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

TL;DR

This paper investigates how centrality bias affects the small-x gluon distribution measurements in pA and pp collisions, predicting an enhanced nuclear modification factor and a possible Cronin peak at small x due to configuration reweighting.

Contribution

It introduces a model for the bias on small-x gluon configurations caused by centrality selection, predicting increased nuclear modification factors and the emergence of a Cronin peak in biased collisions.

Findings

Biased nuclear modification factor ${\ m Q}_{pA}(p_T)$ exceeds $R_{pA}(p_T)$ for minimum bias.

A Cronin peak can reappear at small x in central collisions.

The peak magnitude scales approximately as $1/{A_{\perp}}^{\nu}$ with $\nu\sim0.6\pm0.1$.

Abstract

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.