Measurement of spin effects in p/sub up-arrow/+p/sub up-arrow/. -->. p+p at 18. 5 GeVc

We measured the analyzing power A and the spin-spin correlation parameter A/sub n//sub n/ in medium-P/sub perpendicular//sup 2/ proton-proton elastic scattering, using a polarized-proton target and the 18.5-GeVc Brookhaven Alternating-Gradient Synchrotron polarized-proton beam. We found sharp dips in both A and A/sub n//sub n/, which occur at different P/sub perpendicular//sup 2/ values. The unexpected sharp structure in the spin-spin force occurs near P/sub perpendicular//sup 2/ = 2.3 (GeVc)/sup 2/ where the elastic cross section has no apparent structure.