Comparison of deep inelastic scattering with photoproduction interactions at HERA

Photon-proton (γp) interactions with Q2 < 10−2 GeV2 and deep-inelastic scattering (γ∗p) interactions with photon virtualities Q2 > 5 GeV2 are studied at the high energy electron-proton collider HERA. The transverse energy flow and relative rates of large rapidity gap events are compared in the two event samples. The observed similarity between γp and γ∗p interactions can be understood in a picture where the photon develops as a hadronic object. The transverse energy density measured in the central region of the collision, at η∗ = 0 in the γ∗p centre of mass frame, is compared with data from hadron-hadron interactions as function of the CMS energy of the collision.