Energy losses in the black disc regime and correlation effects in the STAR forward pion production in dAu collisions

We argue that in the small x processes, in the black disc QCD regime (BDR) a very forward parton propagating through the nuclear matter should loose a significant and increasing with energy and atomic number fraction of its initial energy as a result of dominance of inelastic interactions, causality and energy-momentum conservation. We evaluate these energy losses and find them to lead to the significant suppression of the forward jet production in the central NA collisions at collider energies with a moderate suppression of recoiling jet at central rapidities. We confront our expectations with the recent RHIC data of the STAR Collaboration on the probability, P, for emission of at least one fast hadron at a central rapidity in association with production of a very forward high p(t) neutral pion in pp and dAu collisions. We calculate the A-dependence of P, and find that the data imply a strong suppression of leading pion production at central impact parameters. We also conclude that production of recoil jets in the hard subprocess is not suppressed providing further evidence for the dominance of peripheral collisions. Both features of the data are consistent with the onset of BDR. We suggest new phenomena and new observables to investigate BDR at RHIC and LHC. (c) 2007 Published by Elsevier B.V.