Search

The dependence of $K^+$ production on the nuclear equation of state is investigated in heavy ion collisions. An increase of the excitation function of $K^+$ multiplicities obtained in heavy ($Au+Au$) over light ($C+C$) systems when going far below threshold which has been observed by the KaoS Collaboration strongly favours a soft equation of state. This observation holds despite of the influence of an in-medium kaon potential predicted by effective chiral models which is necessary to reproduce the experimental $K^+$ yields., Comment: 4 pages Revtex, 4 PS figures, to appear in Phys. Rev. Lett

The equilibration of hot and dense nuclear matter produced in the central region in central Au+Au collisions at $\sqrt{s}=200$ AGeV is studied within the microscopic transport model UrQMD. The pressure here becomes isotropic at $t \approx 5$ fm/c. Within the next 15 fm/c the expansion of the matter proceeds almost isentropically with the entropy per baryon ratio $S/A \approx 150$. During this period the equation of state in the $(P,\epsilon)$-plane has a very simple form, $P=0.15 \epsilon$. Comparison with the statistical model (SM) of an ideal hadron gas reveals that the time of $\approx 20$ fm/$c$ may be too short to attain the fully equilibrated state. Particularly, the fractions of resonances are overpopulated in contrast to the SM values. The creation of such a long-lived resonance-rich state slows down the relaxation to chemical equilibrium and can be detected experimentally., Comment: Talk at the conference Strangeness'2000, to be published in J. of Phys. G

The dilepton production in elementary ${pp\to e^{+}e^{-}X}$ reactions at BEVALAC energies $T_{lab}=1\div 5$ GeV is investigated. The calculations include direct ${e^{+}e^{-}}$ decays of the vector mesons $\rho ^{0}$, $\omega $, and $\phi $, Dalitz decays of the $\pi ^{0}$-, $\eta $-, $% \rho $-, $\omega $-, and $\phi $-mesons, and of the baryon resonances $% \Delta (1232),N(1520),$ $... $ . The subthreshold vector meson production cross sections in $pp$ collisions are treated in a way sufficient to avoid double counting with the inclusive vector meson production. The vector meson dominance model for the transition form factors of the resonance Dalitz decays $R\to e^{+}e^{-}N$ is used in an extended form to ensure correct asymptotics which are in agreement with the quark counting rules. Such a modification gives an unified and consistent description of both $R\to N\gamma $ radiative decays and $R\to N\rho (\omega)$ meson decays. The effect of multiple pion production on the experimental efficiency for the detection of the dilepton pairs is studied. We find the dilepton yield in reasonable agreement with the experimental data for the set of intermediate energies whereas at the highest energy $T_{lab}=4.88$ GeV the number of dilepton pairs is likely to be overestimated experimentally in the mass range $M=300\div 700$ MeV., Comment: 25 pages (IOP style), 5 figures, revised manuscript accepted for publication in JPG

The equilibration of hot and dense nuclear matter produced in the central cell of central Au+Au collisions at RHIC ($\sqrt{s}=200$ AGeV) energies is studied within a microscopic transport model. The pressure in the cell becomes isotropic at $t\approx 5$ fm/$c$ after beginning of the collision. Within the next 15 fm/$c$ the expansion of matter in the cell proceeds almost isentropically with the entropy per baryon ratio $S/A \approx 150$, and the equation of state in the $(P,\epsilon)$ plane has a very simple form, $P=0.15\epsilon$. Comparison with the statistical model of an ideal hadron gas indicates that the time $t \approx 20$ fm/c may be too short to reach the fully equilibrated state. Particularly, the creation of long-lived resonance-rich matter in the cell decelerates the relaxation to chemical equilibrium. This resonance-abundant state can be detected experimentally after the thermal freeze-out of particles., Comment: LATEX, 21 pages incl. 7 figures

