Dilepton anisotropy from p+Bep + Be and Ca+CaCa + Ca collisions at BEVALAC energies

A full calculation of lepton-pair angular characteristics is carried out for $e^+e^-$ pairs created in $p + Be$ and $Ca + Ca$ collisions from 1.0 to 2.1 GeV/A. It is demonstrated that the dilepton decay anisotropy depends sensitively on the different sources and may be used for their disentangling. Due to the dominance of the $\eta$-and $\Delta$-Dalitz decays and only a small anisotropy coefficient for $\pi^+\pi^-$ annihilation, the expected anisotropy coefficients show a decrease with invariant mass of the dilepton pair and change only moderately when comparing $p + Be$ and $Ca + Ca$ reactions at the same bombarding energy.Comment: 10 pages, LaTeX, plus 4 postscript figures tarred, compressed and uuencoded in separate fil

Low-mass dileptons and dropping rho meson mass

Using the transport model, we have studied dilepton production from heavy-ion collisions at Bevalac energies. It is found that the enhanced production of low-mass dileptons observed in the experiment by the DLS collaboration cannot be explained by the dropping of hadron masses, in particular the $\rho$-meson mass, in dense matter.Comment: 9 pages, LaTeX, including 1 postscript figure, to appear in Phys. Lett.

Dynamics of strange, charm and high momentum hadrons in relativistic nucleus-nucleus collisions

We investigate hadron production and attenuation of hadrons with strange and charm quarks (or antiquarks) as well as high transverse momentum hadrons in relativistic nucleus-nucleus collisions from 2 AGeV to 21.3 ATeV within two independent transport approaches (UrQMD and HSD). Both transport models are based on quark, diquark, string and hadronic degrees of freedom, but do not include any explicit phase transition to a quark-gluon plasma. From our dynamical calculations we find that both models do not describe the maximum in the $K^+/\pi^+$ ratio at 20 - 30 AGeV in central Au+Au collisions found experimentally, though the excitation functions of strange mesons are reproduced well in HSD and UrQMD. Furthermore, the transport calculations show that the charmonium recreation by $D+\bar{D} \to J/\Psi + meson$ reactions is comparable to the dissociation by 'comoving' mesons at RHIC energies contrary to SPS energies. This leads to the final result that the total $J/\Psi$ suppression as a function of centrality at RHIC should be less than the suppression seen at SPS energies where the 'comover' dissociation is substantial and the backward channels play no role. Furthermore, our transport calculations -- in comparison to experimental data on transverse momentum spectra from $pp$, d+Au and Au+Au reactions -- show that pre-hadronic effects are responsible for both the hardening of the hadron spectra for low transverse momenta (Cronin effect) as well as the suppression of high $p_T$ hadrons. The mutual interactions of formed hadrons are found to be negligible in central Au+Au collisions at $\sqrt{s}$ = 200 GeV for $p_T \geq$ 6 GeV/c and the sizeable suppression seen experimentally is attributed to a large extent to the interactions of 'leading' pre-hadrons with the dense environment.Comment: 10 pages, incl. 7 postscript figures, invited talk presented at the 25th Course "Heavy Ion Reactions from Nuclear to Quark Matter", Erice, Sept. 16-24, 2003, to be publ. in Prog. Part. Nucl. Phy

Perspectives of e+e−e^+e^- production in pp, pd and p Be reactions at SIS energies

We study dilepton production from pp, pd and p Be collisions from 1 - 5 GeV including the $\pi^0$, $\eta$, $\omega$ and $\Delta$ Dalitz decays, direct decays of vector mesons ($\rho$, $\omega$) as well as subthreshold $\rho$ production via baryonic resonances (e.g. $D_{13}(1520), P_{11}(1710)$). Our calculations compare rather well with the pp and pd data from the DLS Collaboration, however, overestimate slightly the 'old' p Be data from that group. Futhermore, detailed predictions for differential dilepton spectra at SIS energies are made with a high mass resolution that can be controlled experimentally by the HADES Collaboration in near future.Comment: 27 pages, LaTeX, including 13 postscript figures, to be published in Nucl. Phys.

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