Model-independent measurement of isospin diffusion in Ni-Ni systems at intermediate energy

TL;DR

This study provides a model-independent experimental assessment of isospin diffusion in Ni-Ni collisions at 32 MeV/nucleon, revealing the evolution towards isospin equilibration across different collision centralities, and constraining the symmetry energy in nuclear matter.

Contribution

It introduces a model-independent method to evaluate isospin diffusion in heavy-ion collisions using combined datasets from INDRA and FAZIA detectors.

Findings

Isospin transport ratio varies with impact parameter.

Clear evidence of evolution towards isospin equilibration.

Results can constrain the symmetry energy term of the nuclear Equation of State.

Abstract

In this work we provide a model-independent experimental evaluation of the degree of isospin equilibration taking place in $^{58,64}$Ni+$^{58,64}$Ni collisions at 32 MeV/nucleon across varying reaction centralities. This result has been obtained by combining the complementary information provided by two different datasets, sharing common characteristics. The first dataset has been acquired with the INDRA setup and has been used to implement a model-independent reconstruction of the impact parameter. The second dataset has been acquired in the first experimental campaign of the coupled INDRA-FAZIA apparatus at GANIL. The neutron-to-proton content of the quasiprojectile remnant measured by FAZIA has been employed as isospin observable. The effect of isospin diffusion has been evidenced by means of the isospin transport ratio, reported as a function of the impact parameter of the collision. The evolution towards isospin equilibration from semiperipheral to more central collisions is clearly extracted. This experimental result, expanding our previous works (Phys. Rev. C 106, 024603 (2022) and Phys. Rev. C 108, 054611 (2023)), can be compared with the predictions of any transport model, and can thus be used to set constraints on the behavior of the symmetry energy term of the nuclear Equation of State at sub- to saturation densities.