[Show abstract][Hide abstract] ABSTRACT: Defining two- and three-jet event samples according to an e+e- jet clustering algorithm at a jet resolution scale ycut = y1 ≪ 1, we calculate from perturbative QCD the predicted average cluster (sub-jet) multiplicity in each sample as a function of the cluster resolution y0 < y1. The introduction of the cluster resolution scale y0 makes the comparison of multiplicities in jets less sensitive to hadronization corrections. Our calculation includes leading logarithms of y0 and y1 to order αS2 for the original JADE algorithm, and leading and next-to-leading logarithms to all orders for the recently introduced "k⊥" algorithm, for which we also present results based on the exact O(αS2) matrix elements. We find that the selection at scale y1 has a large effect on the ratio of cluster multiplicities, which consequently does not agree with naive expectations based on colour charge counting. According to local parton-hadron duality, the same is expected to be true of the ratio of hadron multiplicities. This could resolve a long-standing apparent disagreement between QCD and the experimental data.
Nuclear Physics B 09/1992; 383(3). DOI:10.1016/0550-3213(92)90084-O · 3.93 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Using the recently introduced “k⊥” jet clustering algorithm, we are able to compute the average number of hadronic jets produced in e+e− annihilation, as a function of both the centre-of-mass energy and the jet resolution parameter ycut. For large values of ycut, we give complete numerical results to order αS2, using the results of Bethke et al. on multi-jet fractions to this order. For small values, we give the results of resumming leading and next-to-leading logarithms of ycut to all orders in QCD perturbation theory. Our predictions should be useful both for determining αS and for studying the transition from the perturbative to the hadronization regime at very small ycut.
Nuclear Physics B 06/1992; 377(3). DOI:10.1016/0550-3213(92)90296-N · 3.93 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We study in perturbative QCD the initial-state radiation associated to hadron processes in the semi-hard region of small x (x is the Bjorken variable). A recent analysis of the exclusive multi-gluon distributions to double (infrared and collinear) logarithmic accuracy is extended to the case of inclusive distributions, which we evaluate to single (infrared) logarithmic accuracy. Thus the resulting x → 0 structure function or N → 1 gluon anomalous dimension is computed to all-loops accuracy. For the inclusive distributions we are able to perform a calculation to such an accuracy by extensively using cancellations which originate from coherence of QCD radiation and the infrared regularity of real-virtual singularities. We find that the x → 0 structure function satisfies the Lipatov equation. With the present study we therefore provide a new derivation of the Lipatov result in the context of hard collisions together with a fully exclusive description. We discuss the structure of the Lipatov equation in relation with the x → 0 exclusive distributions previously obtained and with the Altarelli-Parisi equation valid for finite values of x.
Nuclear Physics B 09/1991; 361(3):645–672. DOI:10.1016/0550-3213(91)90598-R · 3.93 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Recent results on the small-χ region in perturbative QCD are reviewed. We discuss the structure of the multiparton distributions in the leading logarithmic order, including virtual corrections. The effects of QCD coherence and virtual corrections of non-Sudakov type on the structure of the final states are described. Numerical results of a Monte Carlo simulation at the parton level are presented.
[Show abstract][Hide abstract] ABSTRACT: We schematically describe a recent calculation to all-loops accuracy of the gluon structure function in the semi-hard region of small χ (χ is the Bjorken variable). We also recall the recent results about the associated radiation in this region.
[Show abstract][Hide abstract] ABSTRACT: We analyze in perturbative QCD the asymptotic behaviour of deep inelastic processes in the semi-hard region x→0. The study is done by extending the soft gluon insertion techniques. We confirm and extend the analysis recently performed by Ciafaloni. The main results are the following: (i) Soft gluon emission from the incoming parton takes place in a region where the angles between incoming and outgoing partons are ordered. This is due to coherent effects similar to the ones in the x→1 region. (ii) Virtual corrections involving an internal line with energy fraction x give rise, for x→0, to a new form factor of non-Sudakov type. This regularizes collinear singularities when an emitted gluon is parallel to the incoming parton. (iii) At the complete inclusive level, the new form factor plays the same role as the virtual corrections in the Lipatov equation for the Regge regime. We show that, in the semi-hard regime, the gluon anomalous dimension coincides with the Lipatov ansatz. (iv) We identify the branching structure of initial-state radiation including the semi-hard regime. The branching is formulated as a probability process given in terms of Sudakov and non-Sudakov form factors. This process, in principle, can be used to extend the existing simulations of QCD cascades to the semi-hard regime.
Nuclear Physics B 05/1990; 336(1-336):18-85. DOI:10.1016/0550-3213(90)90342-B · 3.93 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We describe a unified method to analyze the structure of QCD coherence in a hard process with incoming hadrons. In particular we discuss in detail the regimes of x→0 and x→1 where x is the Bjorken variable. The multiparton emission is formulated as a factorized branching process which can be used to extend the existing simulations of QCD cascades to the semi-hard regime.
Physics Letters B 01/1990; 234(3-234):339-345. DOI:10.1016/0370-2693(90)91938-8 · 6.13 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The infrared analysis of perturbative QCD starts from the formula of factorization of the soft-gluon radiation, which has the form of a semi-classical non-abelian current. This formula has been proved within the framework of the old-fashioned perturbation theory but no general proof is available in the more familiar framework of the Feynman-diagram technique. In this paper we present a proof of the factorization formula in Feynman-diagram perturbative amplitudes. This derivation provides further insight into the infrared structure of the theory and in particular into the role of coherence of colour charges. We also discuss the spin- and colour-averaged squared amplitude for the multi-gluon soft emission. We present a new derivation of the planar contribution and show that the first non-planar correction does not contain a leading singularity.
Nuclear Physics B 11/1988; 309(3):439-460. DOI:10.1016/0550-3213(88)90452-X · 3.93 Impact Factor