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This report summarizes the presentations and discussions during the Rapid Reaction Task Force "Dynamics of critical fluctuations: Theory -- phenomenology -- heavy-ion collisions", which was organized by the ExtreMe Matter Institute EMMI and held at GSI, Darmstadt, Germany in April 2019. We address the current understanding of the dynamics of critical fluctuations in QCD and their measurement in heavy-ion collision experiments. In addition, we outline what might be learned from studying correlations in other physical systems, such as cold atomic gases., Comment: 89 pages, 11 figures, report on an ExtreMe Matter Institute EMMI Rapid Reaction Task Force

We present a new approach to identify fragments in computer simulations of relativistic heavy ion collisions. It is based on the simulated annealing technique and can be applied to n-body transport models like the Quantum Molecular Dynamics. This new approach is able to predict isotope yields as well as hyper-nucleus production. In order to illustrate its predicting power, we confront this new method with experimental data and show the sensitivity on the parameters which govern the cluster formation., Comment: 14 pages, 14 figures, submitted to Phys. Rev. C

We use the Isospin Quantum Molecular Dynamics model (IQMD) to analyze the centrality dependence of the isospin ratio of pions, $\pi^-/\pi^+$. We find that the density dependence of the asymmetry potential, the Pauli blocking of the $\Delta$-decay and the thickness of the neutron skin influence in different ways this observable. Using the centrality dependence of this ratio at different beam energies we can disentangle the different contributions and open the way for their experimental determination.

Recently it has been discovered that the elliptic flow, v2, 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 [1] 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 mid-rapidity 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 y-direction accelerates the particles more and creates therefore a negative v2. When using a soft momentum dependent EoS, the QMD calculations reproduce the experimental results., Comment: text substantially revised, most figures exchanged, PRC in print

Comparing K+ spectra at low transverse momenta for different symmetric collision systems at beam energies around 1 AGeV allows for a direct determination of both the strength of the K+ nucleus potential as well as of the K+N rescattering cross section in a hadronic environment. Other little known or unknown quantities which enter the K+ dynamics, like the production cross sections of K+ mesons or the hadronic equation of state, do not spoil this signal as they cancel by using ratios of spectra. This procedure is based on transport model calculations using the Isospin Quantum Molecular Dynamics (IQMD) model which describes the available data quantitatively., Comment: 4 pages, 4 figures

Using the Isospin Quantum Molecular Dynamics (IQMD) model we analyzed the production of pions and kaons in the energy range of 1-2 AGeV in order to study the question why thermal models could achieve a successful description. For this purpose we study the variation of pion and kaon yields using different elementary cross sections. We show that several ratios appear to be rather robust versus their variations., Comment: proceedings to the Workshop on Dense Matter - DM 2010, Stellenbosch (South Africa), April 2010

Recently, is has been observed that events with the {\it same} total transverse energy of light charged particles (LCP) in the quasi target region, $E_{\perp 12}^{QT}$, show two quite distinct reaction scenarios in the projectile domain: multifragmentation and residue production. This phenomenon has been dubbed "bimodality". Using Quantum Molecular Dynamics calculations we demonstrate that this observation is very general. It appears in collisions of all symmetric systems larger than Ca and at beam energies between 50 A.MeV and 600 A.MeV and is due to large fluctuations of the impact parameter for a given $E_{\perp 12}^{QT}$. Investigating in detail the $E_{\perp 12}^{QT}$ bin in which both scenarios are present, we find that neither the average fragment momenta nor the average transverse and longitudinal energies of fragments show the behavior expected from a system in statistical equilibrium, in experiment as well as in QMD simulations. On the contrary, the experimental as well as the theoretical results point towards a fast process. This observation questions the conjecture that the observed bimodality is due to the coexistence of 2 phases at a given temperature in finite systems., Comment: accepted PRC

