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

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

Dilepton production and off-shell transport dynamics at SIS energies

Dilepton production in nucleus-nucleus collisions at 1-2 A GeV as well as in elementary pp and pd reactions is studied within the microscopic HSD transport approach which includes the off-shell dynamics of vector mesons explicitly. The study addresses additionally the production of $\pi^0$ and $\eta$ mesons since their Dalitz decays provide a sizeable contribution to the dilepton invariant mass spectra up to about 0.5 GeV. Our transport results agree with the TAPS experimental data on $\pi$ and $\eta$ multiplicities in C+C collisions from 0.8 to 2 A GeV. We find that the 'DLS-puzzle' - which addresses an underestimation of the $e^+e^-$ yield in the mass range from 0.2 to 0.5 GeV in C+C and Ca+Ca collisions - may be solved when incorporating a stronger bremsstrahlung contribution in line with recent OBE calculations. Moreover, the HSD results with 'enhanced' bremsstrahlung cross sections agree very well with the HADES experimental data for the dilepton mass spectra for C+C at 1 and 2 A GeV, especially when including a collisional broadening in the vector-meson spectral functions. Detailed predictions for dilepton spectra from pp and pn/pd reactions at 1.25 GeV, 2.2 GeV and 3.5 GeV are presented which will allow to verify/falsify the larger bremsstrahlung contributions from the experimental side in the near future.Comment: 43 pages, 22 figures; to be published to Nucl. Phys.

Low mass dilepton production at ultrarelativistic energies

Dilepton production in $pp$ and $Au+Au$ nucleus-nucleus collisions at $\sqrt{s}$ = 200 GeV as well as in $In+In$ and $Pb+Au$ at 158 A$\cdot$GeV is studied within the microscopic HSD transport approach. A comparison to the data from the PHENIX Collaboration at RHIC shows that standard in-medium effects of the $\rho, \omega$ vector mesons - compatible with the NA60 data for $In+In$ at 158 A$\cdot$GeV and the CERES data for $Pb+Au$ at 158 A$\cdot$GeV - do not explain the large enhancement observed in the invariant mass regime from 0.2 to 0.5 GeV in $Au+Au$ collisions at $\sqrt{s}$ = 200 GeV relative to $pp$ collisions.Comment: 13 pages, 6 figures, extended version, Phys. Lett. B, in pres

Observing many body effects on lepton pair production from low mass enhancement and flow at RHIC and LHC energies

The $\rho$ spectral function at finite temperature calculated using the real-time formalism of thermal field theory is used to evaluate the low mass dilepton spectra. The analytic structure of the $\rho$ propagator is studied and contributions to the dilepton yield in the region below the bare $\rho$ peak from the different cuts in the spectral function are discussed. The space-time integrated yield shows significant enhancement in the region below the bare $\rho$ peak in the invariant mass spectra. It is argued that the variation of the inverse slope of the transverse mass ($M_T$) distribution can be used as an efficient tool to predict the presence of two different phases of the matter during the evolution of the system. Sensitivity of the effective temperature obtained from the slopes of the $M_T$ spectra to the medium effects are studied

Electromagnetic Probes

A review is presented of dilepton and real photon measurements in relativistic heavy ion collisions over a very broad energy range from the low energies of the BEVALAC up to the highest energies available at RHIC. The dileptons cover the invariant mass range \mll = 0 - 2.5 GeV/c$^2$, i.e. the continuum at low and intermediate masses and the light vector mesons, $\rho, \omega, \phi$. The review includes also measurements of the light vector mesons in elementary reactions.Comment: To be published in Landolt-Boernstein Volume 1-23A; 40 pages, 24 figures. Final version updated with small changes to the text, updated references and updated figure

Hadronic observables from SIS to SPS energies - anything strange with strangeness ?

We calculate $p, \pi^\pm, K^\pm$ and $\Lambda$(+$\Sigma^0$) rapidity distributions and compare to experimental data from SIS to SPS energies within the UrQMD and HSD transport approaches that are both based on string, quark, diquark ($q, \bar{q}, qq, \bar{q}\bar{q}$) and hadronic degrees of freedom. The two transport models do not include any explicit phase transition to a quark-gluon plasma (QGP). It is found that both approaches agree rather well with each other and with the experimental rapidity distributions for protons, $\Lambda$'s, $\pi^\pm$ and $K^\pm$. Inspite of this apparent agreement both transport models fail to reproduce the maximum in the excitation function for the ratio $K^+/\pi^+$ found experimentally between 11 and 40 A$\cdot$GeV. A comparison to the various experimental data shows that this 'failure' is dominantly due to an insufficient description of pion rapidity distributions rather than missing 'strangeness'. The modest differences in the transport model results -- on the other hand -- can be attributed to different implementations of string formation and fragmentation, that are not sufficiently controlled by experimental data for the 'elementary' reactions in vacuum.Comment: 46 pages, including 15 eps figures, to be published in Phys. Rev.

Analysis of shared heritability in common disorders of the brain

Paroxysmal Cerebral Disorder