79 research outputs found
Enhanced Nonperturbative Effects in Z Decays to Hadrons
We use soft collinear effective field theory (SCET) to study nonperturbative
strong interaction effects in Z decays to hadronic final states that are
enhanced in corners of phase space. These occur, for example, in the jet energy
distribution for two jet events near E_J=M_Z/2, the thrust distribution near
unity and the jet invariant mass distribution near zero. The extent to which
such nonperturbative effects for different observables are related is
discussed.Comment: 17 pages. Paper reorganized, and more discussion and results include
Semi-numerical resummation of event shapes
For many event-shape observables, the most difficult part of a resummation in
the Born limit is the analytical treatment of the observable's dependence on
multiple emissions, which is required at single logarithmic accuracy. We
present a general numerical method, suitable for a large class of event shapes,
which allows the resummation specifically of these single logarithms. It is
applied to the case of the thrust major and the oblateness, which have so far
defied analytical resummation and to the two-jet rate in the Durham algorithm,
for which only a subset of the single logs had up to now been calculated.Comment: 29 pages, 7 figures. Version 2 adds some clarifications, a reference,
as well as corrections to the subleading fixed-order coefficients and to
figures 4 and
The C parameter distribution in e+e- annihilation
We study perturbative and non-perturbative aspects of the distribution of the
C parameter in e+e- annihilation using renormalon techniques. We perform an
exact calculation of the characteristic function, corresponding to the C
parameter differential cross section for a single off-shell gluon. We then
concentrate on the two-jet region, derive the Borel representation of the
Sudakov exponent in the large-beta_0 limit and compare the result to that of
the thrust T. Analysing the exponent, we distinguish two ingredients: the jet
function, depending on Q^2C, summarizing the effects of collinear radiation,
and a function describing soft emission at large angles, with momenta of order
QC. The former is the same as for the thrust upon scaling C by 1/6, whereas the
latter is different. We verify that the rescaled C distribution coincides with
that of 1-T to next-to-leading logarithmic accuracy, as predicted by Catani and
Webber, and demonstrate that this relation breaks down beyond this order owing
to soft radiation at large angles. The pattern of power corrections is also
similar to that of the thrust: corrections appear as odd powers of Lambda/(QC).
Based on the size of the renormalon ambiguity, however, the shape function is
different: subleading power corrections for the C distribution appear to be
significantly smaller than those for the thrust.Comment: 24 pages, Latex (using JHEP3.cls), 1 postscript figur
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
Vector boson production at hadron colliders: hard-collinear coefficients at the NNLO
We consider QCD radiative corrections to vector-boson production in hadron
collisions. We present the next-to-next-to-leading order (NNLO) result of the
hard-collinear coefficient function for the all-order resummation of
logarithmically-enhanced contributions at small transverse momenta. The
coefficient function controls NNLO contributions in resummed calculations at
full next-to-next-to-leading logarithmic accuracy. The same coefficient
function is used in applications of the subtraction method to perform
fully-exclusive perturbative calculations up to NNLO.Comment: 13, pages, no figures. arXiv admin note: text overlap with
arXiv:1106.465
Correcting the Colour-Dipole Cascade Model with Fixed Order Matrix Elements
An algorithm is presented in which the Colour-Dipole Cascade Model as
implemented in the Ariadne program is corrected to match the fixed order
tree-level matrix elements for e+e- -> n jets. The result is a full parton
level generator for e+e- annihilation where the generated states are correct on
tree-level to fixed order in alpha_S and to all orders with modified leading
logarithmic (MLLA) accuracy. In this paper, matrix elements are used up to
second order in alpha_S, but the scheme is applicable also for higher orders.
An improvement to also include exact virtual corrections to fixed order is
suggested and the possibility to extend the scheme to hadronic collisions is
discussed
Resummed event-shape variables in DIS
We complete our study of resummed event-shape distributions in DIS by
presenting results for the class of observables that includes the current jet
mass, the C-parameter and the thrust with respect to the current-hemisphere
thrust axis. We then compare our results to data for all observables for which
data exist, fitting for alpha_s and testing the universality of
non-perturbative 1/Q effects. A number of technical issues arise, including the
extension of the concept of non-globalness to the case of discontinuous
globalness; singularities and non-convergence of distributions other than in
the Born limit; methods to speed up fixed-order Monte Carlo programs by up to
an order of magnitude, relevant when dealing with many x and Q points; and the
estimation of uncertainties on the predictions.Comment: 41 page
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 that follows from a power-enhanced beta function,
and discuss how this that governs unresolved emission may be related
to power corrections. Our findings are in agreement with experimental data.Comment: 37 pages, 20 figure
Matching parton showers to NLO computations
We give a prescription for attaching parton showers to next-to-leading order
(NLO) partonic jet cross sections in electron-positron annihilation. Our method
effectively extends to NLO the scheme of Catani, Krauss, Kuhn, and Webber for
matching between m hard jets and (m+1) hard jets. The matching between parton
splitting as part of a shower and parton splitting as part of NLO matrix
elements is based on the Catani-Seymour dipole subtraction method that is
commonly used for removing the singularities from the NLO matrix elements.}Comment: 45 pages, new introduction, more detailed discussion of the Sudakov
reweightin
Charm-quark fragmentation with an effective coupling constant
We use a recently proposed non-perturbative model, based on an effective
strong coupling constant and free from tunable parameters, to study c-flavoured
hadron production in e+e- annihilation. Charm-quark production is described in
the framework of perturbative fragmentation functions, with NLO coefficient
functions, NLL non-singlet DGLAP evolution and NNLL large-x resummation. We
model hadronization effects by means of the effective coupling constant in the
NNLO approximation and compare our results with experimental data taken at the
Z0 pole and at the Upsilon(4S) resonance. We find that, within the experimental
and theoretical uncertainties, our model is able to give a reasonable
description of D*+-meson spectra from ALEPH for x<1-Lambda/m_c. More serious
discrepancies are instead present when comparing with D and D^* data from BELLE
and CLEO in x-space. Within the errors, our model is nonetheless capable of
reproducing the first ten Mellin moments of all considered data sets. However,
the fairly large theoretical uncertainties call for a full NNLO/NNLL analysis.Comment: 26 pages, 10 figures. Analysis in Mellin space and few references
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