The Isospin Quantum Molecular Dynamics model is used to compare spectra and elliptic flow of kaons produced at subthreshold energies with data taken at the SIS accelerator at GSI. We find that temperatures of the spectra are dominated by the rescattering of the kaons. The study of elliptic flow observables indicates the influence of rescattering as well as of the optical potential of the kaons with increasing dominance of the optical potential at lower incident energies., Comment: contribution to strange Quark Matter 2007, accepted for publication in J. Phys. G 6 pages, 4 figures

Recently the HADES collaboration has published the invariant mass spectrum of $e^+e^-$ pairs, dN/dM$_{e^+e^-}$, produced in C+C collisions at 2 AGeV. Using electromagnetic probes, one hopes to get in this experiment information on hadron properties at high density and temperature. Simulations show that firm conclusions on possible in-medium modifications of meson properties will only be possible when the elementary meson production cross sections, especially in the pn channel, as well as production cross sections of baryonic resonances are better known. Presently one can conclude that a) simulations overpredict by far the cross section at $M_{e^+e^-} \approx M_{\omega}{^0}$ if free production cross sections are used and that b) the upper limit of the $\eta$ decay into $e^+e^-$ is smaller than the present upper limit of the Particle Data Group. This is the result of simulations using the Isospin Quantum Molecular Dynamics (IQMD) approach., Comment: 28 pages, 15 figures, 3 tables, ps file recommended

We compare multiplicities as well as rapidity and transverse momentum distributions of protons, pions and kaons calculated within presently available transport approaches for heavy ion collisions around 1 AGeV. For this purpose, three reactions have been selected: Au+Au at 1 and 1.48 AGeV and Ni+Ni at 1.93 AGeV., Comment: 15 pages, 13 figures, proceedings of Strange Quark Matter 2004, Cape Town, South Africa

We study the reaction Au+Au at 1.48 AGeV and analyze the influence of the KN optical potential on cm spectra and azimuthal distributions at mid-rapidity. We find a significant change of the yields but only slight changes in the shapes of the distributions when turning off the optical potential. However, the spectra show contributions from different reaction times, where early kaons contribute stronger to higher momenta and late kaons to lower momenta. Azimuthal distributions of the kaons at mid-rapidity show a strong centrality dependence. Their shape is influenced by the KN optical potential as well as by re-scattering., Comment: SQM 2003 proceedings, 4 figures, 6 pages

The preliminary experimental data on $\phi$ production in the reaction Ni(1.93 AGeV) + Ni point to a puzzling high $\phi$ yield which can not be reproduced with present transport codes. We survey the experimental situation and present prospects for dedicated measurements of the $\phi$ multiplicities with the $K^+ K^-$ and $e^+ e^-$ channels at HADES and FOPI., Comment: talk at Strange Quarks in Matter 2001, Frankfurt Sep. 24 - 29, 2001

The yields of strange particles are calculated with the UrQMD model for p,Pb(158 AGeV)Pb collisions and compared to experimental data. The yields are enhanced in central collisions if compared to proton induced or peripheral Pb+Pb collisions. The enhancement is due to secondary interactions. Nevertheless, only a reduction of the quark masses or equivalently an increase of the string tension provides an adequate description of the large observed enhancement factors (WA97 and NA49). Furthermore, the yields of unstable strange resonances as the Lambda_star(1520) resonance or the phi meson are considerably affected by hadronic rescattering of the decay products., Comment: Contribution to Strangeness 2000 Conf., published in J.Phys.G.:Nucl.Phys.; 7 pages

Basic problems of the semiclassical microscopic modelling of strongly interactingsystems are discussed within the framework of Quantum Molecular Dynamics (QMD). This model allows to study the influence of several types of nucleonic interactions on a large variety of observables and phenomena occurring in heavy ion collisions at relativistic energies.It is shown that the same predictions can be obtained with several -- numerically completely different and independently written -- programs as far as the same model parameters are employed and the same basic approximations are made. Many observables are robust against variations of the details of the model assumptions used. Some of the physical results, however, depend also on rather technical parameters like the preparation of the initial configuration in phase space. This crucial problem is connected with the description of the ground state of single nuclei,which differs among the various approaches. An outlook to an improved molecular dynamics scheme for heavy ion collisions is given., Comment: 39 pages, 12 figures