We analyze the thermodynamical state of nuclear matter in transport descriptions of heavy ion reactions. We determine thermodynamical variables from an analysis of local momentum space distributions and compare to blast model parameters from an analysis of fragment energy spectra. These descriptions are applied to spectator and fireball matter in semi-central and central Au+Au collisions at SIS-energies, respectively., Comment: 4 pages, 2 postscript-figures, to be published in the proceedings of Bologna2000: Structure of the Nucleus at the Dawn of the Century, Bologna, Italy, 29 May - 3 Jun 2000

We discuss the problems involved in extracting the nuclear equation-of-state from heavy-ion collisions. We demonstrate that the equation of state becomes effectively softer in non-equilibrium and this effect is observable in terms of collective flow effects. Thus, non-equilibrium effects must be included in transport descriptions on the level of the effective mean fields. A comparison with transverse momentum, rapidity, and centrality selected flow data show the reliability and limitations of the underlying interaction which was derived from microscopic Dirac-Brueckner (DB) results., Comment: 6 pages, 3 postscript-figures, to be published in the proceedings of Bologna2000: Structure of the Nucleus at the Dawn of the Century, Bologna, Italy, 29 May - 3 Jun 2000

The transverse momentum ($p_t$) dependence of hadron flow at SPS energies is studied. In particular, the nucleon and pion flow in S+S and Pb+Pb collisions at 160 AGeV is investigated. For simulations the microscopic quark-gluon string model (QGSM) is applied. It is found that the directed flow of pions $v_1(y, \Delta p_t)$ changes sign from a negative slope in the low-$p_t$ region to a positive slope at $p_t \geq 0.6$ GeV/c as recently also observed experimentally. The change of the flow behaviour can be explained by early emission times for high-$p_t$ pions. We further found that a substantial amount of high-$p_t$ pions are produced in the very first primary NN collisions at the surface region of the touching nuclei. Thus, at SPS energies high-$p_t$ nucleons seem to be a better probe for the hot and dense early phase of nuclear collisions than high-$p_t$ pions. Both, in the light and in the heavy system the pion directed flow $v_1(p_t, \Delta y)$ exhibits large negative values when the transverse momentum approaches zero, as also seen experimentally in Pb+Pb collisions. It is found that this effect is caused by nuclear shadowing. The proton flow, in contrary, shows the typical linear increase with rising $p_t$., Comment: REVTEX, 20 pages incl. 6 figures, revised and extended version

We investigate the thermodynamical properties of nuclear matter in heavy ion collisions, in particular with respect to questions of thermodynamical instability, phase transitions and fragmentation. For this we analyze results of relativistic transport calculations of Au+Au collisons at intermediate energies, separately for spectator and participant matter. On one hand, we determine local thermodynamical variables from the analysis of the local momentum distribution; on the other, we analyze fragment energy spectra in a blast model scenario. We find that the spectator represents an instable, equilibrized fragmenting source, while in the participant no such common source can be identified. Our results compare well with experimental determinations of temperatures and flow velocities., Comment: 10 pages, 7 postscript-figures, proceedings of the XXXVIII International Winter Meeting on Nuclear Physics, Bormio, Italy, January 24-29, 2000

The centrality dependence of directed and elliptic flow in light and heavy systems of colliding nuclei is studied within two microscopic transport models at energies from 1 AGeV to 160 A GeV. The pion directed flow has negative slope in the midrapidity range irrespective of bombarding energy and mass number of the colliding ions. In contrast, the directed flow of nucleons vanishes and even develops antiflow in the midrapidity range in (semi)peripheral collisions at energies around 11.6 A GeV and higher. The origin of the disappearance of flow is linked to nuclear shadowing. Since the effect is stronger for a light system, it can be distinguished from the similar phenomenon caused by the quark-gluon plasma formation. In the latter case the disappearance of the flow due to the softening of the equation of state should be most pronounced in collisions of heavy ions. The centrality dependence of the elliptic flow shows that the maximum in the distribution is shifted to very peripheral events with rising incident energy, in accord with experimental data. This is an indication of the transition from baryonic to mesonic degrees of freedom in hot hadronic matter., Comment: 30 pages incl. 17 figures, to be published in the Physical Review C

