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 using an integrated luminosity of ∼10 pb. 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; the second was based on the distribution of MX . 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.