Comparison of Deep Inelastic Scattering with Photoproduction Interactions at HERA

Abstract Photon-proton ( γp ) interactions with Q 2 −2 GeV 2 and deep-inelastic scattering ( γ ∗ p ) interactions with photon virtualities Q 2 > 5 GeV 2 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.