Longitudinal Double-Spin Asymmetries for $\pi^{0}$s in the Forward Direction for 510 GeV Polarized $pp$ Collisions

TL;DR

This paper presents measurements of the longitudinal double-spin asymmetry for neutral pions in forward polarized proton-proton collisions at 510 GeV, providing insights into the polarized gluon distribution at very low Bjorken x.

Contribution

It provides new experimental data on $A_{LL}$ for $0$ mesons at forward angles, probing the polarized gluon distribution at unprecedented low x values.

Findings

Asymmetries are less than b15 d7 10^{-3} in magnitude.

Results help constrain the polarized gluon distribution b1g(x) at x b10^{-3}.

Data will improve global analyses of proton spin structure.

Abstract

The STAR Collaboration reports measurements of the longitudinal double-spin asymmetry, $A_{LL}$, for neutral pions produced at forward directions in polarized proton-proton collisions, at a center-of-mass energy of $510$ GeV. Results are given for transverse momenta in the range $2<p_{T}<10$ GeV/$c$ within two regions of pseudorapidity that span $2.65<\eta<3.9$. These results are sensitive to the polarized gluon parton distribution function, $\Delta g(x)$, down to the region of Bjorken $x \sim 10^{-3}$. The asymmetries observed are less than $\pm 5 \cdot 10^{-3}$ in magnitude, and will help constrain the contribution to the spin of the proton from polarized gluons at low $x$, when combined with other measurements as part of a global analysis.