arXiv subjects
Publications and source records attributed to F. Gelis.
In this talk, I review the status of the calculation of the photon production rate in a hot quark-gluon plasma. Particular emphasis is given to the discussion of the various length scales of the problem.
In this paper, we determine a condition of applicability of a very simple formalism for the calculation of a fast process taking place in a non-equilibrium medium, in which the particle distribution functions are frozen in time. Indeed, a well known obstacle for the use of such a naive formalism is the appearance of the so-called ``pinch singularities''. We argue that these potentially dangerous terms can be regularized, and that they are negligible if the characteristic time-scale of the process under study is small compared to the relaxation time of the system.
Thermal amplitudes with ultrasoft momenta, which are not accessible by standard methods of perturbation theory, have recently attracted a lot of interest. However, the comparison of external momenta with the ultrasoft scale $g^2T\ln(1/g)$ is a too crude criterion, since amplitudes with hard external momenta can also be non perturbative, if these momenta are close enough to the light-cone. In this letter, I give a more refined criterion to decide if an amplitude is non perturbative, that applies to all situations. In physical terms, this condition states that non-perturbative effects appear if the particles running in loops have to travel distances larger than their mean free path.
In recent studies, the production rate of photons or lepton pairs by a quark gluon plasma has been found to be enhanced due to collinear singularities. This enhancement pattern is very dependent on rather strict collinearity conditions between the photon and the quark momenta. It was estimated by neglecting the collisional width of quasi-particles. In this paper, we study the modifications of this collinear enhancement when we take into account the possibility for the quarks to have a finite mean free path. Assuming a mean free path of order $(g^2T\ln(1/g))^{-1}$, we find that only low invariant mass photons are affected. The region where collision effects are important can be interpreted as the region where the Landau-Pomeranchuk-Migdal effect plays a role in thermal photon production by bremsstrahlung. It is found that this effect modifies the spectrum of very energetic photons as well. Based on these results and on a previous work on infrared singularities, we end this paper by a reasonable physical picture for photon production by a quark gluon plasma, that should be useful to set directions for future technical developments.
We critically examine recently published results on the thermal production of massive vector bosons in a quark-gluon plasma. We claim the production rate is a collinear safe observable.
This is the report of the QCD working group at WHEPP 6. Discussions and work on heavy ion collisions, polarised scattering, and collider phenomenology are reported.
In this talk, I review recent progress made in two areas of thermal field theory. In particular, I discuss various approaches for the calculation of the quark gluon plasma thermodynamical properties, and the problem of its photon production rate.
We use the 2-dimensional $σ$ model as a toy model to study the behavior of anomalous amplitudes in the limit where the constituent quark mass is small. Symmetry arguments tell that the $π^o\toγ$ amplitude should vanish if $m\to 0$, but we show that this conclusion is spoiled by infrared singularities. When a proper regularization (resummation of a thermal mass, for instance) is taken into account, this amplitude vanishes as expected. We also study the amplitude $π^oσ\toγ$ and show that it does not vanish in the same limit.
We study the infrared singularities associated to ultra-soft transverse gluons in the calculation of photon production by a quark-gluon plasma. Despite the fact that the KLN theorem works in this context and provides cancellations of infrared singularities, it does not prevent the production rate of low invariant mass dileptons to be sensitive to the magnetic mass of gluons and therefore the rate to be non perturbative.
In this paper, we extend the study of bremsstrahlung photon production in a quark-gluon plasma to the cases of soft static photons and hard real photons. The general framework of this study is the effective perturbative expansion based on the resummation of hard thermal loops. Despite the fact that bremsstrahlung only comes at two loops, we find that in both cases it generates contributions of the same order of magnitude as those already calculated by several other groups at one loop. Furthermore, a new process contained in the two-loop diagrams dominate the emission of a very hard real photon. In all cases, the rate of real or virtual photon production in the plasma is appreciably increased compared to the one-loop predictions.
We discuss the bremsstrahlung production of soft real and virtual photons in a quark-gluon plasma at thermal equilibrium beyond the Hard Thermal Loop (HTL) resummation. The physics is controlled by the ratio $Q^2/q_0^2$ of the virtuality to the energy. When $Q^2/q_0^2$ is much smaller than $g^2$, where $g$ is the strong coupling constant, the emission rate is enhanced by a factor $1/g^2$ over the HTL results due to light-cone singularities and the bremsstrahlung is induced by scattering of the quark via both transverse and longitudinal soft gluon exchanges. When $Q^2/q_0^2$ increases, the enhancement factor is given by $q_0^2/Q^2$. When this ratio is near unity, the bremsstrahlung contribution is of the same order as the rate predicted by the HTL resummation. In that case, the bremsstrahlung is induced by both soft and hard gluon exchanges.
Recent studies of the high temperature soft photon production rate on the light--cone using Braaten--Pisarski resummation techniques have found collinear divergences present. We show that there exist a class of terms outside the Braaten--Pisarski framework which, although also divergent, dominate over these previously considered terms. The divergences in these new terms may be alleviated by application of a recently developed resummation scheme for processes sensitive to the light--cone.