Search

An investigation of color fields that arise in collisions of relativistic heavy ions reveals that, in the non-Abelian case, a change in the color charge leads to the appearance of an extra term that generates a sizable contribution of color-charge glow in chromoelectric and chromomagnetic fields. The possibility of the appearance of a color echo in the scattering of composite color particles belonging to the dipole type is discussed. Arguments are adduced in support of the statement that such effects are of importance in simulating the first stage of ultrarelativistic heavy-ion collisions,where the initial parton state is determined by a high nonequilibrium parton density and by strong local color fluctuations. [ABSTRACT FROM AUTHOR]

The dynamics of hot and dense nuclear matter is discussed from the microscopic transport point of view. The basic concepts of the Hadron-String-Dynamical transport model (HSD)-derived from Kadanoff-Baym equations in phase phase-are presented as well as 'highlights' of HSD results for different observables in heavy-ion collisions from 100 A MeV (SIS) to 21 A TeV(RHIC) energies. Furthermore, a novel extension of the HSD model for the description of the partonic phase-the Parton-Hadron-String-Dynamics (PHSD) approach-is introduced. PHSD includes a nontrivial partonic equation of state-in line with lattice QCD-as well as covariant transition rates from partonic to hadronic degrees of freedom. The sensitivity of hadronic observables to the partonic phase is demonstrated for relativistic heavy-ion collisions from the FAIR/NICA up to the RHIC energy regime. [ABSTRACT FROM AUTHOR]

The dynamics of partons, hadrons and strings in relativistic nucleus-nucleus collisions is analyzed within the novel Parton-Hadron-String Dynamics (PHSD) transport approach, which is based on a dynamical quasiparticle model for partons (DQPM) matched to reproduce recent lattice-QCD results-including the partonic equation of state-in thermodynamic equilibrium. The transition from partonic to hadronic degrees of freedom is described by covariant transition rates for the fusion of quark-antiquark pairs or three quarks (antiquarks), respectively, obeying flavor current-conservation, color neutrality as well as energymomentum conservation. Since the dynamical quarks and antiquarks become very massive close to the phase transition, the formed resonant 'pre-hadronic' color-dipole states ( q $\bar q$ or qqq) are of high invariant mass, too, and sequentially decay to the groundstate meson and baryon octets increasing the total entropy. When applying the PHSD approach to Pb + Pb colllisions at 158 A GeV we find a significant effect of the partonic phase on the production of multi-strange antibaryons due to a slightly enhanced s $\bar q$ pair production from massive time-like gluon decay and a larger formation of antibaryons in the hadronization process. [ABSTRACT FROM AUTHOR]

Using the Quark-Gluon Strings Model --combined with Regge phenomenology-- we perform a comparative analysis of Λ, Σ0, Λ(1520) and Θ+ production in binary reactions induced by photon, pion and proton beams on the nucleon. We find that the existing experimental data on the γ p↦ K+Λ differential and total cross-sections can be described very well by the model for photon energies 1-16 GeV and - t < 2 GeV2 assuming a dominant contribution of the K* Regge trajectory. Moreover, using the same parameters we also reproduce the total γ p↦ K+Σ0 and γ p↦ K+Λ(1520) cross-sections suggesting a “universality” of the Regge model. In order to check the consistency of the approach we evaluate the differential and total cross-sections for the reaction π- p↦ K0Λ which is also found to be dominated by the K* Regge trajectory. Using the apparent “universality” of the Regge model we extend our scheme to the analysis of the binary reactions γ p↦¯0Θ+, π- p↦ K-Θ+ and pp↦Σ+Θ+ as well as the exclusive and inclusive Θ+ production in the reactions pp↦ p¯0Θ+ and pp↦Θ+ X. Our detailed studies demonstrate that Θ+ production does not follow the “universality” principle, thus suggesting an essentially different internal structure of the exotic baryon relative to conventional hyperons or hyperon resonances. [ABSTRACT FROM AUTHOR]

