Review of high energy diffraction in real and virtual photon–proton scattering at HERA

The electron–proton collider HERA at DESY opened the door for the study of diffraction in real and virtual photon–proton scattering at centre-of-mass energies W up to 250 GeV and for large negative mass squared − Q 2 of the virtual photon up to Q 2 = 1600 GeV 2 . At W = 220 GeV and Q 2 = 4 GeV 2 , diffraction accounts for about 15% of the total virtual photon–proton cross section, decreasing to ≈5% at Q 2 = 200 GeV 2 . An overview of the results obtained by the experiments H1 and ZEUS on the production of neutral vector mesons and on inclusive diffraction up to the year 2008 is presented.