Abstract
PHENIX observed a 20% suppression in the production of high pT neutral pions in the most central (0-5%) d+Au collisions at 200 GeV. Through the simultaneous measurement of high pT direct photons (γdir) and π0 production for event samples selected by event activity, the final state effects could be disentangled from cold-nuclear-matter effects and event-selection biases that are inherent in using the standard Glauber model. This isolation of final state effects is achieved by approximating the nuclear modification factor by the double ratio RxA = (γdir/π0)pp/(γdir/π0)xA. While the cold-nuclear-matter effects in x+A collisions cancel in the (γdir/π0) ratio, the effective number of binary collisions is given by NCollEXP = γxAudir / γppdir, which eliminates the dependence on the Glauber model. In addition, many systematic uncertainties cancel in the double ratio.