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Particles and Fields

Eur. Phys. J. C 25, 169-187 (2002)
DOI: 10.1007/s10052-002-01003-1

Measurement of the Q2 and energy dependence of diffractive interactions at HERA

The ZEUS Collaboration

(Received: 27 March 2002 / Published online: 9 August 2002 )

Diffractive dissociation of virtual photons, $\gamma^* p \rightarrow X
p$ , has been studied in ep interactions with the ZEUS detector at HERA. The data cover photon virtualities 0.17 < Q2< 0.70 GeV 2 and 3 < Q2< 80  GeV 2 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 low- and high- Q2 samples used in the LPS-based analysis are $\simeq 0.9$ pb -1 and $\simeq 3.3$ pb -1, respectively. The sample used for the MX-based analysis corresponds to an integrated luminosity of $\simeq 6.2$ pb -1. The dependence of the diffractive cross section on W, the virtual photon-proton centre-of-mass energy, and on Q2 is studied. In the low- Q2 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- Q2 to the high- Q2 region. In the low- Q2 range, little Q2 dependence is found, a significantly different behaviour from the rapidly falling cross section measured for Q2 > 3 GeV 2. The ratio of the diffractive to the virtual photon-proton total cross section is studied as a function of W and Q2. Comparisons are made with a model based on perturbative QCD.

© Società Italiana di Fisica, Springer-Verlag 2002