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 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

Interpretation of Recent SPS Dilepton Data

We summarize our current theoretical understanding of in-medium properties of the electromagnetic current correlator in view of recent dimuon data from the NA60 experiment in In(158 AGeV)-In collisions at the CERN-SPS. We discuss the sensitivity of the results to space-time evolution models for the hot and dense partonic and hadronic medium created in relativistic heavy-ion collisions and the contributions from different sources to the dilepton-excess spectra.Comment: To appear in the proceedings of the 19th International Conference on Ultra-Relativistic Nucleus-Nucleus Collisions (Quark Matter 2006) v2: references added, minor typos correcte

Bottomonium Production at RHIC and LHC

Properties of bottomonia (Upsilon, chi_b and Upsilon') in the Quark-Gluon Plasma (QGP) are investigated by assessing inelastic reaction rates and their interplay with open-bottom states (b-quarks or B-mesons) and color-screening. The latter leads to vanishing quarkonium binding energies at sufficiently high temperatures (close to the dissolution point), which, in particular, renders standard gluo-dissociation, g+Upsilon -> b + b-bar, inefficient due to a substantial reduction in final-state phase space. This problem is overcome by invoking a "quasifree" destruction mechanism, g,q,q-bar + Upsilon -> g,q,q-bar + b + b-bar, as previously introduced for charmonia. The pertinent reaction rates are implemented into a kinetic theory framework to evaluate the time evolution of bottomonia in heavy-ion reactions at RHIC and LHC within an expanding fireball model. While bottom quarks are assumed to be exclusively produced in primordial nucleon-nucleon collisions, their thermal relaxation times in the QGP, which importantly figure into Upsilon-formation rates, are estimated according to a recent Fokker-Planck treatment. Predictions for the centrality dependence of Upsilon production are given for upcoming experiments at RHIC and LHC. At both energies, Upsilon suppression turns out to be the prevalent effect.Comment: 16 Pages, 21 figures, 1 table v2: Manuscript reorganized, several sections moved to appendices, additional comments included, contents unchange