SEMINAR     SEMINAR   Monday, March 5 th, 2007- h. 14.30 Seminar room           Prof. Hans-Juergen Pirner Institut fuer Theoretische Physik, Heidelberg         Energy loss of a fast parton in the Quark Gluon Plasma       I discuss the strong coupling constant in the plasma. Is it changed and how? Then using a R.G. calculation of the strong coupling I calculate the parton-parton cross sections in the plasma and the collisional energy loss. The inverse Compton process is investigated and found to contribute 30%. Next I derive the radiative energy loss from the Bethe Heitler cross section. I work out the temperature dependence of the two contributions and their time dependence during the collision. Finally I emphasize that the fragmentation process also varies with the virtuality of the initial parton.