Spin Dependence of High-P⊥2Elasticp−pScattering

Abstract

We measured $\frac{d\sigma }{\mathrm{dt}}$ for $p_{\uparrow }+p_{\uparrow }\to p+p$ from ${P_{\perp }}^2=4.50\mathrm{} \mathrm{to} 5.09$ ${(\mathrm{G}\mathrm{e}\mathrm{V}/\mathit{c})}^2$ at 11.75 GeV/c. We used a 59%-polarized proton beam and a 71%-polarized proton target with both spins oriented perpendicular to the scattering plane. In these large-${P_{\perp }}^2$ hard-scattering events, spin effects are very large and the ratio ${\left(\frac{d\sigma }{\mathrm{dt}}\right)}_{\uparrow \uparrow }:{\left(\frac{d\sigma }{\mathrm{dt}}\right)}_{\uparrow \downarrow }$ grows rapidly with increasing ${P_{\perp }}^2$, reaching a value of 4 at 90° (c.m.). Thus, hard elastic scattering, which is presumably due to the direct scattering of the protons' constituents, may only occur when the two incident protons' spins are parallel.