Non-equilibrium dynamics in heavy ion collisions at low SIS energies

TL;DR

This paper uses the UrQMD model to analyze non-equilibrium dynamics in low-energy heavy ion collisions, focusing on collective flows and nuclear stopping, and how these are affected by the equation of state and collision terms.

Contribution

It demonstrates the sensitivity of flow observables to the potential and collision terms, highlighting the importance of momentum dependence and medium modifications in transport models.

Findings

Soft EoS with momentum dependence fits flow data well.

Flow observables are sensitive to potential and collision term modifications.

Medium-modified NNECS influences the collectivity in collisions.

Abstract

The Ultrarelativistic Quantum Molecular Dynamics (UrQMD) model, a microscopic transport model, is used to study the directed and elliptic collective flows and the nuclear stopping in Au+Au collisions at incident energies covered by INDRA and lower-energy FOPI experiments. It is seen clearly that these observables are sensitive to both, the potential terms (including iso-scalar and iso-vector parts as well as the momentum dependent term) in the equation of state (EoS) and the collision term (including the Pauli-blocking and the medium-modified nucleon-nucleon elastic cross section (NNECS)). The momentum modifications of both, the mean-field potentials and the density dependent NNECS, are found to be sensitive to the collectivity of heavy-ion collisions. At INDRA energies ($\le 150$~MeV/nucleon), the dynamic transport with a soft EoS with momentum dependence and with the momentum-modified density-dependent NNECS describes the directed flow exhibited by hydrogen isotopes ($Z=1$) emitted at mid-rapidity fairly well.