The directed flow of protons is studied in the quark-gluon string model as a function of the impact parameter for S+S and Pb+Pb reactions at 160 AGeV/c. A significant reduction of the directed flow in midrapidity range, which can lead to the development of the antiflow, is found due to the absorption of early emitted particles by massive spectators (shadowing effect). This effect can mimic the formation of the quark-gluon plasma (QGP). However, in the absorption scenario the antiflow is stronger for the system of light colliding nuclei than for the heavy ones, while in the case of the plasma creation the effect should be opposite., Comment: REVTEX, 11 pages, 5 figures embedded, accepted for publication in Physics Letters B

In a recent paper (quant-ph/9906015), Deutsch claims to derive the "probabilistic predictions of quantum theory" from the "non-probabilistic axioms of quantum theory" and the "non-probabilistic part of classical decision theory." We show that his derivation fails because it includes hidden probabilistic assumptions., Comment: LaTeX, 8 pages, no figures

We analyze the thermodynamical state of nuclear matter in transport calculations of heavy--ion reactions. In particular we determine temperatures and radial flow parameters from an analysis of fragment energy spectra and compare to local microscopic temperatures obtained from an analysis of local momentum space distributions. The analysis shows that the spectator reaches an equilibrated freeze-out configuration which undergoes simultaneous fragmentation. The fragments from the participant region, on the other hand, do not seem to come from a common fragmenting source, but rather heavier fragments seem to be formed earlier., Comment: 16 pages, 5 postscript figures, to appear in Phys. Lett. B

The invariant mass spectrum of the $e^{+}e^{-}$ and $\mu ^{+}\mu ^{-}$ pairs from decays of light unflavored mesons with masses below the $\phi (1020)$-meson mass to final states containing along with a dilepton pair one photon, one meson, and two mesons are calculated within the framework of the effective meson theory. The results can be used for simulations of the dilepton spectra in heavy-ion collisions and for experimental searches of dilepton meson decays., Comment: 73 pages, 19 figures, 3 tables, REVTeX, new references added

An detailed study of the thermodynamical state of nuclear matter in transport calculations of heavy--ion reactions is presented. In particular we determine temperatures from an analysis of the local momentum space distribution on one hand, and from a fit to fragment energy spectra in terms of a blast model with radial flow and temperature on the other. We apply this to spectator and participant matter. In spectator we find regions of spinodal instability with temperatures and densities which are consistent with experiments. In the participant we find different temperatures for different fragment masses, indicating that the fragments are not emitted from a source in thermal and chemical equilibrium., Comment: 6 pages, 3 figures, Proc. of the International Workshop XXVII on Gross Properties of Nuclei and Nuclear Excitations, Hirschegg, Austria, January 17-23, 1999

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., Comment: 10 pages, 5 PS-figures, Proceedings of the Erice School on Nuclear Physics in Erice, Sicily, Italy, September 17 -25 1998; to be published in Progress in Particle and Nuclear Physics Vol. 42

We investigate nuclear matter properties in the relativistic Brueckner approach. The in-medium on-shell T-matrix is represented covariantly by five Lorentz invariant amplitudes from which we deduce directly the nucleon self-energy. To enforce correct Hartree-Fock results we develop a subtraction scheme which treats the bare nucleon-nucleon potential exactly in accordance to the different types of meson exchanges. For the higher order correlations we employ two different covariant representations in order to study the uncertainty inherent in the approach. The nuclear matter bulk properties are only slightly sensitive on the explicit representation used. However, we obtain new Coester lines for the various Bonn potentials which are shifted towards the empirical region of saturation., Comment: 11 pages, 4 PS-figures, Proceedings of the Erice School on Nuclear Physics in Erice, Sicily, Italy, September 17 -25 1998; to be published in Progress in Particle and Nuclear Physics Vol. 42