We investigate the different contributions to the in-plane flow of K+ mesons observed recently by the FOPI collaboration in the reaction Ni(1.93 AGeV)+Ni. Due to the kinematics of the three body phase space decay the flow of kaons produced in baryon-baryon interactions is smaller than that of the baryons in the entrance channel. On the contrary, in pi N interactions the flow of the sources and of the kaons are identical. Therefore the total kaon flow depends on the relative number of Delta N -> K+ and pi N -> K+ reactions and hence on the lifetime of the Delta, in addition to the already known dependence on the potential interaction interaction of the kaons with the nuclear environment., Comment: 11 pages, 10 figures, submitted to NPA

Applying a new method of rescattering which is based on the neural network technique we study the influence of rescattering on the spectra of strange particles produced in heavy ion reactions. In contradistinction to former approaches the rescattering is done explicitly and not in a perturbative fashion. We present a comparison of our calculations for the system Ni(1.93AGeV)+Ni with recent data of the FOPI collaboration. We find that even for this small system rescattering changes the observables considerably but does not invalidate the role of the kaons as a messenger from the high density zone. We cannot confirm the conjecture that the kaon flow can be of use for the determination of the optical potential of the kaon., Comment: 13 pages, 10 figures

The behavior of hadronic matter at high baryon densities is studied within Ultrarelativistic Quantum Molecular Dynamics (URQMD). Baryonic stopping is observed for Au+Au collisions from SIS up to SPS energies. The excitation function of flow shows strong sensitivities to the underlying equation of state (EOS), allowing for systematic studies of the EOS. Dilepton spectra are calculated with and without shifting the $\rho$ pole. Except for S+Au collisions our calculations reproduce the CERES data., Comment: Invited talk at RHIC-theory workshop at BNL july 8-19

The behavior of hadronic matter at high baryon densities is studied within Ultrarelativistic Quantum Molecular Dynamics (URQMD). Baryonic stopping is observed for Au+Au collisions from SIS up to SPS energies. The excitation function of flow shows strong sensitivities to the underlying equation of state (EOS), allowing for systematic studies of the EOS. Effects of a density dependent pole of the $\rho$-meson propagator on dilepton spectra are studied for different systems and centralities at CERN energies., Comment: Proceedings of the Quark Matter '96 Conference, Heidelberg, Germany

We investigate the disappearance of collective flow in the reaction plane in heavy-ion collisions within a microscopic model (QMD). A systematic study of the impact parameter dependence is performed for the system Ca+Ca. The balance energy strongly increases with impact parameter. Momentum dependent interactions reduce the balance energies for intermediate impact parameters $b\approx4.5$ fm. Dynamical negative flow is not visible in the laboratory frame but does exist in the contact frame for the heavy system Au+Au. For semi-peripheral collisions of Ca+Ca with $b\approx6.5$ fm a new two-component flow is discussed. Azimuthal distributions exhibit strong collectiv flow signals, even at the balance energy., Comment: 19 pages, 7 eps-figures, uses psfig.sty; complete postscript file available at ftp://th.physik.uni-frankfurt.de/pub/bass/GSI-preprint_95-11.ps.Z

Triple differential cross sections of pions in heavy ion collisions at 1 GeV/nucl. are studied with the IQMD model. After discussing general properties of $\Delta$ resonance and pion production we focus on azimuthal correlations: At projectile- and target-rapidities we observe an anticorrelation in the in-plane transverse momentum between pions and protons. At c.m.-rapidity, however, we find that high $p_t$ pions are being preferentially emitted perpendicular to the event-plane. We investigate the causes of those correlations and their sensitivity on the density and momentum dependence of the real and imaginary part of the nucleon and pion optical potential., Comment: 40 pages, 18 eps-figures, uses psfig.sty; complete postscript file available at ftp://th.physik.uni-frankfurt.de/pub/bass/GSI-preprint_95-7.ps.Z