Cross-section measurements of hard diffraction at the S$p\overline{p}$S- Collider

The UA8 experiment previously reported the observation of jets in diffractive events containing leading protons (``hard diffraction"), which was interpreted as evidence for the partonic structure of an exchanged Reggeon, believed to be the \pom . An analysis of the longitudinal momentum distribution of the 2-jet system in the \pom -proton rest frame showed that the \pom\ structure is ``hard'', like $x(1-x)^1$, with an additional ``super-hard'' component near $x = 1$. In the present Letter, we report the final UA8 hard-diffractive (jet) cross section results and their interpretation. After corrections, the fraction of single diffractive events with mass from 118 to 189~GeV that have two scattered partons, each with $\etj > 8$~GeV, is in the range 0.002 to 0.003 (depending on \xp ). We determine the product, $fK$, of the fraction by which the \pom 's momentum sum rule is violated and the normalization constant of the \pom -Flux-Factor of the proton. For a pure gluonic- or a pure $q\bar{q}$-\pom , respectively: $fK$ = ($0.30 \pm 0.03 \pm 0.09$) and ($0.56 \pm 0.05 \pm 0.17$) GeV$^{-2}$.