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A relativistic 3-fluid 3D hydrodynamic model has been developed for describing heavy-ion collisions at incident energies between few and $\sim$ 200 A$\cdot$GeV. In addition to two baryon-rich fluids which simulate mutually decelerating counterflows of target and projectile nucleons, the new model incorporates evolution of a third, retarded baryon-free fluid created by this decelerated baryonic matter. Different equations of state, including those with the deconfinement phase transition, are treated. A reasonable agreement with experiment is illustrated by proton rapidity spectra, their dependence on collision centrality and beam energy., Comment: 11 pages, 3 figures, to be published in the Proceedings of the XII International Conference on selected Problems of Modern Physics dedicated to 95th annuversary of D.I. Blokhintsev's birth, Dubna, June 8-11, 2003

Thermodynamical properties of hot and dense nuclear matter are analyzed and compared for different equation of state (EoS). It is argued that the softest point of the equation of state and the strangeness separation on the phase boundary can manifest themselves in observables. The influence of the EoS and the order of the phase transition on the expansion dynamics of nuclear matter and strangeness excitation function is analyzed. It is shown that bulk properties of strangeness production in A-A collisions depend only weakly on the particular form of the EoS. The predictions of different models are related with experimental data on strangeness production., Comment: 38 pages

A 3-fluid hydrodynamic model is introduced for simulating heavy-ion collisions at incident energies between few and about 200 AGeV. In addition to the two baryon-rich fluids of 2-fluid models, the new model incorporates a third, baryon-free (i.e. with zero net baryonic charge) fluid which is created in the mid-rapidity region. Its evolution is delayed due to a formation time $\tau$, during which the baryon-free fluid neither thermalizes nor interacts with the baryon-rich fluids. After formation it thermalizes and starts to interact with the baryon-rich fluids. It is found that for $\tau$=0 the interaction strongly affects the baryon-free fluid. However, at reasonable finite formation time, $\tau$=1 fm/c, the effect of this interaction turns out to be substantially reduced although still noticeable. Baryonic observables are only slightly affected by the interaction with the baryon-free fluid., Comment: 17 pages, 3 figures, submitted to the issue of Phys. of Atomic Nuclei dedicated to S.T. Belyaev on the occasion of his 80th birthday, typos corrected

Entrance-channel potentials in nucleus-nucleus collisions, relevant for the synthesis of superheavy elements, are systematically studied within a semi-microscopic approach, where microscopic nuclear densities of the colliding spherical or deformed nuclei are used in semi-classical expressions of the energy-density functional. From experimental data on fusion windows evidence is found that the existence of pockets in the entrance-channel potentials is crucial for fusion. Criteria for the choice of best collision systems for the synthesis of superheavy elements are discussed., Comment: 15 pages, 8 figures. Accepted for publication in Eur. Phys. J. A. In LaTeX file Springer-Verlag and AMS styles are used

Results on dissipative isoscalar modes of a hot and dilute nuclear droplet are presented. As compared to the adiabatic limit (part I), realistic dissipation yields a substantial reduction of the growth rates for all unstable modes, while the area of spinodal instability in the ($\varrho$,T)-plane remains unchanged. The qualitative features of multifragmentation through spinodal decomposition as obtained in the adiabatic limit are not significantly affected by dissipation., Comment: Latex2e (uses EPJA style, included), 9 pages, 11 postscript figures, submitted to EPJA

The collective motion of nucleons from high-energy heavy-ion collisions is analyzed within a relativistic two-fluid model for different equations of state (EoS). As function of beam energy the theoretical slope parameter F_y of the differential directed flow is in good agreement with experimental data, when calculated for the QCD-consistent EoS described by the statistical mixed-phase model. Within this model, which takes the deconfinement phase transition into account, the excitation function of the directed flow turns out to be a smooth function in the whole range from SIS till SPS energies. This function is close to that for pure hadronic EoS and exhibits no minimum predicted earlier for a two-phase bag-model EoS. Attention is also called to a possible formation of nucleon antiflow (F_y < 0) at energies of the order of 100 A GeV., Comment: 7 pages, 5 figures

The diabatic approach to collective nuclear motion is reformulated in the local-density approximation in order to treat the normal modes of a spherical nuclear droplet analytically. In a first application the adiabatic isoscalar modes are studied and results for the eigenvalues of compressional (bulk) and pure surface modes are presented as function of density and temperature inside the droplet, as well as for different mass numbers and for soft and stiff equations of state. We find that the region of bulk instabilities (spinodal regime) is substantially smaller for nuclear droplets than for infinite nuclear matter. For small densities below 30% of normal nuclear matter density and for temperatures below 5 MeV all relevant bulk modes become unstable with the same growth rates. The surface modes have a larger spinodal region, reaching out to densities and temperatures way beyond the spinodal line for bulk instabilities. Essential experimental features of multifragmentation, like fragmentation temperatures and fragment-mass distributions (in particular the power-law behavior) are consistent with the instability properties of an expanding nuclear droplet, and hence with a dynamical fragmentation process within the spinodal regime of bulk and surface modes (spinodal decomposition)., Comment: 17 pages, 11 figures, LaTeX2e, EPJA style (included)

