Christoph Hartnack

We present a new approach to identify fragments using computer simulations of relativistic heavy ion collisions. It is based on the simulated annealing technique and can be applied to $n$-body transport models such as quantum molecular dynamics. This new approach is able to predict isotope yields as well as hypernucleus production. In order to illustrate its predicting power, we confront this new method with experimental data and show the sensitivity to the parameters which govern cluster formation.

Recently it has been discovered that the elliptic flow, $v_2$, of composite charged particles emitted at midrapidity in heavy-ion collisions at intermediate energies shows the strongest sensitivity to the nuclear equation of state (EoS), which has been observed up to now within a microscopic model. This dependence on the nuclear EoS is predicted by quantum molecular dynamics (QMD) calculations [A. Le Fèvre et al.. Nucl. Phys. A 945, 112 (2016)], which show as well that the absorption or rescattering of in-plane emitted particles by the spectator matter is not the main reason for the EoS dependence of the elliptic flow at midrapidity but different density gradients (and therefore different forces) in the direction of the impact parameter ($x$ direction) as compared to the direction perpendicular to the reaction plan ($y$ direction), caused by the presence of the spectator matter. The stronger density gradient in the $y$ direction accelerates the particles more and creates therefore a negative $v_2$. When using a soft momentum-dependent EoS, the QMD calculations reproduce the experimental results.

Transport simulations are very valuable for extracting physics information from heavy-ion-collision experiments. With the emergence of many different transport codes in recent years, it becomes important to estimate their robustness in extracting physics information from experiments. We report on the results of a transport-code-comparison project. Eighteen commonly used transport codes were included in this comparison: nine Boltzmann-Uehling-Uhlenbeck-type codes and nine quantum-molecular-dynamics-type codes. These codes have been asked to simulate $\mathrm{Au}+\mathrm{Au}$ collisions using the same physics input for mean fields and for in-medium nucleon-nucleon cross sections, as well as the same impact parameter, the similar initialization setup, and other calculational parameters at $100A$ and $400A$ MeV incident energy. Among the codes we compare one-body observables such as rapidity and transverse flow distributions. We also monitor nonobservables such as the initialization of the internal states of colliding nuclei and their stability, the collision rates, and the Pauli blocking. We find that not completely identical initializations may have contributed partly to different evolutions. Different strategies to determine the collision probabilities and to enforce the Pauli blocking also produce considerably different results. There is a substantial spread in the predictions for the observables, which is much smaller at the higher incident energy. We quantify the uncertainties in the collective flow resulting from the simulation alone as about 30% at $100A$ MeV and 13% at $400A$ MeV, respectively. We propose further steps within the code comparison project to test the different aspects of transport simulations in a box calculation of infinite nuclear matter. This should, in particular, improve the robustness of transport model predictions at lower incident energies, where abundant amounts of data are available.

Azimuthal emission patterns of $K^{\pm }$ mesons have been measured in Ni + Ni collisions with the FOPI spectrometer at a beam kinetic energy of 1.91 A GeV. The transverse momentum $p_T$ integrated directed and elliptic flow of $K^{+}$ and $K^{-}$ mesons as well as the centrality dependence of $p_T$ - differential directed flow of $K^{+}$ mesons are compared to the predictions of hadron string dynamics and isospin quantum molecular dynamics transport models. The data exhibits different propagation patterns of $K^{+}$ and $K^{-}$ mesons in the compressed and heated nuclear medium and favor the existence of a kaon-nucleon in-medium potential, repulsive for $K^{+}$ mesons and attractive for $K^{-}$ mesons.