The production of K+-mesons in proton-nucleus collisions from 1.0 to 2.3 GeV is analyzed with respect to one-step nucleon-nucleon (NN?NYK+) and two-step ?-nucleon (?N?K+ YN) or pion-nucleon (pN?K+ Y) production channels on the basis of a coupled-channel (CBUU) transport approach including the kaon final-state-interactions (FSI). Momentum-dependent potentials for the nucleon, hyperon and kaon in the final state are included as well as K+ elastic rescattering in the target nucleus. The transport calculations are compared to the experimental K+ spectra taken at COSY-Jülich. Our systematic analysis of K+ spectra from12C,63Cu,107Ag and197Au targets as well as their momentum differential ratios gives a repulsive K+ potential of 20±5 MeV at normal nuclear matter density. [ABSTRACT FROM AUTHOR]

We propose a nonperturbative resummation scheme for the four-point connected quark-antiquark Green’s functionG4 that shows how the Bethe-Salpeter equation may be “unquenched” with respect to quark-antiquark loops. This mechanism allows to dynamically account for hadronic meson decays and multiquark structures whilst respecting the underlying symmetries. An initial approximation to the four-point Schwinger-Dyson equation - suitable for phenomenological application - is examined numerically in a couple of aspects. It is demonstrated that this approximation explicitly maintains the correct asymptotic limits and contains the physical resonance structures in the near timelike region in the quark-antiquark channel. [ABSTRACT FROM AUTHOR]

The effect of quark-gluon vertex dressing on the ground-state masses of theu/d-quark pseudoscalar, vector and axialvector mesons is considered with the Dyson-Schwinger equations. This extends the ladder-rainbow Bethe-Salpeter kernel to two-loops. To render the calculations feasible for this exploratory study, we employ a simple infrared dominant model for the gluon exchange that implements the vertex dressing. The resulting model, involving two distinct representations of the effective gluon exchange kernel, preserves both the axial-vector Ward-Takahashi identity and charge conjugation symmetry. Numerical results confirm that the pseudoscalar meson retains its Goldstone boson character. The vector meson mass, already at a very acceptable value at ladder level, receives only 30?MeV of attraction from this vertex dressing. For the axial-vector states, which are about 300?MeV too low in ladder approximation, the results are mixed: The 1+- state receives 290?MeV of repulsion, but the 1++ state is lowered further by 30?MeV. The exotic channels 0-- and 1-+ are found to have no states below 1.5?GeV in this model. [ABSTRACT FROM AUTHOR]

Using an effective Lagrangian approach as well as the Quark-Gluon Strings Model, we analyze near-threshold production of a0(980)-mesons in the reaction NN → dK &Kmacr; as well as the background of non-resonant K &Kmacr;-pair production. We argue that the reaction pp → dK+ &Kmacr;0 at an energy release Q ≤ 100MeV is dominated by the intermediate production of the a0(980)-resonance. At larger energies the non-resonant K+ &Kmacr;0-pair production — where the kaons are produced in a relative P-wave — becomes important. The effects of final-state interactions are evaluated in a unitarized scattering length approach and found to be in the order of a 20% suppression close to threshold. Thus, in present experiments at the Cooler Synchrotron COSY-Jülich for Q ≤ 107 MeV the a0+ signal can reliably be separated from the non-resonant K+ &Kmacr;00 background. [ABSTRACT FROM AUTHOR]

Deuteron two-body photodisintegration is analysed within the framework of the Quark-Gluon Strings Model. It is found that the forward-backward angle asymmetry predicted by the model is confirmed by the recent data at different photon energies from JLab. New calculations for polarization observables, the cross-section asymmetry Σ and the polarization transfer Cz', for photon energies (1.2-6) GeV are presented and compared with the data available up to 2 GeV. [ABSTRACT FROM AUTHOR]

The production and decay of vector mesons (p, w) in pA and AA reactions is studied with particular emphasis on their in-medium spectral functions. It is explored within transport calculations if hadronic in-medium decays like Π+π- or π°γ might provide complementary information to their dilepton (e+e-) decays. Whereas the π+&pi,- signal from the p-meson is found to be strongly distorted by pion rescattering, the w-meson Dalitz decay to π°γ appears promising even for more heavy nuclei in 7A and pA reactions. Furthermore, the influence of nucleon and kaon/antikaon potentials on the K± yields and spectra in pA collisions is calculated and compared to the recent data from the ANKE Collaboration. [ABSTRACT FROM AUTHOR]

