CROSS-SECTION MEASUREMENTS OF HARD DIFFRACTION AT THE SP(P)OVER-BARS-COLLIDER

The UA8 experiment previously reported the observation of jets in diff ractive events containing leading protons (''hard diffraction''), whic h was interpreted as evidence for the partonic structure of an exchang ed Reggeon. believed to be the Pomeron. In the present Letter, we repo rt the final UA8 hard-diffractive (jet) cross section results and thei r interpretation. After corrections, the fraction of single diffractiv e events with mass from 118 to 189 GeV that have two scattered partons , each with E-T(jet) > 8 GeV, is in the range 0.002 to 0.003 (dependin g on x(p)). We determine the product, fK, of the fraction by which the Pomeron's momentum sum rule is violated and the normalization constan t of the Pomeron-Flux-Factor of the proton. For a pure gluonic- or a p ure q (q) over bar-Pomeron, respectively: fK = (0.30 +/- 0.05 +/- 0.09 ) and (0.56 +/- 0.09 +/- 0.17) GeV-2. (C) 1998 Published by Elsevier S cience B.V.