Evidence for Nonlinear Gluon Effects in QCD and their $A$ Dependence at STAR

TL;DR

The paper presents evidence of nonlinear gluon effects in QCD through measurements of di-$$ correlations in various collision systems, revealing suppression patterns that depend on the nuclear mass number, consistent with gluon saturation models.

Contribution

It provides the first experimental evidence of nonlinear gluon dynamics and their $A$ dependence at high energies in forward particle production.

Findings

Suppression of back-to-back di-$$ pairs in $p+$Al and $p+$Au compared to $p$+$p$.

Suppression increases with the nuclear mass number $A$, following an $A^{1/3}$ scaling.

Indicates nonlinear gluon effects consistent with gluon saturation models.

Abstract

The STAR Collaboration reports measurements of back-to-back azimuthal correlations of di-$\pi^0$s produced at forward pseudorapidities ($2.6<\eta<4.0$) in $p$+$p$, $p+$Al, and $p+$Au collisions at a center-of-mass energy of 200 GeV. We observe a clear suppression of the correlated yields of back-to-back $\pi^0$ pairs in $p+$Al and $p+$Au collisions compared to the $p$+$p$ data. The observed suppression of back-to-back pairs as a function of transverse momentum suggests nonlinear gluon dynamics arising at high parton densities. The larger suppression found in $p+$Au relative to $p+$Al collisions exhibits a dependence of the saturation scale, $Q_s^2$, on the mass number, $A$. A linear scaling of the suppression with $A^{1/3}$ is observed with a slope of $-0.09$ $\pm$ $0.01$.