Analysis of flow effects in relativistic heavy-ion collisions within the CBUU approach

We study flow phenomena in relativistic heavy-ion collisions, both in transverse and radial direction, in comparison to experimental data. The collective dynamics of the nucleus-nucleus collision is described within a transport model of the coupled channel BUU type (CBUU). This recently developed version includes all nucleonic resonances up to 1.95 GeV in mass and mean-field potentials both of the Skyrme and momentum dependent MDYI type. We find that heavy resonances play an important role in the description of transverse flow above 1 AGeV incident energy. For radial flow we analyse reaction times and equilibration and extract the parameters $T$ and $\beta$ for temperature and collective flow velocity within different prescriptions. Furthermore, we apply a coalescence model for fragment production and check the mass dependence of the flow signals.Comment: 25 pages, 17 figures, submitted to Europ. Phys. J.

Photoabsorption on nuclei

We calculate the total photoabsorption cross section on nuclei in the energy range from 300 MeV to 1 GeV within the framework of a semi-classical phase space model. Besides medium modifications like Fermi motion and Pauli blocking we focus on the collision broadening of the involved resonances. The resonance contributions to the elementary cross section are fixed by fits to partial wave amplitudes of pion photoproduction. The cross sections for $N \, R \to N \, N$, needed for the calculation of collision broadening, are obtained by detailed balance from a fit to $N \, N \to N \, N \, \pi$ cross sections. We show that a reasonable collision broadening is not able to explain the experimentally observed disappearance of the $D_{13}(1520)$-resonance in the photoabsorption cross section on nuclei.Comment: 26 pages Latex including 9 postscript figure

Probing nuclear expansion dynamics with π−/π+\pi^-/\pi^+-spectra

We study the dynamics of charged pions in the nuclear medium via the ratio of differential $\pi^-$- and $\pi^+$-spectra in a coupled-channel BUU (CBUU) approach. The relative energy shift of the charged pions is found to correlate with the pion freeze-out time in nucleus-nucleus collisions as well as with the impact parameter of the heavy-ion reaction. Furthermore, the long-range Coulomb force provides a 'clock' for the expansion of the hot nuclear system. Detailed comparisons with experimental data for $Au + Au$ at 1 GeV/A and $Ni + Ni$ at 2.0 GeV/A are presented.Comment: 21 pages, latex, figures include

Kaon versus Antikaon Production at SIS Energies

We analyse the production and propagation of kaons and antikaons in Ni + Ni reactions from 0.8--1.85 GeV/u within a coupled channel transport approach including the channels $BB \to K^+YN, \pi B\to K^+Y, BB \to NN K \bar{K}, \pi B\to N K\bar{K}, K^+B\to K^+B, \bar{K} B\to \bar{K}B, Y N\to \bar{K} NN, \pi \pi\to K \bar{K}$ as well as $\pi Y\to \bar{K}N$ and $\bar{K} N\to \pi Y$ for the antikaon absorption. Whereas the experimental $K^+$ spectra can be reproduced without introducing any selfenergies for the mesons in Ni + Ni collisions from 0.8 to 1.8 GeV/u, the $K^-$ yield is underestimated by a factor of 5--7 at 1.66 and 1.85 GeV/u. However, introducing density dependent antikaon masses as proposed by Kaplan and Nelson, the antikaon spectra can be reasonably well described.Comment: 16 pages, LaTeX, plus 12 postscript figures, submitted to Nucl. Phys.

Photoproduction of pions and etas in nuclei

We calculate the cross sections for inclusive one-pion, two-pion and eta photoproduction in nuclei in the photon energy range from 300 MeV to 900 MeV within the framework of a semi-classical BUU transport model. Our results are compared with existing experimental data and discussed with respect to a calculation of the total photoabsorption cross section.Comment: 30 pages LaTeX including 13 postscript figure

Reanalysis of antiproton production in proton-nucleus and nucleus-nucleus reactions at subthreshold energies

We reanalyse the production of $p\bar{p}$ pairs in proton-nucleus and nucleus-nucleus collisions employing novel elementary cross sections for baryon-baryon and pion-baryon production channels based on a boson-exchange model. In contrast to previous transport studies performed in the literature the secondary pion induced channel is found to be most important in both p+A and A+A collisions at subthreshold energies. A detailed comparison with the experimental data available indicates that sizeable attractive $\bar{p}$ potentials in the order of - 100 to - 150 MeV at normal nuclear matter density are needed to reproduce the size and shape of the experimental spectra.Comment: 25 pages, latex, including 15 ps-figures, UGI-97-21, submitted to Nucl. Phys.

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

The importance of a 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$-p_t$ 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.Comment: 16 pages, using RevTex, 6 PS-Figure