Collective flow of Lambda hyperons in heavy ion reactions at SIS energies is investigated. It is found that a $\Lambda$ 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 $\Lambda$'s. The attractive mean field can also give rise to an additional "virtual" $\Lambda$ radial flow directed inwards, which is reflected by a "concave" structure of the transverse mass spectrum of the $\Lambda$ hyperons emitted at midrapidity. The $\Lambda$ 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., Comment: 18 pages LeTex, using Elsevier style, 6 PS-figures, accepted for publication in Nuclear Physics A

We investigate nuclear matter properties in the relativistic Brueckner approach. The in-medium on-shell T-matrix is represented covariantly by five Lorentz invariant amplitudes from which we deduce directly the nucleon self-energy. We discuss the ambiguities of this approach and the failure of previously used covariant representations in reproducing the nucleon self-energies on the Hartree-Fock level. To enforce correct Hartree-Fock results we develop a subtraction scheme which treats the bare nucleon-nucleon potential exactly in accordance to the different types of meson exchanges. For the remaining ladder kernel, which contains the higher order correlations, we employ then two different covariant representations in order to study the uncertainty inherent in the approach. The nuclear matter bulk properties are only slightly sensitive on the explicit representation used for the kernel. However, we obtain new Coester lines for the various Bonn potentials which are shifted towards the empirical region of saturation. In addition the nuclear equation-of-state turns out to be significantly softer in the new approach., Comment: 39 pages Latex using Elsevier style, 16 PS figures

The squeeze-out phenomenon of $K^+$ and $K^-$ mesons, i.e. the azimuthal asymmetry of $K^+$ and $K^-$ mesons emitted at midrapidity in heavy ion reactions, is investigated for beam energies of 1-2 A.GeV. It is found that the squeeze-out signal is strongly affected by in-medium potentials of these mesons. The repulsive $K^+$-nucleus potential gives rise to a pronounced out-of-plane emission of $K^+$'s at midrapidity. With the $K^+$ potential we reproduce well the experimental data of the $K^+$ azimuthal distribution. It is found that the attractive $K^-$-nucleus potential cancels to a large extent the influence of rescattering and reabsorption of the $K^-$ mesons on the projectile and target residuals (i.e. shadowing). This results in an azimuthally isotropic emission of the midrapidity $K^-$ mesons with transverse momentum up to 0.8 GeV/c. Since it is well accepted that the shadowing alone would lead to a significant out-of-plane preference of particle emission, in particular at high transverse momenta, the disappearance of the out-of-plane preference for the $K^-$ mesons can serve as an unambiguous signal of the attractive $K^-$ potential. We also apply a covariant formalism of the kaon dynamics to the squeeze-out phenomenon. Discrepancies between the theory and the experiments and possible solutions are discussed., Comment: 24 pages Latex using Elsevier style, 7 PS figures, accepted for publication in Euro. Phys. Jour. A

A mechanism for the effective dissociation of a $c\bar{c}$ 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 (E_T) 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., Comment: 16 pages (RevTex), 6 ps-figures, to be published in J. Phys. G

The production of d' dibaryons in heavy ion collisions due to the elementary process NN -> d' + pion is considered. The cross section NN -> d' + pion is estimated using the vacuum d' width = 0.5 MeV extracted from data on the double charge exchange reactions on nuclei. The d' production rate per single collision of heavy ions is estimated at an incident beam energy of 1 A GeV within the framework of the Quantum Molecular Dynamics transport model. We suggest to analyse the invariant mass spectrum of the NN + pion system in order to search for an abundance of events with the invariant mass of the d' dibaryon. The d' peak is found to exceed the statistical fluctuations of the background at a level of 6 standard deviations for 2 10^5 A central collisions of heavy ions with the atomic number A., Comment: 29 pages including 7 figures, REVTeX