: We study the perspectives of resonant charmonium production in p + A reactions within the Multiple Scattering Monte Carlo (MSMC) approach. We calculate the production of the resonances Ψ(2S) and Ψ(3770) on various nuclei, their propagation and decay to dileptons and D + D in the medium and vacuum, respectively, employing parametrizations for the D,D self-energies taken from QCD sum rule studies. The elastic and inelastic interactions of the charmonia and open-charm mesons in the medium are taken into account, too. It is found that the D,D invariant-mass spectra from light and heavy nuclei are not sufficiently sensitive to the in-medium properties of the Ψ(2S) and Ψ(3770). However, a “suppression” of low-mass dileptons from the Ψ(3770) might be seen experimentally as well as a small broadening of the Ψ(2S) dilepton spectra. [ABSTRACT FROM AUTHOR]

The lifetime of the Λ-hyperon in heavy hypernuclei measured in proton-Au, -Bi and -U collisions by the COSY-13 Collaboration at COSY-Jülich has been analyzed to yield τΛ = (145±11) ps. This value for τΛ is compatible with the lifetime extracted from antiproton annihilation on Bi and U targets, albeit much more accurate. Theoretical models based on the meson exchange picture and assuming the validity of the phenomenological Δ I = 1/2 rule predict the lifetime of heavy hypernuclei to be significantly larger (2-3 standard deviations). Such large differences indicate that at least one of the assumptions in these models is not fulfilled. A much better reproduction of the lifetimes of heavy hypernuclei is achieved in the phase space model, if the Δ I = 1/2 rule is discarded in the nonmesonic Λ decay. [ABSTRACT FROM AUTHOR]

We consider near-threshold a[sub 0](980)-meson production in πN and NN collisions. An effective Lagrangian approach with one-pion exchange is applied to analyze different contributions to the cross section for different isospin channels. The Reggeon exchange mechanism is also evaluated for comparison. The results from πN reactions are used to calculate the contribution of the a[sub 0] meson to the cross sections and invariant KK mass distributions of the reactions pp → pnK[sup +]K[sup 0] and pp → ppK[sup +]K[sup –]. It is found that the experimental observation of a[sub 0][sup +] mesons in the reaction pp → pnK[sup +]K[sup 0] is much more promising than the observation of a[sub 0][sup 0] mesons in the reaction pp → ppK[sup +]K[sup –]. Effects of isospin violation in the reactions pN → da[sub 0], pd → [sup 3]He([sup 3]H)a[sub 0], and dd → [sup 4]Hea[sub 0], which are induced by a[sub 0](980)–f[sub 0](980) mixing, are also analyzed.© 2003 MAIK “Nauka / Interperiodica”. [ABSTRACT FROM AUTHOR]

The production of K+-mesons in proton-nucleus collisions from 1.0 to 2.5GeV is analyzed with respect to one-step nucleon-nucleon ( NN → NYK+) and two-step Δ-nucleon ( Δ N → K+ YN) or pion-nucleon ( π N → K+ Y) production channels on the basis of a coupled-channel transport approach (CBUU) including the kaon final-state interactions. The influence of momentum-dependent potentials for the nucleon, hyperon and kaon in the final state are studied as well as the importance of K+ elastic rescattering in the target nucleus. The transport calculations are compared to the experimental K+ spectra taken at LBL Berkeley, SATURNE, CELSIUS, GSI and COSY-Jülich. It is found that the momentum-dependent baryon potentials affect the excitation function of the K+ cross-section; at low bombarding energies of ∼ 1.0GeV the attractive baryon potentials in the final state lead to a relative enhancement of the kaon yield, whereas the net repulsive potential at bombarding energies ∼ 2GeV causes a decrease of the K+ cross-section. Furthermore, it is pointed out that especially the K+ spectra at low momenta (or kinetic energy TK) allow to determine the in-medium K+ potential almost model independently due to a relative shift of the K+ spectra in kinetic energy that arises from the acceleration of the kaons when propagating out of the nuclear medium to free space, i.e. converting the potential energy to the kinetic energy of the free kaon. [ABSTRACT FROM AUTHOR]

