Energy Dependence of Spin Effects inp↑+p↑→p+p

We measured the analyzing power $A$ and the spin-spin correlation parameter $A_{\mathrm{nn}}$, in large-$P_{\perp }^2$ proton-proton elastic scattering, using a polarized-proton target and the polarized-proton beam at the Brookhaven Alternating-Gradient Synchrotron. We also used our polarimeter to measure $A$ at small $P_{\perp }^2$ at 13 GeV with good precision and found some deviation from the expected $\frac{1}{P_{\mathrm{lab}}}$ behavior. At 18.5 GeV/c we found $A_{\mathrm{nn}}=(-2\mathrm{}\pm 16)\%$ at $P_{\perp }^2=4.7\mathrm{}$ ${(\mathrm{G}\mathrm{e}\mathrm{V}/\mathit{c})}^2$. Comparison with lower-energy data from the Argonne Zero-Gradient Synchrotron shows a sharp and surprising energy dependence for $A_{\mathrm{nn}}$ at large $P_{\perp }^2$.