1,269 research outputs found

    Dileptons from the strongly-interacting Quark-Gluon Plasma within the Parton-Hadron-String-Dynamics (PHSD) approach

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    Dilepton production in In+In collisions at 158 AGeV is studied within the microscopic Parton-Hadron-Strings Dynamics (PHSD) transport approach, which is based on a dynamical quasiparticle model (DQPM) matched to reproduce lattice QCD results in thermodynamic equilibrium. 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 in the nonperturbative QGP. In particular, the observed softening of the mT spectra at intermediate masses is reproduced.Comment: talk given at the 21st International Conference on Ultra-Relativistic Nucleus-Nucleus Collisions (Quark Matter 2009), to be published in Nucl.Phys.A, 4 pages, 3 figures, elsarticle styl

    Dynamics of strange, charm and high momentum hadrons in relativistic nucleus-nucleus collisions

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    We investigate hadron production and attenuation of hadrons with strange and charm quarks (or antiquarks) as well as high transverse momentum hadrons in relativistic nucleus-nucleus collisions from 2 AGeV to 21.3 ATeV within two independent transport approaches (UrQMD and HSD). Both transport models are based on quark, diquark, string and hadronic degrees of freedom, but do not include any explicit phase transition to a quark-gluon plasma. From our dynamical calculations we find that both models do not describe the maximum in the K+/π+K^+/\pi^+ ratio at 20 - 30 AGeV in central Au+Au collisions found experimentally, though the excitation functions of strange mesons are reproduced well in HSD and UrQMD. Furthermore, the transport calculations show that the charmonium recreation by D+DˉJ/Ψ+mesonD+\bar{D} \to J/\Psi + meson reactions is comparable to the dissociation by 'comoving' mesons at RHIC energies contrary to SPS energies. This leads to the final result that the total J/ΨJ/\Psi suppression as a function of centrality at RHIC should be less than the suppression seen at SPS energies where the 'comover' dissociation is substantial and the backward channels play no role. Furthermore, our transport calculations -- in comparison to experimental data on transverse momentum spectra from pppp, d+Au and Au+Au reactions -- show that pre-hadronic effects are responsible for both the hardening of the hadron spectra for low transverse momenta (Cronin effect) as well as the suppression of high pTp_T hadrons. The mutual interactions of formed hadrons are found to be negligible in central Au+Au collisions at s\sqrt{s} = 200 GeV for pTp_T \geq 6 GeV/c and the sizeable suppression seen experimentally is attributed to a large extent to the interactions of 'leading' pre-hadrons with the dense environment.Comment: 10 pages, incl. 7 postscript figures, invited talk presented at the 25th Course "Heavy Ion Reactions from Nuclear to Quark Matter", Erice, Sept. 16-24, 2003, to be publ. in Prog. Part. Nucl. Phy

    Highlights on Dilepton and Photon Observables

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    The highlights of Quark Matter '97 on dileptons and direct photons are presented.Comment: Talk delivered at the summary session of Quark Matter '97, Tsukuba, Japan, Dec. 1-5 97, to be published in Nucl. Phys. A, 7 pages, 7 figure

    Nonequilibrium Quantum-Field Dynamics and Off-Shell Transport for ϕ4\phi^4-theory in 2+1 dimensions

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    We solve the Kadanoff-Baym equations for nonequilibrium initial configurations of the ϕ4\phi^4-theory in 2+1 dimensions and compare to explicit solutions of generalized transport equations for the same theory. The latter transport equations are derived from the Kadanoff-Baym equation in a first order gradient expansion in phase space and explicitly retain the off-shell dynamics as inherent in the time-dependent spectral functions. The solutions of these equations compare very well with the exact solutions of the full Kadanoff-Baym equations with respect to the occupation numbers of the individual modes, the spectral evolution as well as the chemical equilibration process. Furthermore, the proper equilibrium off-shell distribution is reached for large times contrary to the quasiparticle Boltzmann limit. We additionally present a direct comparison of the solution of the generalized transport equations in the Kadanoff-Baym and Botermans-Malfliet form; both solutions are found to agree very well with each other.Comment: 43 pages, LaTex, including 15 postscript figures, submitted to Nucl. Phys.

    Equilibration within a semiclassical off-shell transport approach

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    Equilibration times for nuclear matter configurations -- modelling intermediate and high energy nucleus-nucleus collisions -- are evaluated within the semiclassical off-shell transport approach developed recently. The transport equations are solved for a finite box in coordinate space employing periodic boundary conditions. The off-shell transport model is shown to give proper off-shell equilibrium distributions in the limit tt \to \infty for the nucleon and Δ\Delta-resonance spectral functions. We find that equilibration times within the off-shell approach are only slightly enhanced as compared to the on-shell limit for the momentum configurations considered.Comment: 19 pages, LaTeX, including 4 postscript figures, submitted to Nucl. Phys.
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