Measurement of the Analyzing Power $A_N$ in $pp$ Elastic Scattering in   the CNI Region with a Polarized Atomic Hydrogen Gas Jet Target
  A precise measurement of the analyzing power $A_N$ in proton-proton elastic scattering in the region of 4-momentum transfer squared $0.001 < |t| < 0.032 ({\rm GeV}/c)^2$ has been performed using a polarized atomic hydrogen gas jet target and the 100 GeV/$c$ RHIC proton beam. The interference of the electromagnetic spin-flip amplitude with a hadronic spin-nonflip amplitude is predicted to generate a significant $A_N$ of 4--5%, peaking at $-t \simeq 0.003 ({\rm GeV}/c)^2$. This kinematic region is known as the Coulomb Nuclear Interference region. A possible hadronic spin-flip amplitude modifies this otherwise calculable prediction. Our data are well described by the CNI prediction with the electromagnetic spin-flip alone and do not support the presence of a large hadronic spin-flip amplitude. 
H. Okada, I.G. Alekseev, A. Bravar, G. Bunce, S. Dhawan, R. Gill, W.   Haeberli, O. Jinnouchi, A. Khodinov, Y. Makdisi, A. N...