Neutral pion cross section and spin asymmetries at intermediate pseudorapidity in polarized proton collisions at sqrt{s} = 200 GeV

TL;DR

This paper reports measurements of neutral pion production and spin asymmetries in polarized proton collisions at 200 GeV, providing insights into gluon contributions to proton spin and testing QCD predictions at intermediate pseudorapidity.

Contribution

It presents the first measurements of neutral pion cross sections and various spin asymmetries at intermediate pseudorapidity, extending understanding of proton spin structure and QCD models.

Findings

Cross section aligns with NLO pQCD calculations within uncertainties.

Measured A_LL is consistent with models, constraining gluon polarization.

A_N is small and consistent with twist-3 model predictions.

Abstract

The differential cross section and spin asymmetries for neutral pions produced within the intermediate pseudorapidity range 0.8 < {\eta} < 2.0 in polarized proton-proton collisions at sqrt{s} = 200 GeV are presented. Neutral pions were detected using the endcap electromagnetic calorimeter in the STAR detector at RHIC. The cross section was measured over a transverse momentum range of 5 < p_T < 16 GeV/c and is found to be within the scale uncertainty of a next-to-leading order perturbative QCD calculation. The longitudinal double-spin asymmetry, A_LL, is measured in the same pseudorapidity range. This quantity is sensitive to the gluonic contribution to the proton spin, {\Delta}g(x), at low Bjorken-x (down to x approx 0.01), where it is less constrained by measurements at central pseudorapidity. The measured A_LL is consistent with model predictions. The parity-violating asymmetry, A_L, is also measured and found to be consistent with zero. The transverse single-spin asymmetry, A_N, is measured within a previously unexplored kinematic range in Feynman-x and p_T. Such measurements may aid our understanding of the on-set and kinematic dependence of the large asymmetries observed at more forward pseudorapidity ({\eta} approx 3) and their underlying mechanisms. The A_N results presented are consistent with a twist-3 model prediction of a small asymmetry within the present kinematic range.