Diffraction and Low-Q^2 Physics Including Two-Photon Physics

Recent experimental results on the partonic structure of the photon and on the color singlet exchange in strong interaction processes are reviewed. At the LEP electron-positron and HERA lepton-proton colliders, complementary and consistent measurements have been achieved on the quark-gluon structure of quasi-real and virtual photons. At the HERA lepton-proton and Tevatron proton-antiproton colliders, the quark-gluon configuration of the diffractive exchange is consistently found to have a large gluon component. The rate of diffractive interactions observed by the HERA and Tevatron experiments, however, is largely different and challenges explanation (invited plenary talk at the XXIX International Conference on High Energy Physics, Vancouver, B.C. Canada (1998)).Comment: 36 pages (Latex), 35 figures (Postscript); in this replaced version the report on 'Structure Functions' by A.T.Doyle has been taken out and is available in hep-ex/981202

The proton and the photon, who is probing whom in electroproduction?

The latest results on the structure of the proton and the photon as seen at HERA are reviewed while discussing the question posed in the title of the talk.Comment: 18 pages, including 28 figures. To be published in the proceedings of the 2nd KEK-Tanashi International Symposium on Hadron and Nuclear Physics with Electromagnetic Probes, KEK-Tanashi, Tokyo, Japan, October 25-27, 1999. Typos in eqs (8) and (9) corrected and reference 5 update

Single Spin Asymmetries in Proton-Proton and Proton-Neutron Scattering at 820 GeV

The physics case is summarised for the investigation of high energy spin phenomena by placing an internal polarised target into HERA's unpolarised proton beam. The luminosity and experimental sensitivity are discussed. Estimating the physics reach of single spin asymmetries in different final states reveals a considerable physics potential in testing the spin sector of perturbative QCD.Comment: 10 pages + 5 uuencoded ps figures, late

Photon Structure

The structure of the photon is probed in photon-photon interactions at LEP and in photon-proton interactions at HERA.Comment: 10 pages, submitted to proceedings of DIS2000, Liverpool, UK, 25-30 April 200