A generating functional for ultrasoft amplitudes in hot QCD

The effective amplitudes for gluon momentum p<<gT in hot QCD exhibit damping as a result of collisions. The whole set of n-point amplitudes is shown to be generated from one functional K(x,y;A), in addition to the induced current j(x;A).Comment: 7 pages, no figure (some comments added

Perturbation theory and non-perturbative renormalization flow in scalar field theory at finite temperature

We use the non-perturbative renormalization group to clarify some features of perturbation theory in thermal field theory. For the specific case of the scalar field theory with O(N) symmetry, we solve the flow equations within the local potential approximation. This approximation reproduces the perturbative results for the screening mass and the pressure up to order g^3, and starts to differ at order g^4. The method allows a smooth extrapolation to the regime where the coupling is not small, very similar to that obtained from a simple self-consistent approximation.Comment: 42 pages, 19 figures; v2: typos corrected and references added, version accepted for publication in Nucl. Phys.

Meson correlation functions in a QCD plasma

The temporal pseudoscalar meson correlation function in a QCD plasma is investigated in a range of temperatures exceeding $T_c$ and yet of experimental interest. Only the flavour-singlet channel is considered and the imaginary time formalism is employed for the finite temperature calculations. The behaviour of the meson spectral function and of the temporal correlator is first studied in the HTL approximation, where one replaces the free thermal quark propagators with the HTL resummed ones. This procedure satisfactory describes the soft fermionic modes, but its application to the propagation of hard quarks is not reliable. An improved version of the so-called NLA scheme, which allows a better treatment of the hard fermionic modes, is then proposed. The impact of the improved NLA on the pseudoscalar temporal correlator is investigated.Comment: 35 pages, 15 figure

Heavy ion collisions: puzzles and hopes

This talk is a brief summary of some theoretical issues in the field of hot and dense QCD matter and ultra-relativistic heavy ion collisions.Comment: 6 pages, invited talk presented at the conference "Physics at the LHC 2010", DESY, June 7-12, 2010 Minor corrections in reference

Transverse Momentum Dependence of Anomalous J/\psi Suppression in Pb-Pb Collisions

The recently published data for $$ for $J/\psi$ production in Pb-Pb collisions at 158 A GeV are analyzed. For low values of transverse energy $E_t$, where normal suppression dominates, $(E_t)$ scales with the path length of the gluons which fuse to make the $J/\psi$. In the $E_t$ domain of anomalous suppression $(E_t)$ is found to rise linearly with the relative amount of anomalous suppression. This empirical law is reproduced within an analytically solvable transport model which allows high $p_t$ $J/\psi$'s to escape anomalous suppression. Interpreted in this way, the data for $(E_t)$ lead to an estimate of $t_A \sim 4$ fm/$c$ for the duration of anomalous suppression.Comment: 10 pages and 3 figure

Non-Perturbative Renormalization Group calculation of the scalar self-energy

We present the first numerical application of a method that we have recently proposed to solve the Non Perturbative Renormalization Group equations and obtain the n-point functions for arbitrary external momenta. This method leads to flow equations for the n-point functions which are also differential equations with respect to a constant background field. This makes them, a priori, difficult to solve. However, we demonstrate in this paper that, within a simple approximation which turns out to be quite accurate, the solution of these flow equations is not more complicated than that of the flow equations obtained in the derivative expansion. Thus, with a numerical effort comparable to that involved in the derivative expansion, we can get the full momentum dependence of the n-point functions. The method is applied, in its leading order, to the calculation of the self-energy in a 3-dimensional scalar field theory, at criticality. Accurate results are obtained over the entire range of momenta.Comment: 29 page

High temperature phase of QCD

I give a brief overview of our present understanding of the high temperature phase of QCD, trying to clarify some of the theoretical issues involved in the current discussions that emphasize the strongly coupled character of the quark-gluon plasma produced at RHIC.Comment: To appear in the proceedings of the International Conference on Strong & Electroweak Matter, BNL, May 10-13 (2006), to be published in Nucl. Phys. A 9 pages, 3 figures - late submission to the Archiv