The influence of realistic interactions on the reaction dynamics in intermediate energy heavy ion collisions is investigated. The mean field in relativistic transport calculations is derived from microscopic Dirac-Brueckner (DB) self-energies, taking non-equilibrium effects, in particular the anisotropy of the local phase space configurations, into account. Thus this approach goes beyond the local density approximation. A detailed analysis of various in-plane and out-of-plane flow observables is presented for Au on Au reactions at incident energies ranging from 250 to 800 A.MeV and the results are compared to recent measurements of the FOPI collaboration. An overall good agreement with in-plane flow data and a reasonable description of the out-of-plane emission is achieved. For these results the intrinsic momentum dependence of the non-equilibrium mean fields is important. On the other hand, the local density approximation with the same underlying DB forces as well as a standard non-linear version of the $\sigma\omega$ model are less successful in describing the present data. This gives evidence of the applicability of self energies derived from the DB approach to nuclear matter also far from saturation and equilibrium., Comment: 63 pages Latex, using Elsevier style, 20 ps-figures, to appear in Nucl. Phys. A

This work investigates the collective motion of kaons in heavy ion reactions at SIS energies (about 1-2 GeV/nucleon). A radial collective flow of $K^+$ mesons is predicted to exist in central Au + Au collisions, which manifests in a characteristic "shoulder-arm" shape of the transverse mass spectrum of the midrapidity $K^+$ mesons. The $K^+$ radial flow arises from the repulsive $K^+$ mean field in nuclear matter. In spite of a strong reabsorption and rescattering the attractive $K^-$ mean field leads as well to a collective radial flow of $K^-$ mesons. The $K^-$ radial flow, however, is different from that of $K^+$ mesons and can be observed by a characteristic "concave" structure of the transverse mass spectrum of the $K^-$ mesons emitted at midrapidity. The kaon radial flows can therefore serve as a novel tool for the investigation of kaon properties in dense nuclear matter., Comment: 30 pages RevTex, 5 PS figures, accepted for publication in Phys. Rev. C

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., Comment: 14 pages, RevTex, 3 PS figures, accepted for publication in Phys. Lett. B

We investigate the momentum dependence of the nucleon self-energy in nuclear matter. We apply the relativistic Brueckner-Hartree-Fock approach and adopt the Bonn A potential. A strong momentum dependence of the scalar and vector self-energy components can be observed when a commonly used pseudo-vector choice for the covariant representation of the T-matrix is applied. This momentum dependence is dominated by the pion exchange. We discuss the problems of this choice and its relations to on-shell ambiguities of the T-matrix representation. Starting from a complete pseudo-vector representation of the T-matrix, which reproduces correctly the pseudo-vector pion-exchange contributions at the Hartree-Fock level, we observe a much weaker momentum dependence of the self-energy. This fixes the range of the inherent uncertainty in the determination of the scalar and vector self-energy components. Comparing to other work, we find that extracting the self-energy components by a fit to the single particle potential leads to even more ambiguous results., Comment: 35 pages RevTex, 7 PS figures, replaced by a revised and extended version

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-Figures

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 accout both the Coulomb interaction and a kaon in-medium potential of the strong interaction. It is found that both the calculated kaon flows with only the Coulomb interaction and with both the Coulomb interaction and the strong potential agree within the error bars with the data. The kaon azimuthal distribution exhibits asymmetries of similar magnitude in both theoretical approaches. This means, the inclusion of the Coulomb potential makes it more difficult to extract information of the kaon mean field potential in nuclear matter from the kaon flow and azimuthal distribution data., Comment: 14 pages Latex, 4 PS-files

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 comparision 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 $\pi^-/\pi^+$ 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., Comment: 28 pages Latex, prepared with elsevier-style, 13 PS-figures

