125 research outputs found

### On large angle multiple gluon radiation

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

In a recent paper, Dokshitzer and Marchesini rederived the anomalous
dimension matrix for colour evolution of $gg \to gg$ 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

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 $\sqrt{N_s}$ while
next-to-leading order (NLO) corrections are suppressed by $1/\sqrt{N_s}$, where
the parameter $N_s>1$ 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

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

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

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

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 $\Lambda$ 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

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

We evaluate the multiplicity of hadrons in the $e^+e^-$-annihilation at a
given thrust $T$ in the modified leading-log approximation, including
$O(\sqrt{\alpha_s})$ corrections. The calculation is done at a large value of
$\tau =1-T$ by the use of the factorisation which takes place in the
one-particle-inclusive cross section at a given $\tau$. At a small $\tau$, a
different type of factorisation takes place, which also enable us to evaluate
the multiplicity. Two approaches are compared numerically. Measuring this
quantity near $\tau =1/3$, 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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