Hard parton damping in hot QCD

The gluon and quark collisional widths in hot QCD plasmas are discussed with emphasis on temperatures near Tc, where the coupling is large. Considering the effect on the entropy, which is known from lattice calculations, it is argued that the width of the partons, which in the perturbative limit is given by gamma ~ g^2 ln(1/g) T, should be sizeable at intermediate temperatures but has to be small close to Tc. This behavior implies a substantial reduction of the radiative energy loss of jets near Tc.Comment: invitetd talk given at 'Hot Quarks 04', July 18-24 2004, Taos Valley, NM, US

Resummation of the QCD thermodynamic potential

It is argued why thermodynamic approximations in terms of dressed propagators may, at larger coupling strength, be better behaved than perturbative results, and why in hot QCD the hard thermal loop approximation of the thermodynamic potential cannot be expected to work close to the phase transition.Comment: 5 pages. Talk given at `Statistical QCD', Bielefeld, August 26-30, 200

Collisional jet quenching becomes probable

It was argued recently that loop corrections to tree-level amplitudes are essential in the discussion of the collisional energy loss of energetic partons in the hot quark gluon plasma: Instead of $dE_{\rm coll}^B/dx \sim \alpha^2 T^2 \ln(ET/m_D^2)$, as first derived by Bjorken (assuming a constant $\alpha$), the mean energy loss actually behaves as $dE_{\rm coll}/dx \sim \alpha(m_D^2)T^2$. Here we calculate, within this resummation-improved framework, the probability distribution functions (`quenching weights') of the collisional energy loss. First results from a Monte Carlo implementation of this probabilistic collisional quenching shed new light on the interpretation of jet suppression in heavy ion collisions.Comment: 5 figure

The hot non-perturbative gluon plasma is an almost ideal colored liquid

We study properties of a gluon plasma above the critical temperature $T_c$ in a generalized quasi-particle approach with a Lorentz spectral function. The model parameters are determined by a fit of the entropy $s$ to lattice QCD data. The effective degrees of freedom are found to be rather heavy and of a sizeable width. With the spectral width being closely related to the interaction rate, we find a large effective cross section, which is comparable to the typical distance squared of the quasiparticles. This suggests that the system should be viewed as a liquid as also indicated by an estimate of the plasma parameter $\Gamma$. Furthermore, within the quasiparticle approach we find a very low viscosity to entropy ratio, $\eta/s \sim 0.2$ for $T > 1.05 T_c$, supporting the recent conjecture of an almost ideal quark-gluon liquid seen at RHIC.Comment: 4 pages, 2 eps figures, published versio

Quasi-particle model for deconfined matter

Our quasi-particle model for deconfined matter near T_c is reviewed. The extrapolation of lattice QCD data to a finite baryo-chemical potential is discussed. Determined by the chiral transition temperature T_c, the resulting equation of state of neutral and beta-stable deconfined matter is soft and limits size and mass of pure quark stars.Comment: Contribution to 5th Intern. Conf. "Quark Confinement and the Hadron Spectrum", Gargnano (It), Sep. 10 - 14, 200