The stability of hot expanded nuclear droplets against small bulk and surface oscillations is examined and possible consequences for multifragmentation are discussed., Comment: LaTeX (uses epsfig.sty), 6 pages with 6 eps figures inside text. Talk given at XXVII International Workshop on Gross Properties of Nuclei and Nuclear Excitations, "MULTIFRAGMENTATION", Hirschegg, January 17--23, 1999

We establish a general relation between the diagonal correlator of eigenvectors and the spectral Green's function for non-hermitian random-matrix models in the large-N limit. We apply this result to a number of non-hermitian random-matrix models and show that the outcome is in good agreement with numerical results., Comment: 10 pages with 4 EPS figures included

We compute the modification of the quark condensate $<\bar{q} q>$ in relativistic nucleus-nucleus collisions and estimate the 4-volume, where the quark condensate is small ($<\bar{q}q>/<\bar{q} q>_0\leq$ 0.1--0.3) using hadron phase-space distributions obtained with the quark-gluon string model. As a function of the beam energy the 4-volume rises sharply at a beam energy $E_{lab}/A \simeq$ (2--5) GeV, remains roughly constant up to beam energies $\simeq 20$ GeV and rises at higher energies. At low energies the reduction of the condensate is mainly due to baryons, while at higher energies the rise of the 4-volume is due to the abundant mesons produced. Based on our results we expect that moderate beam energies on the order of 10 GeV per nucleon are favourable for studying the restoration of chiral symmetry in a baryon-rich environment in nucleus-nucleus collisions., Comment: 17 pages, 3 EPS figures, uses article.sty, epsfig.sty, to be published in European Physical Journal A

The thermodynamic potential of a system of pions and nucleons is computed including the piN interactions in the P33 channel. A consistent treatment of the width of the resonance in this channel, the Delta(1232) resonance, is explored in detail. In the low-density limit we recover the leading term of the virial expansion for the thermodynamic potential. An instructive diagrammatic interpretation of the contributions to the total baryon number is presented. Furthermore, we examine within a fireball model the consequences for the pion spectra in heavy-ion collisions at intermediate energies, including the effect of collective flow. A consistent treatment of the Delta width leads to a substantial enhancement of the pion yield at low momenta., Comment: 12 pages, 3 Postscript figures, LaTeX, elsart, epsfig, minor changes, references added, to be published in Physics Letters B

We present a unitary and gauge-invariant model with coupled channels, which provides a consistent description of pion photoproduction off nucleons in the E$_{0+}$ channel and eta-meson photoproduction off protons and deuterons. An effective field theory with hadrons and photons is constructed, which includes non-resonant Born terms as well as the S$_{11}$(1535) and S$_{11}$(1650) baryon resonances. Due to the coupling between the channels, the production of eta-mesons is strongly affected by the S$_{11}$(1650) although its direct coupling to the $\eta$N channel is negligible. The rho- and omega-meson exchange terms are important for achieving a consistent description of both pion- and photon-induced reactions., Comment: 11 pages Latex, 4 Postscript Figures

An effective field theory with hadrons and photons is constructed, where the hadronic parameters are determined by fitting elastic $\pi$N scattering data. The predicted pion-induced $\eta$-production on the nucleon agrees well with the data. The hadronic model is then used to describe the final-state interaction in photoinduced processes. We present a consistent description of pion photoproduction in the E$_0^+$-channel and of the total cross sections for photoinduced $\eta$-production on the proton as well as on the deuteron., Comment: 5 latex pages, 3 postscript figures uuencoded A shorter version will be published in proceedings of the Sixth International Symposium on Meson-Nucleon-Physics and the Structure of the Nucleon, Blaubeuren, Germany,10-14 July 1995

The initial production and dynamical expansion of hot spherical nuclei are examined as the first stage in the projectile-multifragmentation process. The initial temperatures, which are necessary for entering the adiabatic spinodal region, as well as the minimum temperatures and densities, which are reached in the expansion, significantly differ for hard and soft equations of state. Additional initial compression, occurring in central collisions leads most likely to a qualitatively different multifragmentation mechanism. Recent experimental data are discussed in relation to the results of the proposed model., Comment: 9 pages, latex with 3 figures. PS file available at http://www.gsi.de/~papp/papp.html

A model for $\pi$N scattering and $\eta$-meson production in the S$_{11}$ channel is presented. The model includes $\pi$N-scattering Born terms as well as the N$^*${} resonances S$_{11}$(1535) and S$_{11}$(1610). The $T$-matrix is computed in the $K$-matrix approximation. The parameters of the model are determined by fitting the elastic $\pi$N-scattering $T$-matrix to empirical data. We find an excellent fit for all energies up to $\sqrt{s}$ = 1.75 GeV. Furthermore, a good description of the cross section for $\pi^- + p \rightarrow \eta + n$ is obtained without further adjustment of parameters., Comment: LaTex 10 pages, 4 figures (not included;the postscript-figures are available from ftp@tpri6f.gsi.de in subdirectory pub/sauerman, files: fig1.ps, fig2.ps,fig3.ps,fig4.ps- or contact the authors via e-mail), GSI-94-45