Spin effects in large rapidity pi0 production at STAR

TL;DR

This paper reports measurements of neutral pion production and spin asymmetries at large rapidity in polarized proton collisions at RHIC, comparing results with QCD models and discussing future experimental upgrades.

Contribution

It provides new measurements of cross sections and analyzing powers at different energies and rapidities, testing QCD predictions and phenomenological models of spin effects.

Findings

Cross section measurements agree with NLO pQCD calculations.

Analyzing power is zero at negative x_F and rises at positive x_F.

Results support models including Sivers and Collins effects.

Abstract

Measurements by the STAR collaboration of neutral pion production at large Feynman $x$ ($x_F$) in the first polarized proton collisions at $\sqrt{s}=200$ GeV were reported previously. During the following two runs additional statistics were acquired with an improved forward calorimeter for the $\pi^0$ cross-section and analyzing power measurements. First data from $pp$ collisions at $\sqrt{s}=410$ GeV were taken during the RHIC run that ended in June, 2005. The cross section was measured at $\eta=$3.3, 3.8 and 4.0 and was found to be consistent with next-to-leading order perturbative QCD calculations. The analyzing power was found to be zero at negative $x_F$ and at positive $x_F$ up to ~0.3, then increased with increasing $x_F$. This behavior can be described by phenomenological models including the Sivers effect, the Collins effect or higher twist contributions in initial and final states. Results for the analyzing power at $\eta=$3.7 and 4.0 from all data acquired at $\sqrt{s}=200$ GeV and the status of the analysis of the $\sqrt{s}=410$ GeV data will be presented. Future upgrade plans and status will also be discussed.