Influence of the treatment of initialization and mean-field potential on the neutron to proton yield ratios

TL;DR

This study improves the quantum molecular dynamics model by refining initial nuclear density and mean-field potential treatments, then investigates how these affect neutron-proton yield ratios to probe nuclear symmetry energy and effective mass splitting.

Contribution

The paper introduces a new initialization method linked to the mean-field potential and accurately calculates the three-body force term, enhancing the analysis of heavy ion collision observables.

Findings

Neutron to proton yield ratios can probe the slope of symmetry energy.

Low-energy $R(n/p)$ ratios are sensitive to symmetry energy slope.

High-energy $R(n/p)$ ratios can inform on symmetry energy at suprasaturation densities.

Abstract

In this work, we firstly investigate how to reproduce and how well one can reproduce the Woods-Saxon density distribution of initial nuclei in the framework of the improved quantum molecular dynamics model. Then, we propose a new treatment for the initialization of nuclei which is correlated with the nucleonic mean-field potential by using the same potential energy density functional. In the mean field potential, the three-body force term is accurately calculated. Based on the new version of the model, the influences of precise calculations of the three-body force term, the slope of symmetry energy, the neutron-proton effective mass splitting, and the width of the wave packet on heavy ion collision observables, such as the neutron to proton yield ratios for emitted free nucleons [$R(n/p)$] and for coalescence invariant nucleons [$R_{ci}(n/p)$] for $^{124}$Sn+$^{112}$Sn at the beam energy of 200 MeV per nucleon, are discussed. Our calculations show that the spectra of neutron to proton yield ratios [$R(n/p)$] can be used to probe the slope of symmetry energy ($L$) and the neutron-proton effective mass splitting. In detail, the $R(n/p)$ in the low kinetic energy region can be used to probe the slope of symmetry energy ($L$). With a given $L$, the inclination of $R(n/p)$ to kinetic energy ($E_k$) can be used to probe the effective mass splitting. In the case where the neutron-proton effective mass splitting is fixed, $R(n/p)$ at high kinetic energy can also be used to learn the symmetry energy at suprasaturation density.