Christoph Hartnack

Strangeness production close to the threshold in proton–nucleus and heavy-ion collisions

Abstract We discuss strangeness production close to the threshold in p+A and A+A collision. Comparing the body of available K + , K 0 , K − , and Λ data with the IQMD transport code and for some key observables as well with the HSD transport code, we find good agreement for the large majority of the observables. The investigation of the reaction with the help of these codes reveals the complicated interaction of the strange particles with hadronic matter which makes strangeness production in heavy-ion collisions very different from that in elementary interactions. We show how different strange particle observables can be used to study the different facets of this interaction (production, rescattering and potential interaction) which finally merge into a comprehensive understanding of these interactions. We identify those observables which allow for studying (almost) exclusively one of these processes to show how the future high precision experiments can improve our quantitative understanding. Finally, we discuss how the K + xa0xa0multiplicity can be used to study the hadronic equation of state.

Kaon production at subthreshold energies

Abstract We present a comprehensive investigation of subthreshold and threshold kaon production in the framework of the QMD model. The calculation shows that the kaon yield depends strongly on the nuclear equation of state (EOS) and subthreshold kaon experiments may therefore offer the up to now best possibility to determine this yet unknown property of nuclear matter. Whereas static equations of state are in quite good agreement with the data the more realistic momentum dependent EOSs underpredict the data by about a factor of three. Most kaons are produced in a two step process with an intermediate Δ, a process which is absent in NN collisions. Therefore the kaon yield is much higher than expected from the extrapolation of pp data. We discuss in detail the influence of the elementary kaon production cross section, the contribution of different production channels and the properties of the nuclear environment at the place where the kaon is produced. The decrease of the number of produced kaons per participant with decreasing participant number, which is also observed experimentally, has two sources: The nonmaxwellian momentum distribution for small participant numbers as well as a reduction of the ΔΔ channel. This finding is supported by the observed decrease of high momentum pions. We compare our results with other calculations and discuss in detail the theoretical and experimental uncertainties. They include the yet only vaguely known elementary kaon production cross section at the threshold, the lack of a quantum-mechanical treatment of the two step production process as well as the behaviour of the kaon (and therefore also of the kaon production threshold) in the nuclear environment.

Disappearance of flow

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 {ital b}{approx}4.5 fm. For the heavy system Au+Au, dynamical negative flow is not visible in the laboratory frame but does exist if the initial precontact rotation of the system due to the Coulomb potential is subtracted. For semiperipheral collisions of Ca+Ca with {ital b}{approx}6.5 fm a new two-component flow is discussed. Azimuthal distributions exhibit strong collective flow signals, even at the balance energy.

Sideward flow in Au + Au collisions at 400 A.MeV

Abstract We present new experimental data obtained with the FOPI detector at SIS, for the Au + Au heavy-ion collisions at 400 A MeV incident energy. The sideward flow, determined from a method without reaction-plane reconstruction, and the nuclear stopping are studied as a function of the centrality of the collisions. In order to study the nuclear in-medium effects, which act on the NN cross sections and potential and hence on experimental observables like the nuclear-matter flow and stopping, these results are compared with the predictions of two different QMD versions. The first one offers a fully microscopic calculation of the cross sections and potential in the G-matrix formalism and naturally includes the in-medium effects (this version is for the first time confronted with experiment). The second one uses a standard Skyrme potential plus a momentum-dependent term in order to mimic the in-medium effects.

The multifragmentation of spectator matter

Abstract 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.

Analysis of the dilepton invariant mass spectrum in C + C collisions at 2A and 1A GeV

Recently the HADES Collaboration has published the invariant mass spectrum of e{sup +}e{sup -} pairs, dN/dM{sub e{sup +}}{sub e{sup -}}, produced in C + C collisions at 2A GeV. Using electromagnetic probes, one hopes to get information from this experiment 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 (i) simulations overpredict by far the cross section at M{sub e{sup +}}{sub e{sup -}}{approx_equal}M{sub {omega}}{sup 0} if free production cross sections are used and that (ii) the upper limit of the {eta} decay into e{sup +}e{sup -} is smaller than the present upper limit of the Particle Data Group. This is the result of simulations using the isospin quantum molecular dynamics approach.

Azimuthal anisotropies as stringent test for nuclear transport models

Abstract Azimuthal distributions of charged particles and intermediate mass fragments emitted in Au+Au collisions at 600 A MeV have been measured using the FOPI facility at GSI-Darmstadt. Data show a strong increase of the in-plane azimuthal anisotropy ratio with the charge of the detected fragment. Intermediate mass fragments are found to exhibit a strong momentum-space alignment with respect of the reaction plane. The experimental results are presented as a function of the polar centre-of-mass angle and over a broad range of impact parameters. They are compared to the predictions of the Isospin Quantum Molecular Dynamics model using three different parametrisations of the equation of state. We show that such highly accurate data provide stringent test for microscopic transport models and can potentially constrain separately the stiffness of the nuclear equation of state and the momentum dependence of the nuclear interaction.

Kaon production at subthreshold energies: what do we learn about the nuclear medium?

The IQMD 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.

K + mesons, a robust probe to measure the hadronic equation of state?

We demonstrate that the K + mesons are sensitive to nuclear matter properties, especially to the hadronic equation of state (EoS). Once one has understood how K + behave in hadronic matter we can use these mesons to study the hadronic EoS. Several observables point towards a rather soft EoS at densities around three times the normal nuclear matter density ρ0.