We establish the best possible approximation to a perfect quantum cloning machine which produces two clones out of a single input. We analyze both universal and state-dependent cloners. The maximal fidelity of cloning is shown to be 5/6 for universal cloners. It can be achieved either by a special unitary evolution or by a novel teleportation scheme. We construct the optimal state-dependent cloners operating on any prescribed two non-orthogonal states, discuss their fidelities and the use of auxiliary physical resources in the process of cloning. The optimal universal cloners permit us to derive a new upper bound on the quantum capacity of the depolarizing quantum channel., Comment: 30 pages (RevTeX), 2 figures (epsf), further results and further authors added, to appear in Physical Review A

We investigate thermodynamic properties and instability conditions in intermediate energy heavy ion reactions. We define locally thermodynamic variables, i.e. density, pressure and temperature, directly from the phase space distribution of a relativistic transport calculation. In particular, temperatures are determined by a fit to two covariant hot Fermi distributions thus taking into account possible anisotropic momentum configurations. We define instability independent from the nuclear matter spinodal by the criterion that the effective compressibility becomes negative. The method is applied to a semi-central Au on Au reaction at 600 MeV/nucleon. We investigate in particular the center of the participant and the spectator matter. In the latter we find a clear indication of instability with conditions of density and temperature that are consistent with experimental determinations., Comment: 20 pages latex, 5 PS-figures, revised version (minor changes) accepted for publication in Nucl. Phys. A

The effects of the presence of $\Delta$-isobars in nuclear matter are studied in the framework of relativistic mean-field theory. The existence of stable nuclei at saturation density imposes constraints on the $\Delta$-isobar self-energy and thereby on the mean-field coupling constants of the scalar and vector mesons with $\Delta$-isobars. The range of possible values for the scalar and vector coupling constants of $\Delta$-isobars with respect to the nucleon coupling is investigated and compared to recent predictions of QCD sum-rule calculations., Comment: 8 pages, Latex using Elsevier style, 2 PS figures, minor changes in revised version

In the relativistic Quantum Molecular Dynamics (RQMD) approach baryons are described within the framework of covariant hamilton constraint dynamics. The inclusion of a relativistic mean field results in a quasiparticle picture for the baryons. This requires to distinguish between canonical and kinetic variables of the particles. As resonances we include the $\Delta$(1232) and the $N^*$(1440) resonance. The resonance masses are distributed according Breit-Wigner functions. However, the scalar self energy leads to a shift in the masses and introduces an additional medium dependence. Consequences of this description on resonance and pion dynamics are discussed., Comment: 8 pages Latex, 4 PS-figures; To appear in the proceedings of the 25th International Hirschegg Workshop on Properties of Nuclei and Nuclear Excitations, Jan 13-18, 1997 "QCD Phase Transitions"

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--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., Comment: 6 pages Latex using RevTex, revised version accepted for publication in Phys. Rev. C

The formalism of the relativistic (or Dirac-) Brueckner approach in infinite nuclear matter is described. As nucleon-nucleon interaction the one-boson exchange potentials Bonn A,B,C and for comparison the Walecka model are used. The T-matrix is determined from the Thompson equation and is projected onto five covariant amplitudes. By the restriction to positive energy states an ambiguity arises in the relativistic Brueckner approach which is discussed here in terms of the pseudo-scalar and the pseudo-vector projection. The influence of the coupling of the nucleon via the T-matrix as an effective two-nucleon interaction to the nuclear medium is expressed by the self-energy. In particular we investigate the scalar and vector components of the self-energy for the different one-boson exchange potentials and discuss their density and momentum dependence. We estimate the uncertainty of the self-energy due to the pseudo-scalar and the pseudo-vector choice. Usually the momentum dependence of the self-energy is thought to be weak, however, we find that this depends on the one-boson exchange potentials. For the Bonn potentials, in contrast to the $\sigma\omega$-potential, the momentum dependence is strikingly strong above as well as below the Fermi surface. We compare with the results of other groups and study the effects on the equation of state and the nucleon optical potential., Comment: 23 pages Latex, using Revtex style, 10 PS-figures, revised version (only minor changes) accepted for publication in Phys. Rev. C

