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We present a new algorithm 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 to experimental data, and show the sensitivity on the parameters which govern the cluster formation.

Using FOPI data on elliptic flow in Au+Au collisions between 0.4 and 1.5A GeV we extract constraints for the equation of state (EOS) of compressed symmetric nuclear matter using the transport code IQMD by introducing an observable describing the evolution of the size of the elliptic flow as a function of rapidity. This observable is sensitive to the nuclear EOS and a robust tool to constrain the compressibility of nuclear matter up to 2 $\rho_0$.

Here we aim to show that the ratio of the momentum spectra of $K^{+}$ at small transverse momentum measured for symmetric systems of different sizes can be such an observable., Comment: 3 pages, 2 figs, DAE BRNS Symposium on Nuclear Physics Dec 26-30, (2011), Visakhapatnam A.P. INDIA

We aim to show that K+ and K- spectra at low transverse momentum measured in light symmetric systems at around 2AGeV depend strongly on the K potential. The ratio of the spectra can allow therefore for a direct determination of the strength of the K+ as well as that of the K- potential in a hadronic environment., Comment: 3 pages, 3 figs, DAE BRNS Symposium on Nuclear Physics Dec 26-30, (2011), Visakhapatnam A.P. INDIA

We aim to explore the in-medium effects on the transverse momentum ($p_{T}$) spectra of $K^{+}$ and $K^{-}$ in lighter mass system $^{12}C+^{12}C$., Comment: 3 pages, 1 fig. DAE BRNS Symposium on Nuclear Physics Dec 26-30, (2011), Visakhapatnam A.P. INDIA

The comparison of $K^+$ and $K^-$ spectra at low transverse momentum in light symmetric heavy ion reactions at energies around 2 AGeV allows for a direct experimental determination of the strength of the $K^+$ as well as of t he $K^-$ nucleus potential. Other little known or unknown input quantities like the production or rescattering cross sections of $K^+$ and $K^-$ mesons do not spoil this signal. This result, obtained by simulations of these reactio ns with the Isospin Quantum Molecular Dynamics (IQMD) model, may solve the longstanding question of the behaviour of the $K^-$ in hadronic matter and especially whether a $K^-$ condensate can be formed in heavy ion collisions., Comment: 4 Figures

Using the isospin-dependent quantum molecular dynamics model we study the isospin effects on the disappearance of flow for the reactions of 58Ni+58Ni and 58Fe+58Fe as a function of impact parameter. We found good agreement between our calculations and experimentally measured energy of vanishing flow at all colliding geometries. Our calculations reproduce the experimental data within 5%(10%) at central (peripheral) colliding geometries., Comment: Proceedings of the International Symposium on Nuclear Physics, Mumbai (INDIA), Vol. 54 Pg. 454, 2009

Using the isospin-dependent quantum molecular dynamics model we study the isospin effects on the disappearance of flow for the reactions of $^{58}Ni$ + $^{58}Ni$ and $^{58}Fe$ +$^{58}Fe$ as a function of impact parameter. We found good agreement between our calculations and experimentally measured energy of vanishing flow at all colliding geometries. Our calculations reproduce the experimental data within 5%(10%) at central (peripheral) geometries.

In astrophysics as well as in hadron physics progress has recently been made on the determination of the hadronic equation of state (EOS) of compressed matter. The results are contradictory, however. Simulations of heavy ion reactions are now sufficiently robust to predict the stiffness of the (EOS) from (i) the energy dependence of the ratio of $K^+$ from Au+Au and C+C collisions and (ii) the centrality dependence of the $K^+$ multiplicities. The data are best described with a compressibility coefficient at normal nuclear matter density $\kappa$ around 200 MeV, a value which is usually called ``soft'' The recent observation of a neutron star with a mass of twice the solar mass is only compatible with theoretical predictions if the EOS is stiff. We review the present situation., Comment: invited talk Strange Quark Matter Conference SQM06 in Los Angeles

The stiffness of the hadronic equation of state has been extracted from the production rate of $K^+$ mesons in heavy ion collisions around 1 $A$ GeV incident energy. The data are best described with a compressibility coefficient $\kappa$ around 200 MeV, a value which is usually called ``soft''. This is concluded from a detailed comparison of the results of transport theories with the experimental data using two different procedures: (i) the energy dependence of the ratio of $K^+$ from Au+Au and C+C collisions and (ii) the centrality dependence of the $K^+$ multiplicities. It is demonstrated that input quantities of these transport theories which are not precisely known, like the kaon-nucleon potential, the $\Delta N \to N K^+ \Lambda$ cross section or the life time of the $\Delta$ in matter do not modify this conclusion., Comment: 4 pages, 2figures

