Large amplification of the isospin-dependence of proton emitting source size in radioactive heavy-ion collisions: a signal of n-p correlation

TL;DR

This study measures proton-proton correlations in heavy-ion collisions, revealing a significant isospin-dependent difference in proton source sizes linked to neutron-proton correlations beyond mean-field models.

Contribution

It provides experimental evidence of enhanced proton source size in neutron-rich systems due to short-range n-p correlations, challenging existing mean-field transport models.

Findings

Proton source size in neutron-rich system is ~24% larger than in neutron-deficient system.

Transport models based on mean-field dynamics fail to reproduce the observed amplification.

Results suggest short-range n-p correlations significantly influence proton emission in heavy-ion collisions.

Abstract

We report proton-proton correlation function measurements in central $^{132}$Sn+$^{124}$Sn and $^{108}$Sn+$^{112}$Sn collisions at 270 MeV/nucleon. The proton emitting source sizes are extracted for the systems by using femtoscopic imaging technique. The fast dynamic core radius for the neutron-rich system is found to be $2.22 \pm 0.13\ \text{(stat.)} \pm 0.07\ \text{(syst.)}$ fm, which is approximately 24\% larger than that for the neutron-deficient system, $1.74 \pm 0.08\ \text{(stat.)} \pm 0.05\ \text{(syst.)}$ fm. This difference is an order of magnitude larger than the $\sim$3\% difference in the ground-state charge radii of the projectile nuclei. Transport model simulations based on mean-field dynamics cannot reproduce this amplification. The observation reveals a beyond-mean-field mechanism associated to short-range neutron-proton correlations, which dynamically enhance the proton emitting source in the neutron-rich environment. Our results demonstrate that heavy-ion collisions induced by radioactive beam, combined with femtoscopic precision, provide a new hadronic probe of short-range correlation, and that careful treatment of the beyond-mean-field interactions are required in modeling such processes.