Spin effects in large rapidity neutral pion production at STAR

TL;DR

This paper reports measurements of large Feynman x neutral pion production in polarized proton collisions, showing results consistent with QCD predictions and indicating significant spin asymmetries related to underlying partonic processes.

Contribution

It provides new experimental data on neutral pion production at high x_F and analyzes spin asymmetries, advancing understanding of spin effects in high-energy proton collisions.

Findings

Cross section matches NLO pQCD calculations

Large positive analyzing power (A_N) increasing with x_F

Preliminary results from RHIC transverse polarization data

Abstract

First measurements of large Feynman x (x_F) neutral pion production in polarized proton collisions at sqrt(s)=200 GeV were reported previously. Cross section measurements at <eta>=3.3 and 3.8$ and analyzing power (A_N) measurements at <eta>=3.8 were completed for 0.3 < x_F < 0.6. The cross section was found to be consistent with next-to-leading order perturbative QCD calculations. In that framework, the dominant subprocess is partonic collisions of valence quarks on low Bjorken-x gluons. The A_N was found to be large and positive and increasing with x_F, and consistent with phenomonological calculations based on the Collins effect, the Sivers effect and initial-state and final-state higher twist contributions. Subsequent data have been acquired with transverse polarized proton collisions at RHIC at sqrt(s)=200 GeV. The status of the analysis and preliminary results will be presented.