Abstract
Diffractive dissociation of virtual photons, gamma*p --> Xp, has beer, studied ill ep interactions with the ZEUS detector at HERA. The data cover photon virtualities 0.17 < Q(2) < 0.70 GeV and 3 < Q(2) < 80 GeV2 with 3 < M-X < 38 GeV, where M-X 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 M-X. The integrated luminosities of the low- and high-Q(2) samples used in the LPS-based analysis are similar or equal to 0.9 pb(-1) and similar or equal to 3.3 pb(-1), respectively. The sample used for the M-X-based analysis corresponds to an integrated luminosity of similar or equal to 6.2 pb(-1). The dependence of the diffractive cross section on W, the virtual photon-proton centre-of-mass energy. and on Q(2) is studied. In the low-Q(2) 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(2) to the high-Q(2) region. In the low-Q range, little Q(2) dependence is found, a significantly different behaviour from the rapidly falling cross section measured for Q(2) > 3 GeV2. The ratio of the diffractive to the virtual photon-proton total cross sect ion is studied as a function of W and Q(2). Comparisons are made with a model based oil perturbative QCD.