Orateur
Description
The equation of state (EoS) of neutron-rich matter is a cornerstone for understanding the structure and dynamics of neutron stars, from their radii and tidal deformability to cooling mechanisms. However, the density dependence of the nuclear symmetry energy term remains a major source of uncertainty, particularly at densities near, and above, nuclear saturation density. While astrophysical observations (e.g., gravitational waves, pulsar measurements) provide integrated constraints, laboratory experiments at GANIL energies are uniquely suited to probe the EoS between 0.5 and 1.5 saturation density, bridging the gap between nuclear structure and astrophysics.
In this contribution, I will present the EoS constraint recently published by the INDRA-FAZIA collaboration [1]. By combining the excellent isotopic resolution of FAZIA with a model-independent centrality reconstruction, we were able to extract the evolution of the isospin transport ratio (ITR) measured in 58,64Ni+58,64Ni collisions at 32 MeV/nucleon as a function of the impact parameter [2]. We then compared the experimental ITR data to predictions from the BUU@VECC-McGill transport model [3], employing state-of-the-art nuclear functionals, including ab initio chiral-EFT interactions and phenomenological models (SGII, NL3, SAMI) to extract a constraint on the symmetry energy (S(ρ)). A consistent study of the time evolution of baryonic density and isospin current density identifies the density region most sensitive to the ITR, peaking near saturation density.
Our analysis yields a precise constraint on the symmetry energy (S = 29.1±1.1 MeV) and slope parameter (L = 38.3±7.0 MeV) at saturation density. These results exclude stiff symmetry energy behaviors and align with softer ab initio predictions. This work strengthens the synergy between nuclear experiments at GANIL and neutron star physics, offering a critical input for Bayesian inferences of the EoS in astrophysical environments.
[1] Ciampi et al., Phys. Lett. B 868 (2025) 139815
[2] Ciampi et al., Phys. Rev. C 111 (2025) 044601
[3] Mallik et al., J. Phys. G: Nucl. Part. Phys. 49 (2022) 015102.