We investigate the influence of medium corrections to the pion dispersion relation on the pion dynamics in intermediate energy heavy ion collisions. To do so a pion potential is extracted from the in-medium dispersion relation and used in QMD calculations and thus we take care of both, real and imaginary part of the pion optical potential. The potentials are determined from different sources, i.e. from the $\Delta$--hole model and from phenomenological approaches. Depending on the strength of the potential a reduction of the anti-correlation of pion and nucleon flow in non-central collisions is observed as well as an enhancement of the high energetic yield in transverse pion spectra. A comparison to experiments, in particular to $p_t$-spectra for the reaction Ca+Ca at 1 GeV/nucleon and the pion in-plane flow in Ne+Pb collisions at 800 MeV/nucleon, generally favours a weak potential., Comment: 25 pages, using REVTeX, 6 postscript figures; replaced by published version

We investigate the influence of the real part of the in-medium pion optical potential on the pion dynamics in intermediate energy heavy ion reactions at 1 GeV/A. For different models, i.e. a phenomenological model and the $\Delta$--hole model, a pionic potential is extracted from the dispersion relation and used in Quantum Molecular Dynamics calculations. In addition with the inelastic scattering processes we thus take care of both, real and imaginary part of the pion optical potential. A strong influence of the real pionic potential on the pion in-plane flow is observed. In general such a potential has the tendency to reduce the anticorrelation of pion and nucleon flow in non-central collisions., Comment: 12 pages Latex, 4 PS-figures

We study the influence of non-equilibrium phase space effects on the dynamics of heavy ion reactions within the relativistic BUU approach. We use realistic Dirac-Brueckner-Hartree-Fock (DBHF) mean fields determined for two-Fermi-ellipsoid configurations, i.e. for colliding nuclear matter, in a local phase space configuration approximation (LCA). We compare to DBHF mean fields in the local density approximation (LDA) and to the non-linear Walecka model. The results are further compared to flow data of the reaction $Au$ on $Au$ at 400 MeV per nucleon measured by the FOPI collaboration. We find that the DBHF fields reproduce the experiment if the configuration dependence is taken into account. This has also implications on the determination of the equation of state from heavy ion collisions., Comment: Physics Letters B in press; 10 pages, Postscript file replaced by Latex file and 3 Postscript figures

We show that, given a general mixed state for a quantum system, there are no physical means for {\it broadcasting\/} that state onto two separate quantum systems, even when the state need only be reproduced marginally on the separate systems. This result generalizes and extends the standard no-cloning theorem for pure states., Comment: 11 pages, formatted in RevTeX

A fully covariant approach to a density dependent hadron field theory is presented. The relation between in--medium NN interactions and field--theoretical meson--nucleon vertices is discussed. The medium dependence of nuclear interactions is described by a functional dependence of the meson--nucleon vertices on the baryon field operators. As a consequence, the Euler--Lagrange equations lead to baryon rearrangement self--energies which are not obtained when only a parametric dependence of the vertices on the density is assumed. It is shown that the approach is energy--momentum conserving and thermodynamically consistent. Solutions of the field equations are studied in the mean--field approximation. Descriptions of the medium dependence in terms of the baryon scalar and vector density are investigated. Applications to infinite nuclear matter and finite nuclei are discussed. Density dependent coupling constants obtained from Dirac--Brueckner calculations with the Bonn NN-potentials are used. Results from Hartree calculations for energy spectra, binding energies and charge density distributions of $^{16}O$, $^{40,48}Ca$ and $^{208}Pb$ are presented. Comparisons to data strongly support the importance of rearrangement in a relativistic density dependent field theory. Most striking is the simultanuous improvement of charge radii, charge densities and binding energies. The results indicate the appearance of a new "Coester line" in the nuclear matter equation of state., Comment: 48 LateX pages, 12 Figures, figures and full paper are available as postscript files by anonymous ftp at ftp://theorie.physik.uni-giessen.de/ddh