Propagation of energetic partons in matter

We review the properties of energetic parton propagation in hot or cold QCD matter, as obtained in recent work. The medium induced energy loss is stud ied. It has the remarkable feature to grow as L-2, the length of the traver sed matter squared. Numerical estimates suggest that it may be significantl y enhanced in hot matter compared to cold matter, thus pointing towards a p ossible signal for quark-gluon plasma formation. The more realistic case of an expanding (longitudinally) QCD plasma is studied. The resulting radiati ve energy loss can be as large as 6 times the corresponding one in a static plasma at the reference temperature T(L) which is reached after the parton propagates on a distance L. Finally, the spectrum of soft, radiated gluons is studied, leading to the calculation of the medium dependent energy lost by a jet with opening angle theta(cone). It is shown that the fraction of this energy loss to the integrated one exhibits a universal behavior in ter ms of theta(cone)(2)L((q) over cap)(3) where (q) over cap is the transport coefficient characterizing the medium. Phenomenological implications for th e difference between hot and cold matter are discussed.