High pT Physics in the Forward Region Original Research Article

Two of the most interesting results to emerge from RHIC have been suppression of high pT particles produced at y=0 in AuAu collisions and a suppression of high pZ particles in d-Au collisions. These have been explained in terms of quark energy loss in the final state and suppression of the initial soft gluon density respectively. In gold-gold collisions one would naively expect the first effect to get weaker and the second stronger with increasing rapidity. Intriguingly the nuclear modification factor shows almost no change with rapdity. We will use several tools to try to understand this result. The theoretical constructs of color glass condensate and quark recombination have been proposed as answers to this problem. The experimental work begins with operational definitions of high pT and forward rapidity and a review of pp collisions. Using a wide range of data on limiting fragmentation, proton/pion ratios and the dependence of RAuAu on energy density we tentatively conclude that the hot partonic matter extends into the forward region and that high pT particles emerge from the surface of this matter.