The medium-induced p ⊥ -broadening and induced gluon radiation spectrum of a high energy quark or gluon traversing a large nucleus is studied. Multiple scattering of the high energy parton in the nucleus is treated in the Glauber approximation. We show that −dE/dz, the radiative energy loss of the parton per unit length, grows as L, the length of the nuclear matter, as does the characteristic transverse momentum squared of the parton p 2 ⊥W . We find −dE/dz = 1 8 α s N c p 2 ⊥W holds independent of the details of the parton-nucleon scatterings so long as L is large. Numerical estimates suggest that p ⊥ -broadening and energy loss may be significantly enhanced in hot matter as compared to cold matter, thus making the study of such quantities a possible signal for quark-gluon plasma formation.

The medium induced energy loss spectrum of a high energy quark or gluon traversing a hot QCD medium of finite volume is studied. We model the interaction by a simple picture of static scattering centres. The total induced energy loss is found to grow as L 2 , where L is the extent of the medium. The solution of the energy loss problem is reduced to the solution of a Schrödingerlike equation whose "potential" is given by the single-scattering cross section of the high energy parton in the medium. These results should be directly applicable to a quark-gluon plasma.

We propose a version of the QCD-motivated`k ? ' jet-clustering algorithm for hadron-hadron collisions which i s i n v ariant under boosts along the beam directions. This leads to improved factorization properties and closer correspondence to experimental practice at hadron colliders. We examine alternative denitions of the resolution variables and cluster recombination scheme, and show that the algorithm can be implemented eciently on a computer to provide a full clustering history of each e v ent. Using simulated data at p s = 1 : 8 T eV, we study the eects of calorimeter segmentation, hadronization and the soft underlying event, and compare the results with those obtained using a conventional conetype algorithm.

We investigate modifications to the k ⊥ -clustering jet algorithm which preserve the advantages of the original Durham algorithm while reducing non-perturbative corrections and providing better resolution of jet substructure. We find that a simple change in the sequence of clustering (combining smaller-angle pairs first), together with the 'freezing' of soft resolved jets, has beneficial effects.

We consider power-behaved contributions to hard processes in QCD arising from non-perturbative effects at low scales which can be described by introducing the notion of an infrared-finite effective coupling. Our method is based on a dispersive treatment which embodies running coupling effects in all orders. The resulting power behaviour is consistent with expectations based on the operator product expansion, but our approach is more widely applicable. The dispersively-generated power contributions to different observables are given by (log-)moment integrals of a universal low-scale effective coupling, with process-dependent powers and coefficients. We analyse a wide variety of quark-dominated processes and observables, and show how the power contributions are specified in lowest order by the behaviour of one-loop Feynman diagrams containing a gluon of small virtual mass. We discuss both collinear safe observables (such as the e + e − total cross section and τ hadronic width, DIS sum rules, e + e − event shape variables and the Drell-Yan K-factor) and collinear divergent quantities (such as DIS structure functions, e + e − fragmentation functions and the Drell-Yan cross section).

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We determine how the yield of large transverse momentum hadrons is modified due to induced gluon radiation off a hard parton traversing a QCD medium. The quenching factor is formally a collinear-and infrared-safe quantity and can be treated perturbatively. In spite of that, in the p ⊥ region of practical interest, its value turns out to be extremely sensitive to large distances and can be used to unravel the properties of dense quark-gluon final states produced in heavy ion collisions. We also find that the standard modelling of quenching by shifting p ⊥ in the hard parton cross section by the mean energy loss is inadequate.

We study analytically the medium-induced energy loss of a high energy parton passing through a finite size QCD plasma, which is expanding longitudinally according to Bjorken's model. We extend the BDMPS formalism already applied to static media to the case of a quark which hits successive layers of matter of decreasing temperature, and we show that the resulting radiative * Permanent address: Petersburg Nuclear Physics Institute, Gatchina 188350, St. Petersburg, Russia † Supported in part by the U.S. Department of Energy under Grant DE-FG02-94ER-40819 ‡ Laboratoire associé du Centre National de la Recherche Scientifique 1 energy loss can be as large as 6 times the corresponding one in a static plasma at the reference temperature T = T (L), which is reached after the quark propagates a distance L.