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

The nuclear modification factor R(p)A(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 Q(pA)(PT) for central collisions is above R-pA(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(perpendicular to)(nu) , nu similar to 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(perpendicular to.) We predict an enhanced Q(pp)(PT) - 1 similar to 1 /(P-T(2))(nu) and a Cronin peak even for central pp collisions. (C) 2018 Elsevier B.V. All rights reserved.