We study the perspectives of resonant and nonresonant charmed-meson production in ¯p + A reactions within the Multiple Scattering Monte Carlo (MSMC) approach. We calculate the production of the resonances Ψ(3770),Ψ(4040) and Ψ(4160) on various nuclei, their propagation and decay to D,¯D, D*,¯D*, Ds,¯Ds in the medium and vacuum, respectively. The modifications of the open charm vector mesons in the nuclear medium are found to be rather moderate or even small such that dilepton spectroscopy will require an invariant mass resolution of a few MeV. Furthermore, the elastic and inelastic interactions of the open charm mesons in the medium are taken into account, which can be related to ( u, d )-, s- or c-quark exchange with nucleons. It is found that by studying the D/¯D ratio for low momenta in the laboratory ( ? 2 - 2.2 GeV/c) as a function of the target mass A stringent constraints on the c-quark exchange cross-section can be obtained. On the other hand, the ratios D- s/ D+ s as well as D/ D- s and D/ D+ s at low momenta as a function of A will permit to fix independently the strength of the s-quark exchange reaction in D- s N scattering. [ABSTRACT FROM AUTHOR]

The ω resonance production and its π0γ decay in pA reactions close to threshold is considered within the Intranuclear Cascade (INC) model. The π0γ invariant-mass distribution shows two components which correspond to the ω decay “inside” and “outside” the nucleus, respectively. The “inside” component is distorted by medium effects, which introduce a mass shift as well as collisional broadening for the ω-meson and its decaying pion. The relative contribution of the “inside” component is analyzed in detail for different kinematical conditions and nuclear targets. It is demonstrated that a measurement of the correlation in azimuthal angle between the π0 and γ momenta allows to separate events related to the “inside”ω decay from different sources of background when uncorrelated π0's and γ's are produced. [ABSTRACT FROM AUTHOR]

Simulations based on a coupled-channel transport model have been performed to analyze the feasibility to study the in-medium ω mass exploiting the process γ + A→π0γ + X for C, Ca and Nb nuclei. The distortions due to final-state interactions of the π0 and background contributions from the reaction γ + A→π0π0 + X are found to be small in the mass range of interest ( 0.6 < Mπ0γ < 0.8 GeV). Furthermore, the effect of the detector resolution on the π0γ mass determination is discussed. [ABSTRACT FROM AUTHOR]

The lifetime of the Λ-hyperon for the nonmesonic decay ΛN → NN has been determined by a measurement at COSY Jülich of the delayed fission of heavy hypernuclei produced in proton-Au collisions at Tp = 1.9 GeV. It is found that heavy hypernuclei with mass numbers A≈ (180±5) and atomic numbers Z≈ (74±2) fission with a lifetime τΛ = 130±13 (stat.) ±15 (syst.) ps. This value together with the results obtained for other heavy hypernuclei in previous investigations indicates --on the confidence level of 0.9-- a violation of the phenomenological Δ I = 1/2 rule for the ΛN → NN transitions as known from the weak mesonic decays of kaons and hyperons. [ABSTRACT FROM AUTHOR]

We investigate deuteron two-body photodisintegration within the framework of the Quark-Gluon Strings Model with nonlinear baryon Regge trajectories. Special attention is paid to the use of QCD motivated Regge trajectories of the logarithmic and square-root form. We find that the recent experimental data from TJNAF in the few-GeV region can be reasonably described by the model. Angular distributions at different γ-energies are presented and the effect of a forward-backward asymmetry is discussed. Predictions for the energy dependence of dσ/d t at higher energies and different Θc.m. are presented, too. [ABSTRACT FROM AUTHOR]

We investigate the reactions π N↦ a0 N and pp↦ da0 + near threshold and at medium energies. An effective Lagragian approach and the Regge pole model are applied to analyze different contributions to the cross-section of the reaction π N↦ a0 N. These results are used to calculate the differential and total cross-sections of the reaction pp↦ da0 + within the framework of the two-step model in which two nucleons produce an a0-meson via π-meson exchange and fuse to a deuteron. The necessity of new measurements on a0 production and branching fractions (of its decay to the K and πη channels) is emphasized for clarifying the a0 structure. Detailed predictions for the reaction pp→ da0 + are presented for the energy regime of the proton synchrotron COSY-Jülich. [ABSTRACT FROM AUTHOR]

