Decomposition of sensitivity of the symmetry energy observables

TL;DR

This study decomposes the sensitivity of symmetry-energy observables in heavy-ion collisions, revealing they mainly probe densities around 1.25 to 1.5 times nuclear saturation density, with limited sensitivity at higher densities.

Contribution

It provides the first detailed analysis of the density regions that symmetry-energy-sensitive observables probe in heavy-ion collisions.

Findings

Observables mainly probe ~1.25 to 1.5 times saturation density.

Effects of symmetry energy at low densities are small but observable.

Limited sensitivity to symmetry energy at super-density regions.

Abstract

To exactly answer which density region that some frequently used symmetry-energy-sensitive observables probe, for the first time, we make a study of decomposition of the sensitivity of some symmetry-energy-sensitive observables. It is found that for the Au+Au reaction at incident beam energies of 200 and 400 MeV/nucleon, frequently used symmetry-energy-sensitive observables mainly probe the density-dependent symmetry energy around 1.25$\rho_{0}$ (for pionic observables) or 1.5$\rho_{0}$ (for nucleonic observables). Effects of the symmetry energy in the low-density region is in general small but observable. The fact that the symmetry-energy-sensitive observables are not sensitive to the symmetry energy in the maximal baryon-density region increases the difficulty of studying nuclear symmetry energy at super-density.