Z.S. Wang

Consequences of covariant kaon dynamics in heavy ion collisions

Abstract The influence of the chiral mean field on the kaon dynamics in heavy ion reactions is investigated. Inside the nuclear medium the kaons are described as dressed quasi-particles carrying effective masses and momenta. A momentum dependent part of the interaction which resembles a Lorentz force originates from spatial components of the vector field and provides an important contribution to the in-medium kaon dynamics. This contribution is found to counterbalance the influence of the vector potential on the K + in-plane flow to a strong extent. Thus it appears to be difficult to restrict the in-medium potential from the analysis of the corresponding transverse flow.

In-medium dependence and Coulomb effects of the pion production in heavy ion collisions

Abstract The properties of the high energy pions observed in heavy ion collisions, in particular in the system Au on Au at 1 GeV/nucleon are investigated. The reaction dynamics is described within the Quantum Molecular Dynamics (QMD) approach. It is shown that high energy pions freeze out early and originate from the hot, compressed matter. N * -resonances are found to give an important contribution toward the high energy tail of the pion. Further the role of in-medium effects in the description of charged pion yields and spectra is investigated using a microscopic potential derived from the Brueckner G -matrix which is obtained with the Reid soft-core potential. It is seen that the high energy part of the spectra is relatively more suppressed due to in-medium effects as compared to the low energy part. A comparison to experiments further demonstrates that the present calculations describe reasonably well the neutral (TAPS) and charged (FOPI) pion spectra. The observed energy dependence of the π − / π + ratio, i.e. deviations from the isobar model prediction, is due to Coulomb effects and again indicate that high energy pions probe the hot and dense phase of the reaction. These findings are confirmed independently by a simple phase space analysis.

Role of the Coulomb interaction in the flow and the azimuthal distribution of kaons from heavy ion reactions

Coulomb final-state interaction of positive charged kaons in heavy ion reactions and its impact on the kaon transverse flow and the kaon azimuthal distribution are investigated within the framework of QMD (quantum molecular dynamics) model. The Coulomb interaction is found to tend to draw the flow of kaons away from that of nucleons and lead to a more isotropic azimuthal distribution of kaons in the target rapidity region. The recent FOPI data have been analyzed by taking into account both the Coulomb interaction and a kaon in-medium potential of the strong interaction. Although the effect of the kaon Coulomb potential on the kaon flow and azimuthal distribution is much smaller than that of the strong potential, it is found to be visible, and therefore, should be taken into account if one wants to extract unambiguous information about the kaon strong potential in nuclear matter from the kaon flow and azimuthal distribution data.

Lambda collective flow in heavy ion reactions

Abstract Collective flow of lambda hyperons in heavy-ion reactions at SIS energies is investigated. It is found that a Λ mean field constructed on the basis of the quark model leads to a good description of the experimental data of the in-plane transverse flow of Λs. The attractive mean field can also give rise to an additional “virtual” Λ radial flow directed inwards, which is reflected by a “concave” structure of the transverse mass spectrum of the Λ hyperons emitted at midrapidity. The Λ radial flow is found to exhibit a strong mass dependence: The flow is visible in the Ni + Ni system, but is strongly reduced in the system of Au on Au.

Role of isospin dependent mean field in pion production in heavy ion reactions

The importance of an isospin dependent nuclear mean field (IDMF) in regard to the pion production mechanism is studied for the reaction Au+Au at 1 GeV/nucleon using the quantum molecular dynamics (QMD) model. In particular, the effect of the IDMF on pion spectra and the charged pion ratio are analyzed. It is found that the inclusion of a IDMF considerably suppresses the low-

Origin of subthreshold K+ production in heavy ion collisions

{p}_{t}

Chiral Kaon dynamics in heavy ion collisions

pions, thus leading to a better agreement with the data on pion spectra. Moreover, the rapidity distribution of the charged pion ratio appears to be sensitive to the isospin dependence of the nuclear mean field.

J / psi normal and anomalous suppressions in a hadron and string cascade model

We investigate the origin of subthreshold K+ production in heavy ion collisions at intermediate energies. In particular we study the influence of the pion induced K+ creation processes. We find that this channel shows a strong dependence on the size of the system, i.e., the number of participating nucleons as well as on the incident energy of the reaction. In an energy region between 1 and 2 GeV/nucleon the pion induced processes essentially contribute to the total yield and can even become dominant in reactions with a large number of participating nucleons. Thus we are able to reproduce recent measurements of the KaoS Collaboration for 1 GeV/nucleon Au on Au reactions adopting a realistic momentum dependent nuclear mean field.

Origin of subthresholdK+production in heavy ion collisions

Abstract The influence of the chiral mean field on the collective motion of kaons in relativistic heavy ion reactions at SIS energies is investigated. We consider three types of collective motion, i.e. the transverse flow, the out-of-plane flow (squeeze-out) and the radial flow, The kaon dynamics is thereby described with a relativistic mean field as it originates form chiral lagrangiens. For the K mesons inside the nuclear medium we adopt a covariant quasi-particle picture including scalar and vector fields and compare this to a treatment with a static potential like force. The comparison to the available data ( K + ) measured by FOPI and KaoS strongly favor the existence of an in-medium potential. However, using full covariant dynamics makes it more difficult to describe the data which might indicate that the mean field level is not sufficient for a reliable description of the kaon dynamics.

Origin of subthreshold K{sup +} production in heavy ion collisions

A mechanism for the effective dissociation of a c (c) over bar pair in the colour electric field of strings is introduced into a hadron and string cascade model, i.e. JPCIAE, which is based on the LUND model, simulating ultra-relativistic nucleus-nucleus collisions. This new mechanism together with the known mechanism of nuclear absorption (both baryons and mesons) could reproduce fairly the data of the normal and anomalous J/psi suppressions in minimum bias pA, AB (with light projectile), and Pb + Pb collisions at 200 A GeV/c. However the impact parameter (ET) dependence of the J/psi suppression factor, both, in S + U and Pb + Pb reactions at 200 A GeV/c and 158 A GeV/c, respectively, is not well reproduced. We also tested the additional mechanism of the energy degradation of leading particles, with which both, the normal and anomalous J/psi suppressions in minimum bias pA, AB, and Pb + Pb collisions and the E-T dependence of the J/psi suppression factor are better reproduced.