The possibility for an observation of a narrow penta-quark Z+-baryon in NN reactions is discussed. It is shown that the pp↦ nΣ+ K+ reaction at excess energies around 100 MeV above threshold provides optimal conditions for Z+-baryon detection by an analysis of the nK+ invariant mass spectrum, if the Z+ mass is located around 1.5 GeV involving a rather narrow width. [ABSTRACT FROM AUTHOR]

The proton–charged pion correlated emission is studied in the reactions Au (1.06 AGeV) + Au, Ni (1.06 and 1.93 AGeV) + Ni and Ni (1.97 AGeV) + Cu within the BUU approach. The associated invariant mass distributions are shifted to smaller energies with respect to the free Δ(1232) mass distribution due to kinematical reasons. We find that the existing and partly conflicting experimental data do not allow to draw definite conclusions on the in-medium modification of the Δ(1232). [ABSTRACT FROM AUTHOR]

The available data on ω and φ production from πN and pp collisions are reanalyzed with respect to an OZI rule violation on the basis of transition matrix elements. The data are found to be compatible with a constant ratio R , which however, deviates substantially from the SU(3) prediction based on the present knowledge of the φ-ω mixing angle. [ABSTRACT FROM AUTHOR]

We explore the possibility to measure the elastic and inelastic ω N cross section in p+ d→ d+ω+ psp and p+ A reactions. Our studies indicate that the elastic scattering cross sections can be determined for ω momenta above 1 GeV/c in p+ d reactions by gating on high proton spectator momenta whereas the ω N absorption cross section down to low relative ω momenta is most effectively studied in p+ A reactions at beam energies 2.0–2.7 GeV. [ABSTRACT FROM AUTHOR]

We study the possibility to measure the elastic Φ N (Φ≡ J/ψ,ψ(2 S), ψ(3770), χ2c) scattering cross section in the reaction ¯ p+ d→ Φ+ nsp and the elastic D(¯ D) N scattering cross section in the reaction ¯ p+ d→ D− D0 psp. Our studies indicate that the elastic scattering cross sections can be determined for Φ momenta about 4–6 GeV/c and D/¯ D momenta 2–5 GeV/c by selecting events with pt≥ 0.4 GeV/c for Φ's and pt( psp) ≥ 0.5 GeV/c for D/¯ D-meson production. [ABSTRACT FROM AUTHOR]

We calculate the elastic and inelastic ω N→ω N, →π N, →ρ N, →ρπ N, →ππ N, →σ N reactions within a boson exchange approximation where the ωρπ coupling constant and form factor are fixed by the reaction π N→ω N in comparison to the experimental data. We find rather large ωN cross sections at low relative momenta of the ω-meson which leads to a substantial broadening of the ω-meson width in nuclear matter. The implications of the ω N final state interactions are studied for ω production in 12C +12C, 40Ca +40Ca and 58Ni +58Ni reactions at about 2 · A GeV within the HSD transport approach; the drastic changes of the transverse mass spectra relative to a general mT-scaling (for π0 and η mesons) might be controlled experimentally by the TAPS Collaboration. [ABSTRACT FROM AUTHOR]

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 β 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. [ABSTRACT FROM AUTHOR]

We study the mixing and the kinetic equilibration of projectile and target nucleons in relativistic heavy-ion collisions in the energy regime between 150 AMeV and 2 AGeV in a coupled-channel BUU (CBUU) approach. We find that equilibrium in the projectile-target degrees of freedom is in general not reached even for large systems at low energy where elastic nucleon-nucleon collisions dominate. Inelastic nucleon excitations are more favorable for equilibration and their relative abundance increases both with energy and mass. Experimentally, the projectile/target admixture can be determined by measuring the degree of isospin equilibration in isospin asymmetric nuclear collisions. For one of the most promising systems currently under investigation, 96 44Ru +96 40Zr, we investigate the influence of the equation of state and the inelastic in-medium cross section. [ABSTRACT FROM AUTHOR]

