125 research outputs found

    On large angle multiple gluon radiation

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    Jet shape observables which involve measurements restricted to a part of phase space are sensitive to multiplication of soft gluon with large relative angles and give rise to specific single logarithmically enhanced (SL) terms (non-global logs). We consider associated distributions in two variables which combine measurement of a jet shape V in the whole phase space (global) and that of the transverse energy flow away from the jet direction, Eout (non-global). We show that associated distributions factorize into the global distribution in V and a factor that takes into account SL contributions from multi-gluon ``hedgehog'' configurations in all orders. The latter is the same that describes the single-variable Eout distribution, but evaluated at a rescaled energy VQ.Comment: 16 page

    Symmetry of Anomalous Dimension Matrices for Colour Evolution of Hard Scattering Processes

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    In a recent paper, Dokshitzer and Marchesini rederived the anomalous dimension matrix for colour evolution of gggggg \to gg scattering, first derived by Kidonakis, Oderda and Sterman. They noted a weird symmetry that it possesses under interchange of internal (colour group) and external (scattering angle) degrees of freedom and speculated that this may be related to an embedding into a context that correlates internal and external variables such as string theory. In this short note, I point out another symmetry possessed by all the colour evolution anomalous dimension matrices calculated to date. It is more prosaic, but equally unexpected, and may also point to the fact that colour evolution might be understood in some deeper theoretical framework. To my knowledge it has not been pointed out elsewhere, or anticipated by any of the authors calculating these matrices. It is simply that, in a suitably chosen colour basis, they are complex symmetric matrices.Comment: 3 page

    Medium-modified average multiplicity and multiplicity fluctuations in jets

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    The energy evolution of average multiplicities and multiplicity fluctuations in jets produced in heavy-ion collisions is investigated from a toy QCD-inspired model. In this model, we use modified splitting functions accounting for medium-enhanced radiation of gluons by a fast parton which propagates through the quark gluon plasma. The leading contribution of the standard production of soft hadrons is enhanced by a factor Ns\sqrt{N_s} while next-to-leading order (NLO) corrections are suppressed by 1/Ns1/\sqrt{N_s}, where the parameter Ns>1N_s>1 accounts for the induced-soft gluons in the medium. Our results for such global observables are cross-checked and compared with their limits in the vacuum.Comment: 8 pages and 4 figures. Version to be published in EPJ

    Near-to-planar 3-jet events in and beyond QCD perturbation theory

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    We present the results of QCD analysis of out-of-event-plane momentum distribution in 3-jet e+e- annihilation events. We consider the all-order resummed perturbative prediction and the leading power suppressed non-perturbative corrections to the mean value and the distribution and explain their non-trivial colour structure. The technique we develop aims at improving the accuracy of the theoretical description of multi-jet ensembles, in particular in hadron-hadron collisions.Comment: 11 pages, 2 figure

    Revisiting parton evolution and the large-x limit

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    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.Comment: 6 pages, v2 corrects misprints and contains an additional referenc

    Hadron multiplicity induced by top quark decays at the LHC

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    The average charged hadron multiplicities induced by top quark decays are calculated in pQCD at LHC energies. Different modes of top production are considered. Proposed measurements can be used as an additional test of pQCD calculations independent on a fragmentation model.Comment: 12 pages, 12 figures, to be published elsewher

    Universality of 1/Q corrections to jet-shape observables rescued

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    We address the problem of potential non-universality of the leading 1/Q power corrections to jet shapes emerging from the non-inclusive character of these observables. We consider the thrust distribution as an example and analyse the non-inclusive contributions which emerge at the two-loop level. Although formally subleading in \as, they modify the existing na{\"\i}ve one-loop result for the expected magnitude of the power term by a factor of order unity. Such a promotion of a subleading correction into a numerical factor is natural since the non-perturbative power terms are explicitly proportional to powers of the QCD scale Λ\Lambda which can be fixed precisely only at the two-loop level. The ``jet-shape scaling factor'' depends on the observable but remains perturbatively calculable. Therefore it does not undermine the universal nature of 1/Q power corrections, which remain expressible in terms of the universal running coupling and universal soft-gluon emission.Comment: 21 pages, no figures, LaTeX. This revised version corrects a mistake in the calculation of the two-loop correction factor. The conclusions remain unchange

    Heavy quark colorimetry of QCD matter

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    We consider propagation of heavy quarks in QCD matter. Because of large quark mass, the radiative quark energy loss appears to be qualitatively different from that of light quarks at all energies of practical importance. Finite quark mass effects lead to an in-medium enhancement of the heavy-to-light D/\pi ratio at moderately large (5--10 GeV) transverse momenta. For hot QCD matter a large enhancement is expected, whose magnitude and shape are exponentially sensitive to the density of colour charges in the medium.Comment: 15 pages, 4 figures, LaTe

    Multiplicity with a Thrust Cut

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    We evaluate the multiplicity of hadrons in the e+ee^+e^--annihilation at a given thrust TT in the modified leading-log approximation, including O(αs)O(\sqrt{\alpha_s}) corrections. The calculation is done at a large value of τ=1T\tau =1-T by the use of the factorisation which takes place in the one-particle-inclusive cross section at a given τ\tau. At a small τ\tau, a different type of factorisation takes place, which also enable us to evaluate the multiplicity. Two approaches are compared numerically. Measuring this quantity near τ=1/3\tau =1/3, we can determine the multiplicity ratio between a gluon-jet and a quark-jet.Comment: OCHA-PP-32, LATEX FILE, 21
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