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

Electromagnetic probes of the QGP

We investigate the properties of the QCD matter across the deconfinement phase transition in the scope of the parton-hadron string dynamics (PHSD) transport approach. We present here in particular the results on the electromagnetic radiation, i.e. photon and dilepton production, in relativistic heavy-ion collisions. By comparing our calculations for the heavy-ion collisions to the available data, we determine the relative importance of the various production sources and address the possible origin of the observed strong elliptic flow $v_2$ of direct photons. We argue that the different centrality dependence of the hadronic and partonic sources for direct photon production in nucleus-nucleus collisions can be employed to shed some more light on the origin of the photon $v_2$ "puzzle". While the dilepton spectra at low invariant mass show in-medium effects like an enhancement from multiple baryonic resonance formation or a collisional broadening of the vector meson spectral functions, the dilepton yield at high invariant masses (above 1.1 GeV) is dominated by QGP contributions for central heavy-ion collisions at ultra-relativistic energies. This allows to have an independent view on the parton dynamics via their electromagnetic massive radiation.Comment: 12 pages, Proceedings of the 3rd International Conference on 'New Frontiers in Physics 2014', Kolymbari, Cret

Meson mTm_T-scaling in heavy-ion collisions at SIS energies

We perform systematical studies on transverse-mass spectra of $\pi^0$, $\eta$, $\omega$, $\phi$, $K^+$ and $K^-$ mesons for C + C, Ni + Ni, Au + Au collisions at SIS energies within the HSD transport approach. We find that the $m_T$-spectra sensitively reflect the in-medium properties of the mesons. The 'bare mass' scenario leads to a general scaling behaviour for the meson $m_T$-spectra when including a mass shift due to the associate strangeness production threshold for kaons and antikaons whereas a 'dropping' mass scheme violates this $m_T$-scaling. The relative slope parameters, however, provide valuable information about the average meson potentials in the nuclear medium.Comment: 15 pages, LaTeX, including 4 postscript figures, UGI-97-29, submitted to Phys. Lett.

Direct photon production from hadronic sources in high-energy heavy-ion collisions

The low $p_T$ direct photon production from a variety of the hadronic sources is studied within the microscopic HSD transport approach for p+C, p+Pb and Pb+Pb collisions at 160 A GeV. The direct photon emission from elementary hadronic scatterings as well as meson-meson bremsstrahlung are incorporated. The influence of in-medium effects such as a collisional broadening of the vector-meson spectral functions on the photon emission rate is found to be hardly observable in the final spectra which are dominated by bremsstrahlung type processes. The uncertainties in the subtraction of the 'background' from the photon decay of hadronic resonances inside the hot and dense fireball is investigated, additionally. Our findings are relevant for the interpretation and extraction of experimental data on direct photon production at low $p_T$.Comment: 25 pages, 10 figures, to be published in Phys. Rev.