Yu.L. Dokshitzer

Longitudinally-invariant k⊥-clustering algorithms for hadron-hadron collisions☆

Abstract We propose a version of the QCD-motivated “k⊥” jet-clustering algorithm for hadron-hadron collisions which is invariant under boosts along the beam directions. This leads to improved factorization properties and closer correspondence to experimental practice at hadron colliders. We examine alternative definitions of the resolution variables and cluster recombination scheme, and show that the algorithm can be implemented efficiently on a computer to provide a full clustering history of each event. Using simulated data at √S = 1.8 TeV, we study the effects of calorimeter segmentation, hadronization and the soft underlying event, and compare the results with those obtained using a conventional cone-type algorithm.

Radiative energy loss and p⊥-broadening of high energy partons in nuclei

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 p2⊥w. We find −dE/dz = 18αsNcp2⊥w holds independent of the details of the parton-nucleon scatterings so long as L is large and so long as perturbative QCD applies at short distances. 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.

New clustering algorithm for multijet cross sections in e+e− annihilation☆

Abstract Cross sections for e + e − → n - jets , as functions of the jet resolution parameter y cut , are computed according to a new clustering algorithm. The jet multiplicity n is defined in such a way that jets i and j with energies E i and E j at relative angle angle θ ij are resolved if y ij = 2(1− cos θ ij ) min (E i 2 , E j 2 ) s>y cut , where s is the centre-of-mass energy squared. Using this algorithm, large higher-order corrections at small values of y cut can easily be evaluated. Our calculations include resummation of leading and next-to-leading logarithms of y cut to all orders in QCD perturbation theory. This enables us to predict the jet cross sections at small y cut for arbitrary n . Simple analytical results for n ⩽5 are presented.

THE STRING EFFECT AND QCD COHERENCE

Abstract In the framework of the idea of local parton-hadron duality we discuss the asymptotic predictions of QCD perturbation theory for angular distributions of hadron flows in three-jet events, e + e - → q q g → hadrons. The coherence of soft gluon emission provides the QCD explanation of the string effect observed in experiments.

Similarity of parton and hadron spectra in QCD jets

A possible existence of the local correspondence between parton and hadron distributions in hard processes (local parton-hadron duality) is discussed. First comparison is made to check up local duality at modern energies. It is shown, in particular, that an account of essential single-logarithmic corrections to leading double-logarithmic approximation proves sufficient to fit π± spectra ine+e− annihilation atW≳20GeV by theoretical perturbative spectra of QCD partons. Rapidity spectra versus the normalized rapidity appear to be similar for π,K, p. Further tests of local duality as well as effects of chromodynamical coherence are suggested.

The k⊥-clustering algorithm for jets in deep inelastic scattering and hadron collisions

Abstract We propose a new QCD-motivated clustering algorithm to define jets in lepton-hadron and hadron-hadron collisions. It combines the k ⊥ algorithm, proposed earlier for e + e − annihilation, with a pre-clustering procedure that ensures the universal factorization of initial-state collinear singularities.

Revisiting parton evolution and the large-x limit

Abstract This remark is part of an ongoing project to simplify the structure of the multi-loop anomalous dimensions for parton distributions and fragmentation functions. It answers the call for a “structural explanation” of a “very suggestive” relation found by Moch, Vermaseren and Vogt in the context of the x → 1 behaviour of three-loop DIS anomalous dimensions. It also highlights further structure that remains to be fully explained.

On the Transverse Momentum Distribution of Massive Lepton Pairs

Abstract The experimentally measured q t distributions of massive lepton pairs produced in hadron collisions are compared with the predictions of QCD [1]. We demonstrate that a fair agreement may be achieved without the use of large transverse momenta of the constitunt quarks - owing to a specific double-logarithmic form factor which manifests itself in this process. A prediction for the q t distribution in weak intermediate boson production is also given.

Specific features of heavy quark production: Local parton-hadron duality approach to heavy particle spectra.

A perturbative QCD formula for inclusive energy spectra of heavy quarks from heavy quark initiated jets, which takes into account collinear and/or soft logarithms in all orders, the exact first order result, and two-loop effects, is applied to distributions of heavy flavored hadrons in the framework of the local parton-hadron duality concept. {copyright} {ital 1995 The American Physical Society.}

Hard semi-inclusive processes in QCD

Abstract We present formulae resulting from a complete perturbative analysis of differential cross sections of (i) massive lepton pair production in hadron collisions, (ii) leptoproduction of a particle from the current fragmentation region, (iii) production of two particles from opposite jets in e+e− annihilation into hadrons. A new effective double logarithmic quark form factor is derived, which manifests itself in these processes. Experimental investigation of two-particle angle correlations in e+e− annihilation provides a possibility to measure directly this form factor and thus to make a nice test of QCD.