Directly probing existence of $\hat{I}\pm$-cluster structure in ${}^{20}$Ne by relativistic heavy-ion collisions

Abstract

Can relativistic heavy-ion collisions only probe the global shape of colliding nuclei, or their detailed internal structure as well? Taking ${}^{20}$Ne as an example, we attempt to directly probe its internal $\hat{I}\pm$-cluster structure, by comparing experimentally measured observables in collisions at relativistic energies from density distributions of ${}^{20}$Ne with and without $\hat{I}\pm$-cluster structure. Since the two density distributions give the same nucleus size and deformation, they lead to similar mid-rapidity observables. However, the $\hat{I}\pm$-cluster structure may considerably reduce the free spectator nucleon yield and enhance the spectator light nuclei yield, as a result of more compact initial phase-space distribution of nucleons inside $\hat{I}\pm$ clusters. We propose to measure the scaled yield ratio of free spectator neutrons to charged particles with mass-to-charge ratio $A/Z=3$, 3/2, and 2 in ultra-central ${}^{20}$Ne+${}^{20}$Ne collisions, which is found to be reduced by about $25\%$ at $\sqrt{s_{\mathrm{NN}}}=7$ TeV and about $20\%$ at $\sqrt{s_{\mathrm{NN}}}=200$ GeV with $\hat{I}\pm$-cluster structure in ${}^{20}$Ne. This scaled yield ratio thus serves as a robust and direct probe of the existence of $\hat{I}\pm$-cluster structure in ${}^{20}$Ne free from the uncertainty of mid-rapidity dynamics.

6 pages, 4 figures