Forward Neutral Pion Cross Section and Spin Asymmetry Measurements at STAR

TL;DR

This paper reports on measurements of neutral pion cross sections and spin asymmetries in the forward region using the STAR EEMC, providing new insights into gluon polarization and transverse spin effects at low Bjorken-x.

Contribution

It introduces novel measurements of neutral pion cross sections and spin asymmetries in the forward pseudorapidity region, expanding understanding of spin phenomena and gluon polarization.

Findings

Neutral pion cross section consistent with expectations.

Transverse single spin asymmetries observed up to 10%.

Verification of pion reconstruction techniques.

Abstract

The STAR endcap electromagnetic calorimeter (EEMC) was designed to allow measurement of cross sections and spin observables in the forward direction, $1 < \eta < 2$ and with the full azimuth. Using the EEMC to measure double longitudinal spin asymmetries in photonic channels---such as inclusive neutral pions, prompt photon, and prompt photon + jet---allows access to $\Delta G$ within a low Bjorken-$x$ region ($0.01 < x < 0.33$ at $\sqrt{s}=200$) where $\Delta G$ is poorly constrained. Additionally, transverse spin asymmetries, shown to be zero at $\eta$ near zero and as large as 10\% at $\eta$ near 4, occupy a previously unmeasured region in the 3D pseudorapidity, transverse momentum, and $x$-Feynman phase space when measured with the EEMC. The neutral pion cross section measurement verifies that the signal of interest can be properly reconstructed and isolated from the background. Pion reconstruction techniques in the STAR EEMC will be discussed and preliminary cross-section and transverse single spin asymmetry measurements presented.