210 research outputs found

### Soft gluons at large angles in hadron collisions

A general discussion is presented of the single logarithmic soft factor that
appears in two scale QCD observables in processes involving four partons. We
treat it as the ``fifth form factor'', accompanying the four collinear singular
Sudakov form factors attached to colliding and outgoing hard partons. The fifth
form factor is expressed in terms of the Casimir operators (squared colour
charges) of irreducible representations in the crossing $t$- and $u$-channels.
As an application we revisit the problem of large angle radiation in $gg\to gg$
and give a relatively simple solution and interpretation of the results. We
found an unexpected symmetry of the soft anomalous dimension under exchange of
internal and external variables of the problem whose existence calls for
explanation.Comment: 29 pages, pictures generated with feynmf graphic packag

### Local charge compensation from colour preconfinement as a key to the dynamics of hadronization

If, as is commonly accepted, the colour-singlet, `preconfined', perturbative
clusters are the primary units of hadronization, then the electric charge is
necessarily compensated locally at the scale of the typical cluster mass. As a
result, the minijet electric charge is suppressed at scales that are greater
than the cluster mass. We hence argue, and demonstrate by means of Monte Carlo
simulations using HERWIG, that the scale at which charge compensation is
violated is close to the mass of the clusters involved in hadronization, and
its measurement would provide a clue to resolving the nature of the dynamics.
We repeat the calculation using PYTHIA and find that the numbers produced by
the two generators are similar. The cluster mass distribution is sensitive to
soft emission that is considered unresolved in the parton shower phase. We
discuss how the description of the splitting of large clusters in terms of
unresolved emission modifies the algorithm of HERWIG, and relate the findings
to the yet unknown underlying nonperturbative mechanism. In particular, we
propose a form of $\alpha_S$ that follows from a power-enhanced beta function,
and discuss how this $\alpha_S$ that governs unresolved emission may be related
to power corrections. Our findings are in agreement with experimental data.Comment: 37 pages, 20 figure

### Systematic study of the jet fragmentation function for inclusive jet-production in p+p collisions at sqrt{s}=200 GeV in STAR

Jet fragmentation functions measured in e^+e^- and p+\bar{p} experiments are
well-described on an inclusive hadron level by QCD-based calculations.
Fragmentation is expected to be modified by the presence of a strongly
interacting medium, but full theoretical description of this modification must
still be developed. It has recently been suggested that particle-identified
fragmentation functions may provide additional insight into the processes
underlying jet quenching. To assess the applicability of QCD-based
fragmentation calculations to RHIC data, and to provide a baseline with which
to compare fragmentation function measurements in heavy ion collisions, we
present the first measurements of charged hadron and particle-identified
fragmentation functions of jets reconstructed via a midpoint-cone algorithm
from p+p collisions at 200 GeV in STAR. We study the dependence on jet
cone-size and jet-energy, and compare the results to PYTHIA simulations based
on the Modified Leading Log Approximation (MLLA).Comment: 6 pages, 5 figures, proceedings of Hard Probes 2008 conferenc

### 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

### Gluon Distribution Functions in the kT-factorization Approach

At small x, the effects of finite transverse momenta of partons inside a
hadron become increasingly important, especially in analyses of jets and
heavy-quark production. These effects can be systematically accounted for in a
formalism based on kT-factorization and unintegrated distribution functions. We
present results for the unintegrated distribution function, together with the
corresponding integrated one, obtained within the framework of the Linked
Dipole Chain model. Comparisons are made to results obtained within other
approaches

### Azimuthal correlation in DIS

We introduce the azimuthal correlation for the deep inelastic scattering
process. We present the QCD prediction to the level of next-to-leading log
resummation, matching to the fixed order prediction. We also estimate the
leading non-perturbative power correction. The observable is compared with the
energy-energy correlation in e+e- annihilation, on which it is modelled. The
effects of the resummation and of the leading power correction are both quite
large. It would therefore be particularly instructive to study this observable
experimentally.Comment: 33 pages, 4 figures, JHEP class included. One figure and some
clarifications adde

### Single Inclusive Distribution and Two-Particle Correlations Inside One Jet at "Modified Leading Logarithmic Approximation" of Quantum Chromodynamics II : Steepest Descent Evaluation at Small X

The MLLA single inclusive distribution inside one high energy (gluon) jet at
small x is estimated by the steepest descent method. Its analytical expression
is obtained outside the "limiting spectrum". It is then used to evaluate
2-particle correlations at the same level of generality. The dependence of both
observables on the ratio between the infrared cutoff Q\_0 and Lambda\_QCD is
studied. Fong & Webber's results for correlations are recovered at the limits
when this ratio goes to 1 and when one stays close to the peak of the single
inclusive distribution.Comment: LaTeX, 22 pages, 18 .eps figure

### Scaling Rule for Nonperturbative Radiation in a Class of Event Shapes

We discuss nonperturbative radiation for a recently introduced class of
infrared safe event shape weights, which describe the narrow-jet limit.
Starting from next-to-leading logarithmic (NLL) resummation, we derive an
approximate scaling rule that relates the nonperturbative shape functions for
these weights to the shape function for the thrust. We argue that the scaling
reflects the boost invariance implicit in NLL resummation, and discuss its
limitations. In the absence of data analysis for the new event shapes, we
compare these predictions to the output of the event generator PYTHIA.Comment: 23 pages, 3 figures, uses JHEP3.cls (included); v2 - version to
appear in JHE

### Color evolution of 2 -> 3 processes

The color structure needed for resummation of all colored 2 -> 3 processes is
calculated using multiplet inspired s-channel bases. In this way the resulting
matrices, describing the color structure, are guaranteed to obey simplifying
symmetries.Comment: 25 page

### Color structure for soft gluon resummation - a general recipe

A strategy for calculating the color structure needed for soft gluon
resummation for processes with any number of colored partons is introduced
using a N_c --> infinity inspired basis. In this basis a general formalism can
be found at the same time as the calculations are simplified.
The advantages are illustrated by recalculating the soft anomalous dimension
matrix for the processes gg --> gg, q\qbar --> q \qbar g and q\qbar --> ggg.Comment: 16 page

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