The small-x gluon distribution in centrality biased pA and pp collisions

The nuclear modification factor RpA(pT) provides information on the small-x gluon distribution of a nucleus at hadron colliders. Several experiments have recently measured the nuclear modification factor not only in minimum bias but also for central pA collisions. In this paper we analyze the bias on the configurations of soft gluon fields introduced by a centrality selection via the number of hard particles. Such bias can be viewed as reweighting of configurations of small-x gluons. We find that the biased nuclear modification factor QpA(pT) for central collisions is above RpA(pT) for minimum bias events, and that it may redevelop a “Cronin peak” even at small x. The magnitude of the peak is predicted to increase approximately like 1/A⊥ν, ν∼0.6±0.1, if one is able to select more compact configurations of the projectile proton where its gluons occupy a smaller transverse area A⊥. We predict an enhanced Qpp(pT)−1∼1/(pT2)ν and a Cronin peak even for central pp collisions.