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

TL;DR

The paper tackles constraining neutron skin thickness $delta_np$ in heavy nuclei by exploiting extreme collision geometries in heavy-ion collisions. It introduces two observables tied to the initial neutron-proton asymmetry: (1) the count of spectator neutrons in ultra-central collisions measured by ZDCs, and (2) a peripheral collision pion yield asymmetry $R = (Y_{pi_minus} - Y_{pi_plus})/(Y_p - Y_pbar)$, analyzed with MC Glauber and AVFD hydrodynamics. It demonstrates that these observables are sensitive to $delta_np$ for $^{208}$Pb and $^{48}$Ca and further proposes asymmetric $^{40}$Ca+$^{48}$Ca collisions to provide an additional constraint, notably via forward-backward spectator-neutron asymmetry. The results indicate that these measurements could substantially improve current $delta_np$ constraints from PREX/CREX and offer an independent experimental probe using existing detectors, with potential extensions to other neutron-rich nuclei using radioactive beams.

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.

Figures (5)

• Figure 1: Histogram for the number of spectator neutrons in super-central ($0\sim0.1\%$) PbPb collisions at $\sqrt{s_{NN}}=5.02~\rm TeV$ from simulations with thickness $\delta_{np}=0.3\ \rm fm$ (red) and $\delta_{np}=0\ \rm fm$ (black), respectively.
• Figure 2: The average number of spectator neutrons (either forward or backward) versus neutron skin thickness $\delta_{np}$ in super-central PbPb and CaCa collisions at $\sqrt{s_{NN}}=5.02~\rm TeV$, with red curve for $0\sim0.1\%$, black curve for $0\sim 1\%$. The green (blue) shaded bands indicate projected observable range based on current PREX (CREX) results for $\delta_{np}$ of $^{208}$Pb ($^{48}$Ca), respectively.
• Figure 3: The yield asymmetry observable $R$ (as defined in text) versus neutron skin thickness $\delta_{np}$ in peripheral PbPb and CaCa collisions at $\sqrt{s_{NN}}=5.02 ~\rm TeV$, with red curve for $70\sim 80 \%$, black curve for $50\sim 60\%$. The green (blue) shaded bands indicate projected observable range based on current PREX (CREX) results for $\delta_{np}$ of $^{208}$Pb ($^{48}$Ca), respectively.
• Figure 4: Histogram of event-by-event difference between forward ($^{48}$Ca-going direction) and backward ($^{40}$Ca-going direction) spectator neutrons in $0\sim 0.1 \%$ centrality $^{40}$Ca$^{48}$Ca collisions at $\sqrt{s_{NN}}=5.02~\rm TeV$ from simulations with thickness $\delta_{np}=0.2\ \rm fm$ (red) and $\delta_{np}=0\ \rm fm$ (black), respectively.
• Figure 5: The ratio between event-averaged forward and backward spectator neutrons versus neutron skin thickness $\delta_{np}$ in $^{40}$Ca$^{48}$Ca collisions at $\sqrt{s_{NN}}=5.02~\rm TeV$, with red curve for $0\sim 0.1 \%$, black curve for $0\sim 1\%$, and green shaded bands for projected range based on current CREX results for $\delta_{np}$.