Energy Dependence of Spin Effects in<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi>p</mml:mi></mml:mrow><mml:mrow><mml:mi>↑</mml:mi></mml:mrow></mml:msub></mml:mrow><mml:mo>+</mml:mo><mml:mrow><mml:msub><mml:mrow><mml:mi>p</mml:mi></mml:mrow><mml:mrow><mml:mi>↑</mml:mi></mml:mrow></mml:msub></mml:mrow><mml:mo>→</mml:mo><mml:mi>p</mml:mi><mml:mo>+</mml:mo><mml:mi>p</mml:mi></mml:math>

We measured the analyzing power $A$ and the spin-spin correlation parameter ${A}_{\mathrm{nn}}$, in large-${P}_{\ensuremath{\perp}}^{2}$ 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 ${P}_{\ensuremath{\perp}}^{2}$ 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 ${A}_{\mathrm{nn}}=(\ensuremath{-}2\ifmmode\pm\else\textpm\fi{}16)%$ at ${P}_{\ensuremath{\perp}}^{2}=4.7$ ${(\mathrm{G}\mathrm{e}\mathrm{V}/\mathit{c})}^{2}$. Comparison with lower-energy data from the Argonne Zero-Gradient Synchrotron shows a sharp and surprising energy dependence for ${A}_{\mathrm{nn}}$ at large ${P}_{\ensuremath{\perp}}^{2}$.