Transverse Single-Spin Asymmetries of Midrapidity Direct Photons and Neutral Mesons at PHENIX

TL;DR

This paper reports measurements of transverse single-spin asymmetries for direct photons and neutral mesons at midrapidity in polarized proton collisions, providing insights into gluon spin effects in QCD.

Contribution

It presents new experimental data on spin asymmetries for direct photons and mesons, constraining gluon-related functions in proton spin structure.

Findings

Direct photon asymmetries are sensitive to initial-state gluon effects.

Neutral meson asymmetries involve both initial- and final-state effects.

Results help constrain gluon Sivers and trigluon correlation functions.

Abstract

Results are presented for the transverse single-spin asymmetries of direct photons, neutral pions, and eta mesons for $|\eta|<0.35$ from $p^\uparrow + p$ collisions with $\sqrt{s} = 200$ GeV at PHENIX. As hadrons, $\pi^0$ and $\eta$ mesons are sensitive to both initial- and final-state effects and at midrapidity probe the dynamics of gluons along with a mix of quark flavors. Because direct photon production does not include hadronization, the direct photon TSSA is only sensitive to initial-state effects and at midrapidity provides a clean probe of the gluon dynamics in transversely polarized protons. All three of these results will help constrain the collinear twist-3 trigluon correlation function as well as the gluon Sivers function, improving our knowledge of spin-dependent gluon dynamics in QCD.