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

TL;DR

This paper proposes a method to directly detect alpha-cluster structures in $^{20}$Ne nuclei through specific spectator nucleon yield ratios in relativistic heavy-ion collisions, providing a new probe of nuclear internal structure.

Contribution

It introduces a novel experimental observable, the scaled yield ratio of spectator neutrons to charged particles, as a direct probe of alpha-cluster structures in nuclei during relativistic collisions.

Findings

Alpha-cluster structure reduces free spectator neutron yield by about 20-25%.

The scaled yield ratio is sensitive to alpha-cluster presence and is unaffected by mid-rapidity dynamics.

Proposed measurement can confirm alpha-cluster existence in $^{20}$Ne.

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 $\alpha$-cluster structure, by comparing experimentally measured observables in collisions at relativistic energies from density distributions of $^{20}$Ne with and without $\alpha$-cluster structure. Since the two density distributions give the same nucleus size and deformation, they lead to similar mid-rapidity observables. However, the $\alpha$-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 $\alpha$ 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 $\alpha$-cluster structure in $^{20}$Ne. This scaled yield ratio thus serves as a robust and direct probe of the existence of $\alpha$-cluster structure in $^{20}$Ne free from the uncertainty of mid-rapidity dynamics.