Evidence for x -dependent proton color fluctuations in pA collisions at the CERN Large Hadron Collider

The centrality dependence of forward jet production in pA collisions at the Large Hadron Collider (LHC) has been found to grossly violate the Glauber model prediction in a way that depends on the x in the proton. We argue that this modification pattern provides the first experimental evidence for x-dependent proton color fluctuation effects. On average, parton configurations in the projectile proton containing a parton with large x interact with a nuclear target with a significantly smaller than average cross section and have smaller than average size. We implement the effects of fluctuations of the interaction strength and, using the ATLAS analysis of how hadron production at backward rapidities depends on the number of wounded nucleons, make quantitative predictions for the centrality dependence of the jet production rate as a function of the x-dependent interaction strength σ(x). We find that σ(x)∼0.6σ gives a good description of the data at x=0.6. These findings support an explanation of the European Muon Collaboration effect as arising from the suppression of small-size nucleon configurations in the nucleus.