Within the meson-exchange model we calculate f0-meson production cross section in π N and NN reactions and investigate the possibility for f0 observation via the K¯ K decay mode in pp collisions. Our studies indicate that an extraction of the f0 signal is unlikely due to the large background from other reaction channels. [ABSTRACT FROM AUTHOR]

Using the method of correlation dynamics we investigate the properties of a field-theory for fermions and scalar bosons coupled via a Yukawa interaction. Within this approach, which consists in an expansion of full equal-time Green functions into connected equal-time Green functions and a corresponding truncation of the hierarchy of equations of motion we carry out calculations up to 4th order in the connected Green functions and evaluate the effective potential of the theory in 1+1 dimensions on a torus. Comparing the different approximations we find a strong influence of the connected 4-point functions on the properties of the system. [ABSTRACT FROM AUTHOR]

The two-dimensional Fokker-Planck equation describing the phase-space evolution of the reduced density matrix f( r, v; t) with respect to radial distance r and radial velocity v is numerically solved for various cases of inelastic heavy-ion collisions. A comparison of the exact solution with the global moment approach (GMA) used so far by several authors clearly shows that the GMA is not applicable for deeply inelastic collisions. A local moment approach (LMA) is proposed as an approximate solution to the Fokker-Planck equation. [ABSTRACT FROM AUTHOR]

The quantal double differential cross-section averaged over micro channels is parametrized in a way that accounts for the transfer of radial momentum and orbital angular momentum of relative motion of two colliding heavy ions into intrinsic degrees of freedom. The formfactors introduced are related to the solutions of classical Fokker-Planck equations and to semiclassical approximations of the phase shifts. An application of the model toAr→Th at E=388 MeV which is based on familiar classical friction models and contains no further parameters yields a cross section d σ/dθ dE in good overall agreement with the experimental data. [ABSTRACT FROM AUTHOR]

The quantal description of the relative motion of two complex nuclei is discussed on the basis of a Wigner transformed reduced von-Neumann equation which reduces to a transport equation of the Fokker-Planck type in the classical limit. For relative motion in three spatial dimensions a partial waves expansion of the reduced density operator leads to a set of equations for a matrix of quantal Wigner functions ρ( R, K) which decouple if angular momentum transfer is neglected. It is shown that the evolution in R-space of the radial momentum distributions ρ( R, K) is sufficiently well described by incorporating only statistical fluctuations, and that the effect of quantal fluctuations in the double differential cross section d σ/dθdE is mainly due to the interference of the contributions from various partial waves. An application to the reactionAr→Th at E=388 MeV based on the same nucleus-nucleus potential and the same transport coefficients as classical models shows that the coherent summation of the contributions of all partial waves reproduces in a natural way the large experimentally observed spreading in the Wilczynski plot in the quasi-elastic region. [ABSTRACT FROM AUTHOR]

A partial integro-differential equation is formulated for the Wigner transform of the quantum mechanical reduced density operator describing the time evolution of a 'macroscopic' coordinate under the influence of coupling to a large number of 'intrinsic' degrees of freedom. The equation contains integral operators which lead to energy dissipation and diffusion and reduces to a transport equation of the Fokker-Planck type if the form factors in the integrands are treated in appropriate (harmonic) approximations. The stationary solution of the partial integro-differential equation is obtained numerically for scattering by a conservative potential and by a dissipative and diffusive scattering centre in one spatial dimension. [ABSTRACT FROM AUTHOR]

The dynamical description of strongly interacting finite Fermi systems is based on coupled equations of motion for the one-body density matrix ρ(11′) and the two-body correlation function c(12, 1′2′) as obtained from the density-matrix hierarchy. The truncation schemes considered exceed the conventional Brueckner-Hartree-Fock scheme and also apply for nonstationary problems. In the limit of slow processes in time the equation of motion for the two-body correlation function c can be integrated in time and closed expressions can be given for the dynamical evolution of ρ(11′). When performing a Wigner transformation and adopting semiclassical limits this gives a transport equation of the Uehling-Uhlenbeck type for the one-body phase-space distribution f( x, p; t). Furthermore, in the small amplitude limit, we obtain a set of coupled equations for particle-hole ( p-h), 2p-2p, 2h-2h, 2p-2h, 1p-3h, 1h-3p amplitudes beyond the level of second RPA which provide a genuine basis for the description of giant resonances and their damping width. [ABSTRACT FROM AUTHOR]

