The nuclear modification factor RpA provides insights into the small-x gluon distribution of a nucleus at high energy hadron colliders. Several experimental collaborations have recently measured the nuclear modification factor not only in minimum bias but also in central pA collisions. Their main observation is that the nuclear modification factor increases with centrality and that it displays a Cronin-like peak at transverse momenta of order 3 GeV.
In this talk, in the saturation/CGC framework, I analyze the bias on the configurations of soft gluon fields introduced by a centrality selection via the number of hard particles. I demonstrate that, in contrast to a naive expectation, the biased nuclear modification factor QpA for central collisions is above RpA for minimum bias events, and that the bias may lead to a Cronin peak even at small x.