Introduction

Abstract

I give a brief introduction to diffraction in lepton-hadron and hadron-hadron collisions, the subject of this special issue of the International Journal of Modern Physics A.The theme of this collection of papers is diffraction in lepton-hadron and hadronhadron collisions in view of the new results at the LHC. These reactions are very different from typical inelastic pp reactions at colliders, where the hadrons are completely destroyed after the interaction and one observes only part of the proton remnants at the edge of the detector.Historically, pp → pp elastic interactions where both protons remain intact were first measured. They represent for instance about 20% of produced events at LHC energies. In addition, hard diffraction where both protons are intact in the final state and where a hard scale given by the presence of jets, vector mesons, W/Z bosons . . . , was observed in the UA8 experiment at CERN, and then in the H1 and ZEUS experiments at HERA and in the D0 and CDF experiments at the Tevatron. Such phenomena amount about 10% of the events at HERA and 1% at the Tevatron.Different methods have been developed experimentally to measure this kind of events, namely the existence of a rapidity gap in the detector devoid of any activity in the direction of the outgoing hadron or the detection of the intact scattered hadron in dedicated detectors (usually roman pots). These events can be explained by the exchange of a colorless object, the pomeron.Many different results have been obtained at previous colliders before the LHC. In this report, we will start by a review of the results at HERA 1 which led to the determination of the hard pomeron structure in terms of quarks and gluons. The results from the Tevatron D0 and CDF experiments and especially the discussion of factorization breaking between HERA and Tevatron and the measurement of the total cross-section are given in Refs. 2 and 3.1502002-1 C. RoyonThe description of the first LHC results start with the measurement of the total and elastic cross-section as well as the theoretical developments in soft diffraction. 4-6 The description of the most recent measurements from the ATLAS (ALFA) and TOTEM collaborations are given in Ref. 7. At the LHC, it is possible to measure the structure of the pomeron in terms of quark and gluon densities and compare the results to HERA in order to study further if the same object (the pomeron) is exchanged at lepton-hadron and hadron-hadron colliders, 8 including some nonperturbative effects such as the survival probability. 9 The most recent results at the LHC from the ATLAS and CMS experiments are given in Refs. 10 and 11. The results and prospects in the heavy ion mode are described in Ref. 12. A very special category of events exists, namely the exclusive diffractive events. In this kind of events at the LHC, both protons are intact in the final state and the full energy is used to produce the final state object such as jets, W/Z bosons, the Higgs boson . . . In other words there is no energy loss in the pomeron remnants....