We present an extension of relativistic mean-field theories within the framework of quantum correlation dynamics starting from an effective hadronic Lagrangian density with minimal coupling between baryons and mesons. When neglecting retardation for the meson fields the conventional meson-exchange picture is obtained for the baryon-baryon interaction. The resulting equations of motion for the baryonic one-body density matrix and two-body correlation spinor matrix provide a nonperturbative framework for the description of relativistic heavy-ion collisions. Adopting semiclassical limits for the Wigner transform of the one-body density matrix a covariant transport equation is derived in the limit of time-dependent Dirac-Brueckner theory. It describes the mean-field dynamics including momentum-dependent potentials as well as in-medium nucleon-nucleon collisions on the basis of the same meson fields without double counting of interactions. [ABSTRACT FROM AUTHOR]

We derive a transport equation for the description of non-relativistic heavy-ion collisions from time-dependent G-matrix theory. The approximations introduced are investigated within a modified Lipkin model on the A-body level and by one-dimensional calculations for colliding finite fermion systems on the two-body level. Both approximations are found to hold reasonably well. The classical limit of these equations in phase space yields a transport equation of the Vlasov-Uehling-Uhlenbeck type without double counting of interactions and uniquely determines the time-dependent mean field as well as an in-medium nucleon-nucleon cross-section on the basis of the same G-matrix. [ABSTRACT FROM AUTHOR]

A selfconsistent quantal two-body theory as obtained from the density-matrix hierarchy is solved numerically for the first time for a one-dimensional system modelling α + α collisions at 80 MeV/u. Various truncation schemes for the two-body correlation function are investigated in order to explore the convergence properties of the theory. We find that perturbative treatments with respect to two-body processes do not yield reliable results in the energy regime investigated and that the nuclear stopping power sensitively depends on the order of the two-body correlations considered. [ABSTRACT FROM AUTHOR]

The Uehling-Uhlenbeck collision term is derived from coupled equations of motion for the one-body density matrix and the two-body correlation function as obtained from the density-matrix hierarchy. The respective approximations introduced are related with energy and momentum conservation in single nucleon-nucleon collisions and found numerically to be approximately valid for energetic nucleon-nucleon collisions in case of intermediate energy heavy-ion reactions. Primary collision times of nucleons in the dinuclear reaction zone are calculated for various laboratory energies and the approach to local equilibrium is followed in time. Average relaxation times for highly excited finite nuclear systems are found to be considerably shorter than for related infinite nuclear matter problems, however, still too large as to allow for the formation of hot spots in nuclei. [ABSTRACT FROM AUTHOR]

We describe the effective static quark-(anti)quark colour potential inside hadronic bags in terms of linear strings and semiclassical corrections of order ħ/r. According to (chromo-) electrostatics we generalize this potential to colour non-neutral configurations making use of the mirror charge method. A thermodynamical study of quark-antiquark pairs and many-quark systems shows a virtually higher order phase transition from the hadronic phase to colour deconfinement. The free plasma stage, furthermore, is achieved only at temperatures much higher than T≈200 MeV. Hadrons increase in size by a factor of 1.5 from T=0 to T=T. [ABSTRACT FROM AUTHOR]

A one-dimensional toy model of two moving finite boxes is analysed with respect to quantal phenomena associated with heavy-ion dynamics at low and intermediate energies. Special attention is payed to the relation between energy and momentum of the nucleons inside and outside the time-dependent mean field. A Wigner transformation of the one-body density matrix in space and time allows for a unique comparison with classical phase-space dynamics. It is found that high momentum components of the nuclear groundstate wave function approximately become on-shell during the heavy-ion reaction. This leads to the emission of energetic nucleons which do not appear classically. It is furthermore shown, that the low lying eigenstates of the dinuclear system for fixed time are only partly occupied throughout the reaction at intermediate energies. This opens up final phase space for nucleons after producing e.g. a pion or energetic photon. Though the present model does not allow for a reliable calculation of double differential nucleon spectra, pion or photon cross sections, it transparently shows the peculiar features of quantum dynamics in heavy-ion collisions. [ABSTRACT FROM AUTHOR]

