Forward Neutral Pion Transverse Single Spin Asymmetries in p+p Collisions at \sqrt{s}=200 GeV

TL;DR

Measurements of the Feynman x (xF) dependence, and first measurements of the transverse momentum (pT) dependence), of transverse single-spin asymmetries for the production of pi0 mesons from polarized proton collisions at sqrt[s] = 200 GeV are reported.

Abstract

We report precision measurements of the Feynman-x dependence, and first measurements of the transverse momentum dependence, of transverse single spin asymmetries for the production of π^0 mesons from polarized proton collisions at \sqrt{s}=200 GeV. The x_F dependence of the results is in fair agreement with perturbative QCD model calculations that identify orbital motion of quarks and gluons within the proton as the origin of the spin effects. Results for the p_T dependence at fixed x_F are not consistent with pQCD-based calculations.

Figures (4)

• Figure 1: Correlation between pion longitudinal momentum scaled by $\sqrt{s}/2$ ($x_F$) and transverse momentum ($p_T$) for all events. Bins in $x_F$ used in Figs. \ref{['fig:2']} and \ref{['fig:4']} are indicated by the vertical lines. There is a strong correlation between $x_F$ and $p_T$ at a single pseudorapidity ($\langle\eta\rangle$).
• Figure 2: Analyzing powers in $x_F$ bins (see Fig. \ref{['fig:1']}) at two different $\langle\eta\rangle$. Statistical errors are indicated for each point. Systematic errors are given by the shaded band, excluding normalization uncertainty. The calculations are described in the text. The inset shows examples of the spin-sorted invariant mass distributions. The vertical lines mark the $\pi^0$ mass.
• Figure 3: Analyzing powers versus $\pi^0$ transverse momentum ($p_T$) for events with scaled $\pi^0$ longitudinal momentum $|x_F| > 0.4$. Errors are as described for Fig. \ref{['fig:2']}.
• Figure 4: Analyzing powers versus $\pi^0$ transverse momentum ($p_T$) in fixed $x_F$ bins (see Fig. \ref{['fig:1']}). Errors are as described for Fig. \ref{['fig:2']}. The calculations are described in the text.