Charm elliptic flow in heavy-ion collisions at the Relativistic Heavy Ion Collider is studied in a multiphase transport model. Assuming that the cross section for charm quark scattering with other light quarks is the same as that between light quarks, we find that both charm and light quark elliptic flows are sensitive to the value of the cross section. Compared to that of light quarks, the elliptic flow of charm quarks is smaller at low transverse momentum but approaches comparable values at high transverse momentum. Similar features are seen in the elliptic flow of charmed mesons as well as that of the electrons from their semileptonic decays when the charmed mesons are produced from quark coalescence during hadronization of the partonic matter. To describe the large electron elliptic flow observed in available experimental data requires a charm quark-scattering cross section that is much larger than that given by the perturbative quantum chromodynamics.

We address dilepton production in Au+Au collisions at √ sNN = 200 GeV by employing the parton-hadron-string dynamics (PHSD) off-shell transport approach. Within the PHSD one goes beyond the quasiparticle approximation by solving generalized transport equations on the basis of the off-shell Kadanoff-Baym equations for the Green's functions in the phase-space representation. The approach consistently describes the full evolution of a relativistic heavy-ion collision from the initial hard scatterings and string formation through the dynamical deconfinement phase transition to the quark-gluon plasma (QGP) as well as hadronization and to the subsequent interactions in the hadronic phase. With partons described in the PHSD by the dynamical quasiparticle model (DQPM) -matched to reproduce lattice QCD results in thermodynamic equilibrium -we calculate, in particular, the dilepton radiation from partonic interactions through the reactions qq → γ * , qq → γ * + g and qg → γ * q (qg → γ * q ) in the early stage of relativistic heavy-ion collisions. By comparing our results to the data from the PHENIX Collaboration, we study the relative importance of different dilepton production mechanisms and point out the regions in phase space where partonic channels are dominant. Furthermore, explicit predictions are presented for dileptons within the acceptance of the STAR detector system and compared to the preliminary data.

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Dilepton production in In+In collisions at 158 A·GeV is studied within the microscopic partonhadron-string dynamics (PHSD) transport approach that incorporates explicit partonic degrees-offreedom, dynamical hadronization as well as the more familiar hadronic dynamics in the final reaction stages. A comparison to the data of the NA60 Collaboration shows that the measured dilepton yield is well described by including the collisional broadening of vector mesons, while simultaneously accounting for the electromagnetic radiation of the strongly coupled quark-gluon plasma (sQGP) via off-shell quark-antiquark annihilation, quark annihilation with additional gluon Bremsstrahlung and the gluon-Compton scattering mechanisms. In particular, the spectra in the intermediate mass range (1 GeV ≤ M ≤ 2.5 GeV) are dominated by quark-antiquark annihilation in the nonperturbative QGP. Also, the observed softening of the transverse mass spectra at intermediate masses (1 GeV ≤ M ≤ 2.5 GeV) is approximately reproduced. Furthermore, for dileptons of low masses (M < 0.6 GeV), we find a sizeable contribution from the quark annihilation with additional gluon bremsstrahlung, thus providing another possible window for probing the properties of the sQGP.PACS numbers: 25.75.Cj, 25.75.Nq, 24.85.+p,

The relativistic transport model is extended to include the kaon degree of freedom. We also take into account the density dependence of the kaon effective mass in nuclear matter and the rescattering of kaons by nucleons. We find that the inclusion of kaon self-energy due to the attractive scalar mean field leads to an enhanced kaon yield in heavy-ion collisions at subthreshold energies. Also, kaon rescatterings are found to affect significantly the final kaon xnomentum spectra. We coxnpare the theoretical results with recent experimental data from the Heavy-Ion Synchrotron (SIS) at GSI. PACS number(s): 25.75.+r, 21.65.+f, 24.10.Cn, 25.80.Nv Experiments on kaon production &om heavy-ion collisions are being carried out at SIS [1]. The incident energy per nucleon in the collision is around 1 GeV and is below the threshold energy of 1.56 GeV for kaon production in the nucleon-nucleon collision in &ee space. One of the motivations for this study is to learn about the nuclear equation of state at high densities. As 6rst pointed out in Ref. [2], using the Vlasov-Uehling-Uhlenbeck (VUU) model, kaon production &om heavy-ion collisions at subthreshold energies offers the possibility of extracting the information about the nuclear equation of state as its yield can differ by a factor of three depending on the stiffness of the nuclear equation of state at high densities. A similar conclusion has been obtained recently using the covariant Boltzmann-Uehling-Uhlenbeck (BUU) model [3] and the quantum molecular dynamics [4,5].In the transport model for heavy-ion collisions, kaons are usually treated as free particles. In Ref. [6] the relativistic transport model has been generalized to include both the kaon mean-6eld potential and the collisions of kaons with other particles. In this Rapid Communication we shall use the generalized relativistic transport model to study subthreshold kaon production in heavy-ion collisions.From the quantum hadrodynamics [7] in which the nuclear matter is treated as a system of interacting baryons and mesons, one can derive a relativistic transport equation for the phase space distribution function f(x, p') of nucleons [8]. This transport equation can be solved using the method of pseudoparticles in which each nucleon is replaced by a collection of test particles. The propagation of these test particles is described by the classical equations of motion, dxwith p~b eing the nuclear matter density and E' (m' + p' )~2 . The nucleon efFective mass m' and ki-Permanent address: Institute of Atomic Energy, Beijing 102413, China. netic momentum p' are defined by m'=mg (o), P = P g~(~). (3) This gives rise to an attractive s-wave interaction for the kaon. In the above, f~i s the kaon decay constant and Z~~is the KN sigma term. Their values are taken to be flc 93 MeV and Zatv 350MeV as in Ref [6]. There. is also a vector interaction in the chiral Lagrangian, Lv -2 N NKO(K 3i 8 zwhich leads to a repulsive s-wave interaction for a kaon in the nuclear matter. We note that the resulting vectorexchange mean-6el...