Hard diffraction: results, comparison and conclusions

Experimental results on hard diffraction from HERA and the Tevatron are summarized and compared, and conclusions are drawn about factorization and scaling in diffraction and about the nature of the pomeron. Results on hard diffraction from HERA and from the Tevatron indicate that the pomeron structure is composed of mainly hard gluons and quarks in a ratio of approximately 3 / 1. A comparison of hard diffraction rates at HERA with rates at the Tevatron confirms the hypothesis embedded in the generic definition of diffraction that the gap probability must be normalized to unity, i.e. scaled to its integral over all available phase space for gap formation. (author)