Production mechanisms for subthreshold pions in nucleus-nucleus collisions are studied on a microscopic level. The non-relativistic expansion of the pseudoscalar π NN-vertex provides a basis for the construction of several two-nucleon models. These are incorporated into a dynamical description of the heavy-ion collision by boosted harmonic oscillators that adopts essential features from TDHF. The approach allows for a separate analysis of the various production processes and the influence of the collision dynamics on the total pion yield. Results are presented for the systemO +O at laboratory energies of 40 and 80 MeV per nucleon. [ABSTRACT FROM AUTHOR]

We investigate the production of e e pairs from p + Be and nucleus-nucleus collisions from 2 GeV/A to 200 GeV/A within a covariant transport approach and focus on the dilepton angular anisotropies as a function of the dilepton invariant mass. Whereas the low mass regime yields information about the Dalitz decays of the Δ, η and ω, above M ≈ 0.5 GeV the angular anisotropies provide additional information about the π π → ρ → e e channel in the medium. The anisotropy coefficient is found to change its sign for M > 0.5 GeV in case of nucleus-nucleus reactions when increasing the bombarding energy from 2 GeV/A to 200 GeV/A which sheds some light on the ππ → ρ dynamics. [ABSTRACT FROM AUTHOR]

We apply a truncated set of dynamical equations of motion for connected equal-time Green functions up to the 4-point level to the investigation of spontaneous ground state symmetry breaking in Φ quantum field theory. Within our momentum space discretization we obtain a second order phase transition as soon as the connected 3-point function is included. However, an additional inclusion of the connected 4-point function still shows a significant influence on the shape of the effective potential and the critical coupling. [ABSTRACT FROM AUTHOR]

We study the production of K+, ρ, ω and ϕ mesons in p+12C reactions on the basis of empirical spectral functions. The high momentum, high removal energy part of the spectral function is found to be negligible in all cases close to the absolute threshold. Furthermore, the two-step process (pN → π N N; π N → N +K+, ρ,ω,ϕ) dominates the cross section at threshold energies in line with earlier calculations based on the folding model. [ABSTRACT FROM AUTHOR]

We investigate the convergence properties of the cluster expansion of equal-time Green functions in scalar theories with quartic self-coupling in (0 + 1), (1 + 1), and (2 + 1) space-time dimensions. The computations are carried out within the equal-time correlation dynamics approach, which consists in a closed set of coupled equations of motion for connected Green functions as obtained by a truncation of the BBGKY hierarchy. We find that the cluster expansion shows good convergence as long as the system is in a localized state (single phase configuration) and that it breaks down in a non-localized state (two phase configuration), as one would naively expect. Furthermore, in the case of dynamical calculations with a time dependent Hamiltonian for the evaluation of the effective potential we find two timescales determining the adiabaticity of the propagation; these are the time required for adiabaticity in the single phase region and the time required for tunneling into the non-localized lowest energy state in the two phase region. Our calculations show a good convergence for the effective potentials in (1 + 1) and (2+1) space-time dimensions since tunneling is suppressed in higher space-time dimensions. [ABSTRACT FROM AUTHOR]

We investigate the production of pions in heavyion collisions in the energy range of 1-2 GeV/A. The dynamics of the nucleus-nucleus collisions is described by a set of coupled transport equations of the Boltzmann-Uehling-Uhlenbeck type for baryons and mesons. Besides the N(938) and the Δ(1232) we also take into account nucleon resonances up to masses of 1.95 GeV/c as well as π-, η- and ρ-mesons. We study in detail the influence of the higher baryonic resonances and the 2 π-production channels ( NN→NNππ) on the pion spectra in comparison to π data from Ar+ KCl collisions at 1.8 GeV/A and π -data for Au+Au at 1.0GeV/A. We, furthermore, present a detailed comparison of differential pion angular distributions with the BEVALAC data for Ar+ KCl at 1.8 GeV/A. The general agreement obtained indicates that the overall reaction dynamics is well described by our novel transport approach. [ABSTRACT FROM AUTHOR]