thickness of nuclei like $^{208}$Pb and $^{48}$Ca, by utilizing the asymmetry between neutrons and protons of spectator nucleons in super-central collisions as well as that of participant nucleons in peripheral collisions. Using quantitative simulations, we demonstrate their sensitivity and great potential in constraining neutron skin thickness for both $^{208}$Pb and $^{48}$Ca nuclei in these extreme event geometries. Furthermore, we propose the asymmetric collisions between $^{48}$Ca and $^{40}$Ca nuclei as a unique and powerful way to nail down the neutron skin thickness.