The STAR collaboration reports a measurement of the transverse single-spin asymmetries, AN, for neutral pions produced in polarized proton collisions with protons (pp), with aluminum nuclei (pAl) and with gold nuclei (pAu) at a nucleon-nucleon center-of-mass energy of 200 GeV. Neutral pions are observed in the forward direction relative to the transversely polarized proton beam, in the pseudorapidity region 2.7<η<3.8. Results are presented for $\pi^{0}$s observed in the STAR forward meson spectrometer electromagnetic calorimeter in narrow Feynman x ($x_F$) and transverse momentum ($p_{T}$) bins, spanning the range $0.17<x_F<0.81$ and $1.7<p_{T}<6.0$ GeV/c. For fixed $x_F<0.47$, the asymmetries are found to rise with increasing transverse momentum. For larger $x_F$, the asymmetry flattens or falls as $p_T$ increases. Parametrizing the ratio $r(A)≡A_{N}(pA)/A_{N}(pp)=A^{P}$ over the kinematic range, the ratio $r(A)$ is found to depend only weakly on $A$, with $⟨P⟩=−0.027\pm0.005$. No significant difference in $P$ is observed between the low-$p_T$ region, $p_T<2.5$ GeV/c, where gluon saturation effects may play a role, and the high-$p_T$ region, $p_T>2.5$ GeV/c. It is further observed that the value of $A_{N}$ is significantly larger for events with a large-$p_T$ isolated $\pi^{0}$ than for events with a nonisolated $\pi^{0}$ accompanied by additional jetlike fragments. The nuclear dependence $r(A)$ is similar for isolated and nonisolated $\pi^{0}$ events.
Rongrong Ma