78 research outputs found
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 -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 ( 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 , and (b) the early temperatures of the fireball
through the slope of the invariant-mass spectrum in the intermediate-mass
region (). 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
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