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Physics of Large-x Nuclear SuppressionJ. Nemchik a,b and M. ˇ Sumbera ca Institute of Experimental Physics SAS, Watsonova 47, 04001 Koˇ sice, Slovakiab Czech Tech. Univ. in Prague, FNSPE, Bˇ rehová 7, 11519 Prague, Czech Republicc Nuclear Physics Institute AS CR, 25068 ˇReˇ z/Prague, Czech RepublicAbstractWe discuss a commonfeatureofall knownreactionsonnucleartargets-a significantsuppressionatlarge x. Simpleinterpretationofthiseffectisbasedonenergyconservationrestrictionsininitialstate parton rescatterings. Using the light-cone dipole approach this mechanism is shown tocontrol variety of processes on nuclear targets: high-p T particle production at different rapiditiesas well as direct and virtual (Drell-Yan) photon production. We demonstrate universality andwide applicability of this mechanism allowing to describe large-x effects also at SPS and FNALenergies too low for the onset of coherent effects or shadowing.1. IntroductionRecent measurements of high-p T hadrons produced in the beam fragmentation region ind + Au collisions at RHIC [1, 2, 3] allow to reach the smallest values of Bjorken x in the nucleusand thus to reachmaximalcoherenceeffects leadingeventuallyto nuclearsuppression. Observedsuppression is usually interpreted within the models based on color glass condensate (CGC).However,such an interpretationmisses a globalapplicability. For example,a similar suppressionlike at RHIC was measured also in p + Pb collisions at SPS where no effects of coherence arepossible.Therise ofthe suppressionwith Feynman x F forhadronsproducedin p+Pbcollisions at SPS[4] or for the Drell-Yan (DY) pairs at FNAL [5] has a similar pattern as seen at RHIC. All theseexamples and another reactions treated in [6] favor the same large-x mechanism independentlyof the energy and type of reaction.Such a common mechanism was proposed in [6, 7] where large-x F nuclear suppression wasshown to be caused by the energyconservationin multiple parton rescatterings. This mechanismis a leading twist effect giving rise to the breakdown of QCD factorization and exhibits also thex F -scaling [7].Another consequenceof this treatment discussed in this paper is the manifestation of nucleareffects also at midrapidities, i.e. at large x T = 2 p T /√ s. We expect a suppression pattern similarto that at large x F and the nucleus-to-nucleonratio below one. Similarly to x F -scaling at forwardrapidities x T -scaling of this effect is predicted.In this paper we further exploit proposed model [6, 7] to analyze and quantify the nuclearsuppression at large x for a variety of processes occurring in p(d) + A and A + B collisions.2. Sudakov suppression, production cross sectionIn any hard reaction in the limit x → 1 gluon radiation is forbidden by the energy conserva-tion. Then the probability to have a large rapidity gap (LRG) Δy = −lnS(x) between the leadingNuclear Physics A 830 (2009) 611c–614c0375-9474/$ – see front matter © 2009 Elsevier B.V. All rights reserved.www.elsevier.com/locate/nuclphysadoi:10.1016/j.nuclphysa.2009.09.052