Large rapidity gap reactions on nuclei: breakdown of QCD factorization

The relative production rate of high-pT hadrons in deuteron-gold and proton-proton collisions measured at forward rapidities in the BRAHMS experiment is found to be suppressed. Observing other reactions, one may notice that any reaction (forward production of light hadron, Drell-Yan process, heavy flavor production, etc.) expose the same property: the cross section at large xF is suppressed by nuclei. Although it is tempting to interpret this effect as a result of coherence, or color glass condensate, it takes place even at low energies. We demonstrate here that there is a suppression mechanism common for all reactions which can be understood as Sudakov suppression or as a reduced survival probability of a large rapidity gap process in nuclei. This is confirmed by calculations, which provide a good description for all the reactions under consideration.