Small $P_{\perp}^2$ Elastic Proton-Proton Polarization near 300 GeV
  Using two hopefully reasonable assumptions plus existing data on the analyzing power in high energy proton-proton elastic scattering, we estimate that the hadronic helicity-flip amplitude at $P^2_{\perp}$ of 0.003 (GeV/c)$^2$ is neglible near 300 GeV. This would allow the absolute calibration of a CNI polarimeter. 
A. D. Krisch and S. M. Troshin