Core - Corona Model analysis of the Low Energy Beam Scan at RHIC (Relativistic Heavy Ion Collider) in Brookhaven (USA)

The centrality dependence of spectra of identified particles in collisions between ultrarelativistic heavy ions with a center of mass energy ($\sqrt{s}$) of 39 and 11.5 $AGeV$ is analyzed in the core - corona model. We show that at these energies the spectra can be well understood assuming that they are composed of two components whose relative fraction depends on the centrality of the interaction: The core component which describes an equilibrated quark gluon plasma and the corona component which is caused by nucleons close to the surface of the interaction zone which scatter only once and which is identical to that observed in proton-proton collisions. The success of this approach at 39 and 11.5 $AGeV$ shows that the physics does not change between this energy and $\sqrt{s}=200~ AGeV$ for which this model has been developed (Aichelin 2008). This presents circumstantial evidence that a quark gluon plasma is also created at center of mass energies as low as 11.5 $AGeV$.Comment: Proceedings of STARS2013 4-10 May 2013 Havana/Varadero - CUB

Beyond Mean Field Confrontation of Different Models with High Transverse Momentum Proton Spectra

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), revte

On the flow of kaons produced in relativistic heavy ion collisions

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 NP

Is the centrality dependence of the elliptic flow v2v_2 and of the average <pT><p_T> in RHIC experiments more than a Core-Corona Effect?

Recently we have shown that the centrality dependence of the multiplicity of different hadron species observed in RHIC and SPS experiments can be well understood in a simple model, dubbed core-corona model. There it is assumed that those incoming nucleons which scatter only once produce hadrons as in pp collisions whereas those which scatter more often form an equilibrated source which decays according to phase space. In this article we show that also kinematical variables like $v_2/\epsilon_{part} (N_{part})$ as well as $v_2^i/\epsilon_{part} (N_{part})$ and $$of identified particles are well described in this model. The correlation of$$ between peripheral heavy ion collisions and pp collisions for different hadrons, reproduced in this model, questions whether hydrodynamical calculations are the proper tool to describe non-central heavy ion collision. The model explains as well the centrality dependence of $v_2/\epsilon_{part}$ of charged particles, considered up to now as an observable which allows to determine the viscosity of the quark gluon plasma. The observed dependence of $v_2^i/\epsilon_{part}(N_{part})$ on the particle species is a simple consequence of the different ratios of core to corona particles.Comment: Figure added, text partially rewritten, interpretation of v2 of identified particle

Thermodynamics - a valuable approach to multifragmentation?

Since years it has been vividly debated whether multifragmentation is a thermal or a dynamical process. Recently it has been claimed \cite{toek1,po} that new data allow to decide this question. The conclusion, drawn in these papers, are, however, opposite. Whereas \cite{toek1} states that the behavior of different observables as a function of the fragment multiplicity excludes a thermal origin of the fragments in \cite{po} it has been argued that data show a first order phase transition between a liquid and a gaseous phase. It is the aim of this paper to show that both conclusions are premature. They are based on the salient assumption, that the system is sufficiently large to be susceptible to a canonical description. We will show that this is not the case. A micro canonical approach describes the data as good as dynamical calculations. Hence the quest for the physical origin of multifragmentation continues.Comment: 17 pages, 4 figures, completely revised, accepted for publication in NP

Dynamical fragment production in central collisions Xe(50 A.MeV)+Sn

For central collisions Xe(50 A.MeV)+Sn we compared experimental data from the INDRA detector with QMD simulations. Theory as well as experiment show a clear binary character of the fragment emission even for very central collisions. From the time evolution of the reaction (QMD simulation) we could built up a scenario for the dynamical emission of fragmentsComment: To appear in the Proceedings of the 36th International Winter Meeting on Nuclear Physics, Bormio, Italy, Jan. 26-31 199

Molecular dynamics description of an expanding qq/qˉ\bar{q} plasma with the Nambu--Jona-Lasinio model and applications to heavy ion collisions at RHIC and LHC energies

We present a relativistic molecular dynamics approach based on the Nambu--Jona-Lasinio Lagrangian. We derive the relativistic time evolution equations for an expanding plasma, discuss the hadronization cross section and how they act in such a scenario. We present in detail how one can transform the time evolution equation to a simulation program and apply this program to study the expansion of a plasma created in experiments at RHIC and LHC. We present first results on the centrality dependence of $v_2$ and of the transverse momentum spectra of pions and kaons and discuss in detail the hadronisation mechanism.Comment: 25 pages, 28 figure

Multifragmentation - what the data tell us about the different models

We discuss what the presently collected data tell us about the mechanism of multifragmentation by comparing the results of two different models, which assume or show an opposite reaction scenario, with the recent high statistics $4\pi$ experiments performed by the INDRA collaboration. We find that the statistical multifragmentation model and the dynamical Quantum Molecular Dynamics approach produce almost the same results and agree both quite well with experiment. We discuss which observables may serve to overcome this deadlock on the quest for the reaction mechanism. Finally we proof that even if the system is in equilibrium, the fluctuation of the temperature due to the smallness of the system renders the caloric curve useless for the proof of a first order phase transition.Comment: Proceedings CRIS 200