Small-${\it x}$ and Forward Measurements at STAR

TL;DR

This paper investigates forward di-pion azimuthal correlations in d+Au and p+Au collisions at STAR to explore low-x gluon saturation and multi-parton interactions, revealing similar correlation features in both collision types.

Contribution

It provides new insights into gluon saturation and multi-parton interactions by comparing d+Au and p+Au collisions and differentiating collision events using neutron tagging.

Findings

Gluon saturation signals may be present at STAR in forward di-pion measurements.

Multi-parton interactions are more significant in d+Au than p+Au collisions.

Similar azimuthal correlation features are observed in both d+Au and p+Au collisions.

Abstract

Measurements of azimuthal differences between forward di-pions are sensitive to the low-${\it x}$ gluon content of the proton and provide the best opportunity to probe for gluon saturation in nuclei. Previously reported analyses have shown that the gluon saturation regime may have been reached at STAR by looking at forward di-pions in d+Au collisions. Further insight into the uncorrelated pedestal below the near-side and away-side peaks in azimuthal correlations may be provided by differentiating between d+Au and p+Au collisions, by tagging on intact neutrons in the deuteron beam in d+Au collisions. Comparisons to recent theories indicate that multi-parton interactions play a more significant role in d+Au collisions than p+Au collisions and offer a unique opportunity to study correlations between leading partons inside nucleons. The general features found for the peaks in forward di-pion azimuthal correlations in d+Au collisions are also present in p+Au collisions.