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

The STAR collaboration
Phys.Rev.D 89 (2014) 012001, 2014.

Abstract (data 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.

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