Understanding Nuclear Effects with QCD

Jet quenching and partonic energy loss High-pT hadron production is expected to provide a sensitive probe for dense QCD matter created in heavy ion collisions. This probe, usually called jet quenching, exploits the theoretical expectation of an enhanced energy loss of a parton propagating through hot matter. The effect is predicted to be proportional to the density of the medium. In spite of much theoretical work done over the last decade, no experimental evidence for energy loss even in a cold nuclear medium has been found so far. Moreover, the existing analysis of nuclear effects in Drell-Yan reactions which is sensitive to energy loss detected no effect at all. This confusing conclusion contradicting theoretical expectations has raised doubts about the correctness of our understanding of the QCD dynamics of induced gluon radiation. The main difficulty one faces trying to single out the effect of energy loss from Drell-Yan reactions is a strong overlap with the phenomenon of nuclear shadowing. This problem needs a more profound analysis employing the best of our knowledge of the dynamics of shadowing. A new analysis based on reliable and well tested predictions for the net shadowing effect was undertaken in [1, 2]. The combined analysis of data from the E772 and E866 experiments at Fermilab resulted in a rather large energy loss rate, − dz = 2.73± 0.37± 0.5GeV/fm , (1)