Correlations and Equilibration in Relativistic Quantum Systems

In this article we study the time evolution of an interacting field theoretical system, i.e. \phi^4-field theory in 2+1 space-time dimensions, on the basis of the Kadanoff-Baym equations for a spatially homogeneous system including the self-consistent tadpole and sunset self-energies. We find that equilibration is achieved only by inclusion of the sunset self-energy. Simultaneously, the time evolution of the scalar particle spectral function is studied for various initial states. We also compare associated solutions of the corresponding Boltzmann equation to the full Kadanoff-Baym theory. This comparison shows that a consistent inclusion of the spectral function has a significant impact on the equilibration rates only if the width of the spectral function becomes larger than 1/3 of the particle mass. Furthermore, based on these findings, the conventional transport of particles in the on-shell quasiparticle limit is extended to particles of finite life time by means of a dynamical spectral function A(X,\vec{p},M^2). The off-shell propagation is implemented in the Hadron-String-Dynamics (HSD) transport code and applied to the dynamics of nucleus-nucleus collisions.Comment: 20 pages, 7 figures to appear in "Nonequilibrium at short time scales - Formation of correlations", edited by K. Morawetz, Springer, Berlin (2003), p16

Correlation Dynamics of Yukawa-theory in 1+1 dimensions

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.Comment: LaTeX, 38 pages, 12 figure

Dileptons from the nonequilibrium Quark-Gluon Plasma

According to the dynamical quasiparticle model (DQPM) -- matched to reproduce lattice QCD results in thermodynamic limit, -- the constituents of the strongly interacting quark-gluon plasma (sQGP) are massive and off-shell quasi-particles (quarks and gluons) with broad spectral functions. In order to address the electromagnetic radiation of the sQGP, we derive off-shell cross sections of $q\bar q\to\gamma^*$, $q\bar q\to\gamma^*+g$ and $qg\to\gamma^*q$($\bar q g\to\gamma^* \bar q$) reactions taking into account the effective propagators for quarks and gluons from the DQPM. Dilepton production in In+In collisions at 158 AGeV is studied by implementing these processes into the parton-hadron-string dynamics (PHSD) transport approach. The microscopic PHSD transport approach describes the full evolution of the heavy-ion collision: from the dynamics of quasi-particles in the sQGP phase (when the local energy density is above $\sim 1$ GeV/fm$^3$) through hadronization and to the following hadron interactions and off-shell propagation after the hadronization. A comparison to the data of the NA60 Collaboration shows that the low mass dilepton spectra are well described by including a collisional broadening of vector mesons, while the spectra in the intermediate mass range are dominated by off-shell quark-antiquark annihilation, quark Bremsstrahlung and gluon-Compton scattering in the nonperturbative QGP. In particular, the observed softening of the $m_T$ spectra at intermediate masses (1 GeV $\le M \le$ 3 GeV) is approximately reproduced.Comment: 8 pages, 8 figures, to be published in the Proceedings of the 26th Winter Workshop on `Nuclear Dynamics', Ochto Rios, Jamaica, 2-9 January, 201

Dilepton production in p+p, Cu+Cu and Au+Au collisions at 200 AGeV

We study dilepton production in proton-proton, Cu+Cu as well as in Au+Au collisions at the center-of-mass energy 200 GeV per participating nucleon pair within an extended statistical hadronization model. In extension to earlier studies we incorporate transport calculations for an estimate of uncorrelated e+e- -pairs from semileptonic D meson decays. While the invariant mass spectrum of dielectrons is well understood in the p+p collisions, severe discrepancies among different model scenarios based on hadronic degrees of freedom and recent data from the PHENIX Collaboration are found in heavy-ion collisions in the low mass region from 0.15 to 0.6 GeV as well as in the intermediate mass regime from 1.1 to 3 GeV when employing the standard dilepton sources. We investigate, furthermore, the background from correlated dileptons that are not emitted as a pair from a parent hadron but emerge from semileptonic decays of two correlated daughter hadrons. Our calculations suggest a sizeable contribution of such sources in central heavy-ion collisions in the low mass region. However, even the upper limits of our calculations are found to be far below the dilepton mass spectra of the PHENIX Collaboration.Comment: revised version, 17 pages, 13 figure

Dilepton production in proton-proton collisions at top RHIC energy

We study dielectron production in proton-proton collisions at top RHIC beam energy within an extended statistical hadronization model. The invariant mass spectrum of correlated dielectron pairs is evaluated in the low invariant mass region and calculated results are compared with the PHENIX experiment. The model is found to be able to describe the data very well up to invariant masses of 1 GeV with few adjustable parameters.Comment: Proceedings of Hot Quarks 201

Charmonium Suppression with cc~ Dissociation by Strings

We study the production of cc~ pairs in nuclear reactions at SPS energies within the covariant transport approach HSD. The production of cc~ is treated perturbatively employing experimental cross sections while the interactions of cc~ pairs with baryons are included by conventional cascade-type two-body collisions. Adopting 6 mb for the cc~-baryon cross sections the data on J/\Psi suppression in p+A reactions are reproduced in line with calculations based on the Glauber model. Additionally the dissociation of the cc~ pairs by strings is included in a purely geometrical way. We find good agreement with experimental data from the NA38 and NA50 collaboration with an estimate for the string radius of ~0.2-0.3 fm.Comment: 9 pages (LaTeX), 5 PS figure

Constraint Correlation Dynamics of SU(N) Gauge Theories

A constraint correlation dynamics up to 4-point Green functions is proposed for SU(N) gauge theories which reduces the N-body quantum field problem to the two-body level. The resulting set of nonlinear coupled equations fulfills all conservation laws including fermion number, linear and angular momenta as well as the total energy. Apart from the conservation laws in the space-time degrees of freedom the Gauss law is conserved as a quantum expectation value identically for all times. The same holds for the Ward identities as generated by commutators of Gauss operators. The constraint dynamical equations are highly non-perturbative and thus applicable also in the strong coupling regime, as e.g. low-energy QCD problems.Comment: 26 pages, LATEX, UGI-94-0

Studying the ω\omega-mass in-medium in γ+A→π0γ+X\gamma + A\to \pi^0\gamma +X reactions

Simulations based on a coupled-channel transport model have been performed to analyze the feasibility to study the in-medium $\omega$ mass exploiting the process $\gamma+A\to\pi^0\gamma+X$ for C, Ca and Nb nuclei. The distortions due to final-state interactions of the $\pi^0$ and background contributions from the reaction $\gamma+A\to\pi^0\pi^0+X$ are found to be small in the mass range of interest ($0.6<M_{\pi^0\gamma}<0.8$ GeV). Furthermore, the effect of the detector resolution on the $\pi^0\gamma$-mass determination is discussed