  The nuclear modification factor $R_{pA}(p_T)$ 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 ${\cal Q}_{pA}(p_T)$ for central collisions is
above $R_{pA}(p_T)$ 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 $1/{A_{\perp}}^\nu$, $\nu\sim0.6\pm0.1$, if one is
able to select more compact configurations of the projectile proton where its
gluons occupy a smaller transverse area $A_\perp$. We predict an enhanced
${\cal
  Q}_{pp}(p_T)-1 \sim 1/(p_T^2)^\nu$ and a Cronin peak even for central $pp$
collisions.
