Abstract:
At large collision energy sqrt(s) and relatively low momentum transfer Q, one expects a new regime of Quantum Chromo-Dynamics (QCD) known as "saturation". This kinematical range is characterized by a very large occupation number for gluons inside hadrons and nuclei; this is the region where higher twist contributions are as large as the leading twist contributions incorporated in collinear factorization. In this talk, I discuss the onset of and dynamics in the saturation regime, some of its experimental signatures, and its implications for the early stages of Heavy Ion Collisions.

Abstract:
In this paper, we review the set of rules specific to the calculation of the imaginary part of a Green's function at finite temperature in the real-time formalisms. Emphasis is put on the clarification of a recent controversy concerning these rules in the "1/2" formalism, more precisely on the issue related to the interpretation of these rules in terms of cut diagrams, like at T=0. On the second hand, new results are presented, enabling one to calculate the imaginary part of thermal Green's functions in other formulations of the real-time formalism, like the "retarded/advanced" formalism in which a lot of simplifications occur.

Abstract:
A lot of work has been devoted in the past to understand the role of vertical branch of the time path in thermal field theories, and in particular to see how to deal with it in the real-time formalism. Unlike what is commonly believed, I emphasize on the fact that the vertical part of the path contributes to real-time Green's functions, and I prove that this contribution is taken into account simply by the substitution $n(\omega_{\boldsymbol k})\to n(|k_o|)$ in the real time Feynman rules. This new proof is based on very simple algebraic properties of the contour integration.

Abstract:
At large collision energy sqrt(s) and relatively low momentum transfer Q, one expects a new regime of Quantum Chromo-Dynamics (QCD) known as "saturation". This kinematical range is characterized by a very large occupation number for gluons inside hadrons and nuclei; this is the region where higher twist contributions are as large as the leading twist contributions incorporated in collinear factorization. In this talk, I discuss the onset of and dynamics in the saturation regime, some of its experimental signatures, and its implications for the early stages of Heavy Ion Collisions.

Abstract:
Strong collinear divergences, although regularized by a thermal mass, result in a breakdown of the standard hard thermal loop expansion in the calculation of the production rate of photons by a plasma of quarks and gluons using thermal field theory techniques.

Abstract:
We calculate within the Hard Thermal Loop expansion the production rate of soft photons and lepton pairs by a hot quark-gluon plasma at thermal equilibrium, up to the 2-loop order. Strong collinear divergences appear to mix the orders of the perturbative expansion so that the 2-loop order is in fact dominant compared to the 1-loop order. More precisely, angular integrals that would otherwise be of order 1 are found to behave as powers of $1/g$ when the produced photon is massless, therefore breaking the standard HTL power counting rules.

Abstract:
In this talk, I review new developments of QCD calculations of photon and dilepton production rates in a Quark-Gluon plasma. All the rates are now known up to $O(\alpha_s)$ both for photons and dileptons, thanks to the resummation of multiple scatterings. For dileptons, a direct numerical calculation on the lattice attempted recently will also be discussed.

Abstract:
The production rate of soft real photons or soft lepton pairs by a hot QCD plasma is dominated by strong collinear divergences. As a consequence, it appears that the effective theory based on the resummation of hard thermal loops fails to handle properly these light-cone sensitive processes since some formally higher order diagrams are in reality the dominant ones.

Abstract:
In this proceedings, I consider two-loop contributions to real photon production in thermal QCD. If the photon is strictly massless, strong collinear divergences appear in this calculation. These singularities are regularized by the quark thermal mass of order $gT$, which generates powers of $1/g$, so that the corresponding terms are strongly enhanced with respect to naive expectations of their order of magnitude.

Abstract:
Modifications of the $\pi^o\to 2\gamma$ decay amplitude by thermal effects have already been considered by several authors, leading to quite different results. I consider in this paper the triangle diagram connecting a neutral pion to two photons in a constituent quark model, within the real-time formulation of thermal field theory and study the zero external momentum limit of this diagram. It appears that this limit is not unique and depends strongly on the kinematical configuration of the external particles. This non-uniqueness is shown to explain the contradiction between existing results. I end with some considerations suggesting that this decay amplitude may be significantly modified by the resummation of hard thermal loops, due to infrared singularities.