Conclusions and Outlook

Evidence is being accumulated which indicates that there is a strongly interacting colour singlet object which can mediate high-t interactions and which also contributes through its partonic content to low-t interactions. Further work to extrapolate the diffractive cross section to intermediate t ranges by determining its t dependence may bring about a confrontation of the experimental results in these complementary regimes. The Tevatron and HERA results pertaining to hard diffractive scattering cannot be directly compared at the moment, due to a lack of understanding of the gap survival probabilities. One possible route to achieve a more stringent comparison of the Tevatron and HERA data may be for the Tevatron experiments to try to measure the diffractive contribution to their data in a regime where the survival probability is expected to be high, i.e., for a sample with very high x_p. (The HERA experiments cannot do the converse and go to very low $x_{\gamma}$ while remaining in the regime of applicability of perturbative QCD.) The Tevatron constraint on the $\Sigma_{I\!\!P} - c_g$ plane from measurements of diffractive hard scattering is only barely consistent with the HERA constraint at present. (They find, for instance, that $\Sigma_{I\!\!P}$ must be less than 0.5 if $c_g \sim 0.5$ [23].) We look forward to an exciting comparison in the near future. As for the confrontation between experiment and theory, neither are presently precise enough for any strong statements to be made and much work remains to be done.