Directly probing existence of α -cluster structure in Ne 20 by relativistic heavy-ion collisions

Can relativistic heavy-ion collisions only probe the global shape of colliding nuclei, or their detailed internal structure as well Taking Ne20 as an example, we attempt to directly probe its internal α-cluster structure, by comparing experimentally measured observables in collisions at relativistic energies from density distributions of Ne20 with and without α-cluster structure. Since the two density distributions give the same nucleus size and deformation, they lead to similar midrapidity observables. However, the α-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 α 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 ultracentral Ne20+Ne20 collisions, which is found to be reduced by about 25% at sNN=7 TeV and about 20% at sNN=200 GeV with α-cluster structure in Ne20. This scaled yield ratio thus serves as a robust and direct probe of the existence of α-cluster structure in Ne20 free from the uncertainty of midrapidity dynamics.