In heavy ion reactions at energies around 1-2 AGeV the measured $K^-$ yields appear rather high as compared to pp collisions as shown by the KaoS collaboration. Employing IQMD simulations, we show that this is caused by the fact that the dominant production channel is not $BB\to BBK^+K^-$ but the mesonic $\Lambda(\Sigma) \pi \to K^-B$ reaction. Because the $\Lambda $ ($\Sigma$) stem from the reaction $BB \to \Lambda(\Sigma) K^+ B$, the $K^+$ and the $K^-$ yield are strongly correlated, i.e. the $K^-/K^+$ ratio occurs to be nearly independent of the impact parameter as found experimentally. $K^-$ are continuously produced but also very quickly reabsorbed leading to an almost identical rate for production and reabsorption. The final $K^-$ yield is strongly influenced by the $K^+N$ (due to their production via the $\Lambda(\Sigma)$) but very little by the $K^-$N potential., Comment: 6 pages, 4 figure, RevTeX, revised version, accepted for publication in PRL

The Quantum Molecular Dynamics Model (IQMD) is used to investigate the origin of the collective transverse velocity observed in heavy ion experiments. We find that there are three contributions to this effect: initial-final state correlations, potential interactions and collisions. For a given nuclear equation of state (eos) the increase of the transverse velocity with increasing beam energy is caused by the potential part. For a given beam energy the collective transverse velocity is independent of the nuclear eos but the relative contributions of potential and collisions differ. In view of the importance of the potential interactions between the nucleons it is not evident that the similarity of the radial velocities measured for fragments at beam energies below 1 AGeV and that for mesons at beam energies above 2 AGeV is more than accidental., Comment: 5 pages, 5 figures, revtex, OASIS ref PLB17008

Several models have been proposed to simulate heavy ion reactions beyond the mean field level. The lack of data in phase space regions which may be sensitive to different treatments of fluctuations made it difficult to judge these approaches. The recently published high energy proton spectra, measured in the reaction 94 AMeV Ar + Ta, allow for the first time for a comparison of the models with data. We find that these spectra are reproduced by Quantum Molecular Dynamics (QMD) and Boltzmann Uehling Uhlenbeck (BUU) calculations. Models like Boltzmann Langevin (BL) in which additional fluctuations in momentum space are introduced overpredict the proton yield at very high energies. The BL approach has been successfully used to describe the recently measured very subthreshold kaon production assuming that the fluctuations provide the necessary energy to overcome the threshold in two body collisions. Our new findings suggest that the very subthreshold kaon production cannot be due to two body scattering and thus remains a open problem., Comment: 5 pages, 3 figures (eps), revtex

In this paper, the concepts of microscopic transport theory are introduced and the features and shortcomings of the most commonly used ansatzes are discussed. In particular, the Ultrarelativistic Quantum Molecular Dynamics (UrQMD) transport model is described in great detail. Based on the same principles as QMD and RQMD, it incorporates a vastly extended collision term with full baryon-antibaryon symmetry, 55 baryon and 32 meson species. Isospin is explicitly treated for all hadrons. The range of applicability stretches from $E_{lab}< 100$ MeV/nucleon up to $E_{lab}> 200$ GeV/nucleon, allowing for a consistent calculation of excitation functions from the intermediate energy domain up to ultrarelativistic energies. The main physics topics under discussion are stopping, particle production and collective flow., Comment: 129 pages, pagestyle changed using US letter (8.5x11 in) format. The whole paper (13 Mb ps file) could also be obtained from ftp://ftp.th.physik.uni-frankfurt.de/pub/urqmd/ppnp2.ps.gz

We present the first microscopic calculation of the spectator fragmentation observed in heavy ion reactions at relativistic energies which reproduces the slope of the kinetic energy spectra of the fragments as well as their multiplicity, both measured by the ALADIN collaboration. In the past both have been explained in thermal models, however with vastly different assumptions about the excitation energy and the density of the system. We show that both observables are dominated by dynamical processes and that the system does not pass a state of thermal equilibrium. These findings question the recent conjecture that in these collisions a phase transition of first order, similar to that between water and vapor, can be observed., Comment: 7 pages