Dilepton production by dynamical quasiparticles in the strongly interacting quark gluon plasma

The dilepton production by the constituents of the strongly interacting quark-gluon-plasma (sQGP) is addressed. In order to make quantitative predictions at realistically low plasma temperatures (O(T_c)), experimentally relevant low dilepton mass (O(1 GeV)) and strong coupling (alphaS=0.5-1), we take into account not only the higher order pQCD reaction mechanisms, but also the non-perturbative spectral functions (off-shellness) and self-energies of the quarks, anti-quarks and gluons thus going beyond the leading twist. For this purpose, our calculations utilize parametrizations of the non-perturbative propagators for quarks and gluons provided by the dynamical quasi-particle model (DQPM) matched to reproduce lattice data. The DQPM describes QCD properties in terms of single-particle Green's functions (in the sense of a two-particle irreducible approach) and leads to the notion of the constituents of the sQGP being effective quasiparticles, which are massive and have broad spectral functions (due to large interaction rates). In the present work, we derive the off-shell cross sections of dilepton production in the reactions q+qbar->l^+l^- (Drell-Yan mechanism), q+qbar->g+l^+l^- (quark annihilation with the gluon Bremsstrahlung in the final state), q(qbar)+g->q(qbar)+l^+l^- (gluon Compton scattering), g->q+qbar+l^+l^- and q(qbar)->q(qbar)+g+l^+l^- (virtual gluon decay, virtual quark decay) in the sQGP by dressing the quark and gluon lines in the perturbative diagrams with the DQPM propagators for quarks and gluons.Comment: RevTex style, 16 pages, 16 figure

Modelling Drell-Yan pair production in pbar-p

We predict the triple differential cross section of the unpolarized Drell-Yan process at sqrt(S)=6 GeV. The model incorporates primordial parton transverse momentum and quark off-shellness effects caused by the initial state interaction.Comment: 4 pages, AIP style, talk given at the conference LEAP'05 (Low Energy Antiproton Physics

Quark Initial State Interaction in Deep Inelastic Scattering and the Drell-Yan process

We pursue a phenomenological study of higher twist effects in high energy processes by taking into account the off-shellness (virtuality) of partons bound in the nucleon. The effect of parton off-shellness in deep inelastic ep->eX scattering and the Drell-Yan process (pp->l+ l- X) is examined. Assuming factorization and a single-parameter Breit-Wigner form for the parton spectral function, we develop a model to calculate the corresponding off-shell cross sections. Allowing for a finite parton width of the order of 100 MeV, we reproduce the data of both DIS and the triple differential Drell-Yan cross section without an additional K-factor. The results are compared to those from perturbative QCD and the intrinsic-kT approach.Comment: 31 page, 12 figures, accepted to Phys.Rev.D; in v2: references added, typos corrected, minor changes to the tex

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

Effect of quark off-shellness in DIS and the Drell-Yan process

We study higher twist corrections to the perturbative QCD cross sections for ep deep inelastic scattering and the Drell-Yan lepton pair production in pp collision. The corrections arise due to the initial state interaction of the active quarks with the spectator partons. The effect of this interaction is calculated by dressing the incoming quark lines with phenomenological single parameter Breit-Wigner spectral functions and taking into account the full off-shell kinematics. The quark width in the proton is estimated by analyzing the data on Drell-Yan triple differential cross section from the experiment E866 at Fermilab.Comment: talk on the conference "Lepton Scattering and the Structure of Hadrons and Nuclei," Erice, 16 - 24 Sept. 2004; 2 pages, 4 figures, to be published in "Progress in Particle and Nuclear Physics

Dilepton radiation by vector mesons and off-shell partons in the hot and dense medium

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+qbar->gamma*, q+qbar->gamma*+glue and q+glue->gamma*+q(qbar+glue->gamma*qbar) 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 ~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 mT spectra at intermediate masses (1 GeV < M < 3 GeV) is approximately reproduced.Comment: Talk given at the XIII International Conference on Hadron Spectroscopy (HADRON 09), November 29 - December 4, 2009, Tallahassee, Florida; two references added in version