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 Physics , 2005, DOI: 10.1016/j.nuclphysa.2005.03.125 Abstract: We present a unified description of the vector meson and dilepton production in elementary and in heavy ion reactions. The production of vector mesons is described via the excitation of nucleon resonances. Medium effects in heavy ion reactions are discussed.
 Physics , 2015, Abstract: In this review we address the dynamics of relativistic heavy-ion reactions and in particular the information obtained from electromagnetic probes that stem from the partonic and hadronic phases. The out-of-equilibrium description of strongly interacting relativistic fields is based on the theory of Kadanoff and Baym. For the modeling of the partonic phase we introduce a dynamical quasiparticle model (DQPM) for QCD in equilibrium. The widths and masses of the quasiparticles are controlled by transport coefficients in comparison to lattice QCD results. The resulting off-shell transport approach - denoted by Parton-Hadron-String Dynamics (PHSD) - also includes covariant dynamical hadronization and keeps track of the hadronic interactions in the final phase. We show that PHSD captures the bulk dynamics of heavy-ion collisions from SPS to LHC energies and provides a basis for the evaluation of the electromagnetic emissivity, using the same dynamical parton propagators as for the system evolution. Direct photon production in elementary processes and heavy-ion reactions at RHIC and LHC energies is investigated and the status of the photon v2 puzzle - a large elliptic flow of the direct photons observed in A+A collisions - is addressed. We discuss the roles of hadronic and partonic sources for the photon spectra and the flow coefficients v2 and v3 and also the possibility to subtract the QGP signal from observables. Furthermore, the production of dilepton pairs is addressed from SIS to LHC energies. The low-mass dilepton yield is enhanced due to the in-medium modification of the rho-meson and at the lowest energy also due to a multiple regeneration of Delta-resonances. In addition, a signal of the partonic degrees-of-freedom is found in the intermediate dilepton mass regime (1.2GeV
 Physics , 2003, DOI: 10.1103/PhysRevC.68.014904 Abstract: We present a unified description of the vector meson and dilepton production in elementary and in heavy ion reactions. The production of vector mesons ($\rho,\omega$) is described via the excitation of nuclear resonances ($R$). The theoretical framework is an extended vector meson dominance model (eVMD). The treatment of the resonance decays $R\longmapsto NV$ with arbitrary spin is covariant and kinematically complete. The eVMD includes thereby excited vector meson states in the transition form factors. This ensures correct asymptotics and provides a unified description of photonic and mesonic decays. The resonance model is successfully applied to the $\omega$ production in $p+p$ reactions. The same model is applied to the dilepton production in elementary reactions ($p+p, p+d$). Corresponding data are well reproduced. However, when the model is applied to heavy ion reactions in the BEVALAC/SIS energy range the experimental dilepton spectra measured by the DLS Collaboration are significantly underestimated at small invariant masses. As a possible solution of this problem the destruction of quantum interference in a dense medium is discussed. A decoherent emission through vector mesons decays enhances the corresponding dilepton yield in heavy ion reactions. In the vicinity of the $\rho/\omega$-peak the reproduction of the data requires further a substantial collisional broadening of the $\rho$ and in particular of the $\omega$ meson.
 Volker Metag Physics , 2007, DOI: 10.1016/j.ppnp.2007.12.041 Abstract: Experimental searches for modifications of vector mesons in the nuclear medium are reviewed. Data on $\rho,\omega$ and $\Phi$ mesons are presented. The results have been obtained in elementary reactions with proton and photon beams as well as in heavy-ion collisions. Compared to the free particle properties, the $\omega$ and $\Phi$ meson are found to drop in mass at normal nuclear matter density by 9-14% and 3.5% whereas their widths are reported to increase by factors of about 16 and 3.6, respectively. For the $\rho$ meson, conflicting results on in-medium mass shifts and broadening have been published. The experimental data are compared to recent model calculations.
 Physics , 1998, DOI: 10.1103/PhysRevLett.81.1572 Abstract: Through the analysis of HELIOS-3 data obtained at the CERN SPS, we demonstrate the importance of secondary processes for dilepton production in heavy-ion collisions in the intermediate invariant mass region. We find that while the dilepton spectra between 1 to 2.5 GeV from proton-induced reactions can be attributed to the decay of primary vector mesons, charmed hadrons, and initial Drell-Yan processes; the strong enhancement seen in the heavy-ion data as compared to the background comes mainly from the secondary processes which are germane to heavy-ion collisions. Furthermore, we find $\pi a_1\to l{\bar l}$ to be the most important process in this mass region, as was found by thermal rate calculations. We emphasize the constraints on the elementary cross sections by the experimental data from $e^+e^-$ annihilation.
 U. Mosel Physics , 2000, Abstract: The sensitivity of dilepton production in elementary reactions of photons and pions with nuclei on in-medium changes of hadronic properties is studied. It is shown that this sensitivity is comparable to that encountered in ultrarelativistic heavy-ion collisions. It is also shown how a significant broadening of the mesons affects other photonuclear processes.
 Physics , 2006, DOI: 10.1140/epja/i2006-10048-2 Abstract: We consider the role of rescattering of secondary kaons on the dilepton branching ratio of the phi meson. In-medium mass modifications and broadening of kaons and phi mesons are taken into account. We find in the framework of a Bjorken scenario for the time evolution of the expanding fireball that the phi yield from dimuons is moderately or at least only slightly enhanced compared to that from kaon-antikaon pairs. The relation to experimental yields measured by the NA49, NA50 and CERES Collaborations at CERN SPS and the PHENIX Collaboration at RHIC is discussed.
 Physics , 1997, DOI: 10.1016/S0375-9474(98)00338-8 Abstract: Through the analysis of HELIOS-3 data on dilepton spectra, we demonstrate the importance of secondary processes for the dilepton production in heavy-ion collisions in the intermediate mass region. We find that, while the dilepton spectra in proton-induced reactions can be nicely explained by the decay of primary vector mesons, charmed hadrons, and initial Drell-Yan processes, the strong enhancement seen in the heavy-ion data comes mainly from the secondary meson-meson interactions which are unique to heavy-ion collisions.
 Physics , 2013, DOI: 10.1088/1742-6596/503/1/012039 Abstract: We present transport calculations with the Ultra-relativistic Quantum Molecular Dynamics approach (UrQMD) for dilepton spectra at SIS energies. While we obtain a good agreement with experiment for elementary reactions, in heavy-ion collisions an excess in the invariant mass spectra is observed which cannot be described by the model. As the pure transport calculations do not include any in-medium effects and are limited to hadronic degrees of freedom, we present an alternative approach that uses coarse-grained output from transport calculations to determine thermal dilepton emission rates. For this we apply the medium-modified $\rho$ spectral function by Eletsky et al. In a first exemplary comparison to data from the NA60 experiment we find that the coarse-graining approach gives reasonable results.
 Physics , 1996, DOI: 10.1103/PhysRevC.56.453 Abstract: We calculate electromagnetic production from highly excited hadronic matter created in heavy ion collisions. The rates include the usual lowest order processes in quark-gluon plasma plus the usual reactions in the hadronic phase, related with rho,a_1 mesons. The space-time integration is done using a hydrodynamical model. We have found that dilepton mass spectrum agrees with results of other previous works, but *disagrees* with the CERES dilepton data. In order to explain these data, some "unconventional" production mechanism need to be incorporated: we discuss especially the notion of modified rho,a_1 masses, which indeed may explain the data. Other suggestions (e.g. the longer-lived fireball) to increase the production of low mass dileptons seem to be insufficient for the task. The results for direct photon production are below the current WA80 experimental bounds, for all variants considered.
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