Photon production in a quark-gluon plasma

Lise Meitner Position M 590 Photon production in a quark-gluon Plasma Haintham ZARAKET 09.10.2000 A new state of matter called the quark-gluon plasma (QGP) is expected to be formed in the ultra relativistic heavy ion collisions presently studied at RHIC (Brookhaven) and later at LHC (Geneva). In this state the quarks and gluons behave almost as a free "gas" of particles. The transparency of QGP to electromagnetic waves gives the detection of photons or dileptons produced in the plasma a special status as a probe on the early history of the plasma. The theoretical framework in which the study is carried out is Thermal Field Theory. Although the photon will not have further interactions in the plasma, the quark that emits it is affected by collective effects. These effects can he taken into account by using the hard thermal loop effective theory (HTL) (a la Braaten and Pisarski). Of utmost importance for the discovery of the QGP is the real and quasi-real (low mass dilepton) photon production. However, their rate is plagued by collinear divergences. At one loop level these divergences are treated using the modified HTL effective theory (a la Flechsig and Rebhan). But our work on these rates show that the modified effective theory does not prevent the two loop diagrams to be among the dominant contributions: new physical processes like bremsstrahlung and the annihilation of an off-shell quark stand out as dominant one. Moreover higher loop diagrams are shown to be essential when one approaches the light cone (i.e. the photon mass approaches zero). Getting closer and closer to the light cone we explore new physical mechanisms of photon production: first, the Landau-Pomeranchuk-Migdal-effect (LPM) shows up, with complete new feature like the importance of transverse gluon exchanges or that of spectrum modification of highly energetic photon, second, still nearer the light cone, a dependence on the (non perturbative) magnetic mass enters into the game, despite the thermal field theory version of the Kinoshita-Lee-Naunberg theorem. To calculate, in practice, the LPM effect and the magnetic mass dependence it is required to develop new techniques like functional integration methods. Using eikonal approximation technique we show complete cancellations to all orders between different diagrams and hence going beyond this approximation is mandatory. Kinetic equations methods, or new effective theories are the building blocks of a solution of the underlined problems. These methods will be at the basis of my near future research and collaboration. Beside the development needed to solve the above mentioned problem, I have started working on the World line formalism at finite temperature namely the problem of analytical continuation, long time raised by Mckeon and Rebhan.