Introduction

In the H1 Diffraction Physics Working Group, ep collisions exhibiting a large phase space area devoid of particle production in the region of the outgoing proton ('rapidity gap events') are studied. This is unlike "normal" ep interactions, where the proton remnant and the struck quark are thought to be connected by QCD colour fields, resulting in a relatively homogeneous production of hadrons. Regge theory has since long been able to describe the cross section for diffractive dissociation as a function of centre-of-mass energy (W) and squared momentum transfer (t) using the concept of reggeon and pomeron exchanges. The elastically scattered proton in Diffractive events can also be directly measured using "roman pot" detectors (FPS) very close to the proton beam far downstream of the interaction point. A new detector - the VFPS - has been installed in 2003.

In addition to the study of rapidity gap events, we measure cross sections for events with high energy forward proton or neutron production ('leading baryons'), we study the exclusive production of light vector mesons and photons ('Deeply Virtual Compton Scattering - DVCS') - see here, and we search for the Odderon. Note that heavy vector meson production (J/Psi and Upsilon) is studied in the H1 Heavy Quark Group.

The novelty at HERA lies in the fact that these processes can be studied as function of several hard scales like photon virtuality(Q²), squared momentum transfer (t) or quark mass, thus enabling perturbative QCD calculations for (at least part of) the interaction mechanism. The challenge in modern research of diffractive phenomena is therefore to reconcile the concepts of Regge theory with a microscopic description in terms of QCD.