x / 02 03 03 9 v 2 2 3 Ju n 20 02 Measurement of the Q 2 and energy dependence of diffractive interactions at HERA ZEUS Collaboration DESY - 02 - 029 March 2002

Diffractive dissociation of virtual photons, γp → Xp, has been studied in ep interactions with the ZEUS detector at HERA. The data cover photon virtualities 0.17 < Q < 0.70 GeV and 3 < Q < 80 GeV with 3 < MX < 38 GeV, where MX is the mass of the hadronic final state. Diffractive events were selected by two methods: the first required the detection of the scattered proton in the ZEUS leading proton spectrometer (LPS); the second was based on the distribution of MX . The integrated luminosities of the lowand high-Q samples used in the LPS-based analysis are ≃ 0.9 pb and ≃ 3.3 pb, respectively. The sample used for theMX -based analysis corresponds to an integrated luminosity of ≃ 6.2 pb. The dependence of the diffractive cross section on W , the virtual photon-proton centre-of-mass energy, and on Q is studied. In the low-Q range, the energy dependence is compatible with Regge theory and is used to determine the intercept of the Pomeron trajectory. The W dependence of the diffractive cross section exhibits no significant change from the low-Q to the high-Q region. In the low-Q range, little Q dependence is found, a significantly different behaviour from the rapidly falling cross section measured for Q > 3 GeV. The ratio of the diffractive to the virtual photon-proton total cross section is studied as a function of W and Q. Comparisons are made with a model based on perturbative QCD.