Quantum Chromodynamics at Colliders

QCD is the accepted (that is, the effective) theory of the strong interaction; studies at colliders are no longer designed to establish this. Such studies can now be divided into two categories. The first involves the identification of observables which can be both measured and predicted at the level of a few percent. Such studies parallel those of the electroweak sector over the past fifteen years, and deviations from expectations would be a sign of new physics. These observables provide a firm " place to stand " from which to extend our understanding. This links to the second category of study, where one deliberately moves to regions in which the usual theoretical tools fail; here new approximations in QCD are developed to increase our portfolio of understood processes, and hence our sensitivity to new physics. Recent progress in both these aspects of QCD at colliders is discussed. 1 The Data and the Experiments QCD studies at colliders involve measurements of the hadronic final state in e + e − , lepton-hadron and hadron-hadron collisions. The lepton colliders also allow the study of effective photon-photon, lepton-photon and photon-hadron collisions, due to the almost-on-shell photon beam which accompanies lep-ton beams. In collisions involving these pho-tons, the photon may participate directly in the hard process, or it may act as a source of partons much like a hadron. Together, this array of different colliding beams provides us with many data and rich opportunities to learn from cross-comparison between experiments. Data presented at this meeting include precise measurements of a great number of properties of the final state, and these measurement are used to demonstrate and improve our understanding of the physics. With the confidence that this is understood, it then becomes possible to infer, from an increasing number of measurements, information about the initial state; that is, quarks and gluons in their natural habitat inside hadrons. This in turn enables us to predict effects at future colliders, particularly the Large Hadron Col-lider under construction at CERN 1. In sections 2-5, the final state measurements are discussed. In the subsequent section, some experimental advances in the current knowledge of parton densities within the Dokshitzer-Gribov-Lipatov-Altarelli-Parisi (DGLAP) paradigm are presented. Following that, some measurements in regions of phase space where DGLAP evolution is not applicable are discussed. This includes low x and diffractive effects, at which point I conclude this contribution and hand over to the …