Thermalization of Heavy Quarks in the Quark-Gluon Plasma

Charm- and bottom-quark rescattering in a Quark-Gluon Plasma (QGP) is investigated with the objective of assessing the approach towards thermalization. Employing a Fokker-Planck equation to approximate the collision integral of the Boltzmann equation we augment earlier studies based on perturbative parton cross sections by introducing resonant heavy-light quark interactions. The latter are motivated by recent QCD lattice calculations which indicate the presence of "hadronic" states in the QGP. We model these states by colorless (pseudo-) scalar and (axial-) vector D- and B-mesons within a heavy-quark effective theory framework. We find that the presence of these states at moderate QGP temperatures substantially accelerates the kinetic equilibration of c-quarks as compared to using perturbative interactions. We also comment on consequences for $D$-meson observables in ultra-relativistic heavy-ion collisions.Comment: 14 pages, 5 figures, v2: Added references, v2: Added further references, some typos correcte

Thermal Dileptons as Fireball Thermometer and Chronometer

Thermal dilepton radiation from the hot fireballs created in high-energy heavy-ion collisions provides unique insights into the properties of the produced medium. We first show how the predictions of hadronic many-body theory for a melting $\rho$ meson, coupled with QGP emission utilizing a modern lattice-QCD based equation of state, yield a quantitative description of dilepton spectra in heavy-ion collisions at the SPS and the RHIC beam energy scan program. We utilize these results to systematically extract the excess yields and their invariant-mass spectral slopes to predict the excitation function of fireball lifetimes and (early) temperatures, respectively. We thereby demonstrate that future measurements of these quantities can yield unprecedented information on basic fireball properties. Specifically, our predictions quantify the relation between the measured and maximal fireball temperatures, and the proportionality of excess yields and total lifetime. This information can serve as a "caloric" curve to search for a first-order QCD phase transition, and to detect non-monotonous lifetime variations possibly related to critical phenomena.Comment: 4 pages 3 figure

Finite pion width effects on the rho-meson and di-lepton spectra

Within a field theoretical model where all damping width effects are treated self-consistently we study the changes of the spectral properties of rho-mesons due to the finite damping width of the pions in dense hadronic matter at finite temperature. The corresponding effects in the di-lepton yields are presented. Some problems concerning the self consistent treatment of vector or gauge bosons are discussed.Comment: Invited talk given at International Workshop "Gross properties of Nuclei and Nuclear Exitations", Hirschegg, Austria, 16-22.01.2000, Latex, 7 page

Thermal Electromagnetic Radiation in Heavy-Ion Collisions

We review the potential of precise measurements of electromagnetic probes in relativistic heavy-ion collisions for the theoretical understanding of strongly interacting matter. The penetrating nature of photons and dileptons implies that they can carry undistorted information about the hot and dense regions of the fireballs formed in these reactions and thus provide a unique opportunity to measure the electromagnetic spectral function of QCD matter as a function of both invariant mass and momentum. In particular we report on recent progress on how the medium modifications of the (dominant) isovector part of the vector current correlator ($\rho$ channel) can shed light on the mechanism of chiral symmetry restoration in the hot and/or dense environment. In addition, thermal dilepton radiation enables novel access to (a) the fireball lifetime through the dilepton yield in the low invariant-mass window $0.3 \; \mathrm{GeV} \leq M \leq 0.7 \; \mathrm{GeV}$, and (b) the early temperatures of the fireball through the slope of the invariant-mass spectrum in the intermediate-mass region ($1.5 \; \mathrm{GeV} <M< 2.5 \; \mathrm{GeV}$). The investigation of the pertinent excitation function suggests that the beam energies provided by the NICA and FAIR projects are in a promising range for a potential discovery of the onset of a first order phase transition, as signaled by a non-monotonous behavior of both low-mass yields and temperature slopes.Comment: 5 pages, 4 figures; contribution to the NICA White Paper (EPJA topical issue

Comprehensive Interpretation of Thermal Dileptons at the SPS

Employing thermal dilepton rates based on medium-modified electromagnetic correlation functions we show that recent dimuon spectra of the NA60 collaboration in central In-In collisions at the CERN-SPS can be understood in terms of radiation from a hot and dense hadronic medium. Earlier calculated \rho-meson spectral functions, as following from hadronic many-body theory, provide an accurate description of the data up to dimuon invariant masses of about M\simeq 0.9 GeV, with good sensitivity to details of the predicted \rho-meson line shape. This, in particular, identifies baryon-induced effects as the prevalent ones. We show that a reliable description of the \rho contribution opens the possibility to study further medium effects: at higher masses (M \simeq 0.9-1.5 GeV) 4-pion type annihilation is required to account for the experimentally observed excess indicating precursor effects of chiral symmetry restoration (``chiral mixing''), while remaining structures in the \omega and \phi region are suggestive for modifications in their line shapes as well.Comment: 4 pages, 4 figures, v2: slightly improved estimate of four-pion contributions; accepted for publication in Phys. Rev. Let

Electromagnetic emission from hot medium measured by the PHENIX experiment at RHIC

Electromagnetic radiation has been of interest in heavy ion collisions because they shed light on early stages of the collisions where hadronic probes do not provide direct information since hadronization and hadronic interactions occur later. The latest results on photon measurement from the PHENIX experiment at RHIC reflect thermodynamic properties of the matter produced in the heavy ion collisions. An unexpectedly large positive elliptic flow measured for direct photons can not be explained by any of the current models.Comment: Talk contributed to Rutherford Centennial Conference, Aug 8-12, 2011, held in Manchester, U