Cross Section and Transverse Single-Spin Asymmetry of $\eta$ Mesons in   $p^{\uparrow}+p$ Collisions at $\sqrt{s}=200$ GeV at Forward Rapidity
  We present a measurement of the cross section and transverse single-spin asymmetry ($A_N$) for $\eta$ mesons at large pseudorapidity from $\sqrt{s}=200$~GeV $p^{\uparrow}+p$ collisions. The measured cross section for $0.5<p_T<5.0$~GeV/$c$ and $3.0<|\eta|<3.8$ is well described by a next-to-leading-order perturbative-quantum-chromodynamics calculation. The asymmetries $A_N$ have been measured as a function of Feynman-$x$ ($x_F$) from $0.2<|x_{F}|<0.7$, as well as transverse momentum ($p_T$) from $1.0<p_T<4.5$~GeV/$c$. The asymmetry averaged over positive $x_F$ is $\langle{A_{N}}\rangle=0.061{\pm}0.014$. The results are consistent with prior transverse single-spin measurements of forward $\eta$ and $\pi^{0}$ mesons at various energies in overlapping $x_F$ ranges. Comparison of different particle species can help to determine the origin of the large observed asymmetries in $p^{\uparrow}+p$ collisions. 
A. Adare, C. Aidala, N.N. Ajitanand, Y. Akiba, R. Akimoto, H.   Al-Bataineh, J. Alexander, M. Alfred, A. Angerami, K. Aoki, ...
hep-ex nucl-ex