New Symmetry Energy Constraint from a Model-Independent Measurement of Isospin Diffusion with INDRA-FAZIA

Caterina Ciampi (GANIL, France)

Heavy-ion collisions at intermediate energies are an essential tool for probing the properties of nuclear matter in far-from-equilibrium conditions. Among other topics, they allow the investigation of isospin transport phenomena, which can provide information on the symmetry energy term of the Nuclear Equation of State (NEoS). This area of research holds significant interest due to its implications in both nuclear physics and astrophysics [1]. However, extracting quantitative constraints from comparisons with transport model predictions requires careful consideration of several factors, such as the choice of observables and ensuring comparable conditions between experimental and simulated data. In particular, an accurate treatment of reaction centrality is crucial to properly account for the latter aspect.

In this seminar, a model-independent experimental measurement of isospin diffusion in ^58,64Ni+^58,64Ni collisions at 32MeV/nucleon is presented. This result has been obtained by combining two datasets having common characteristics, but bearing complementary information. The first dataset has been acquired with the INDRA setup [2] and used to implement a model-independent reconstruction of the impact parameter [3], while the second one has been acquired in the first experimental campaign of the coupled INDRA-FAZIA apparatus at GANIL [4]. The neutron-to-proton content of the quasiprojectile remnant measured by FAZIA [5] has been employed as isospin observable. The isospin transport ratio technique [6] has been employed to highlight the effect of isospin diffusion, and the evolution towards equilibration as a function of the impact parameter of the collision is clearly extracted [7].

The experimental result is then compared with predictions from the BUU@VECC-McGill transport model [8], which, through the metamodeling approach [9], employs various nuclear equation of state parametrizations from the literature. In particular, two extreme χ-EFT interactions were tested, and a good agreement is found within the chiral constraint [10]. Additionally, the BUU@VECC-McGill model was used to study the isospin current and baryonic densities evolution during the collision, in order to provide a consistent determination of the density region significantly probed by the experiment.

Finally, we present the resulting symmetry energy constraint from the new INDRA-FAZIA isospin diffusion experimental assessment, which can be used to inform Bayesian inference of the neutron star EoS.

References

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[10] C. Ciampi, S. Mallik et al., submitted (2025).