Single Spin Asymmetry $A_N$ in Polarized Proton-Proton Elastic   Scattering at $\sqrt{s}=200$ GeV
  We report a high precision measurement of the transverse single spin asymmetry $A_N$ at the center of mass energy $\sqrt{s}=200$ GeV in elastic proton-proton scattering by the STAR experiment at RHIC. The $A_N$ was measured in the four-momentum transfer squared $t$ range $0.003 \leqslant |t| \leqslant 0.035$ $\GeVcSq$, the region of a significant interference between the electromagnetic and hadronic scattering amplitudes. The measured values of $A_N$ and its $t$-dependence are consistent with a vanishing hadronic spin-flip amplitude, thus providing strong constraints on the ratio of the single spin-flip to the non-flip amplitudes. Since the hadronic amplitude is dominated by the Pomeron amplitude at this $\sqrt{s}$, we conclude that this measurement addresses the question about the presence of a hadronic spin flip due to the Pomeron exchange in polarized proton-proton elastic scattering. 
STAR Collaboration: L. Adamczyk, G. Agakishiev, M. M. Aggarwal, Z.   Ahammed, A. V. Alakhverdyants, I. Alekseev, J. Alford, ...
nucl-ex hep-ex