2,403 research outputs found
Principles of general final-state resummation and automated implementation
Next-to-leading logarithmic final-state resummed predictions have
traditionally been calculated, manually, separately for each observable. In
this article we derive NLL resummed results for generic observables. We
highlight and discuss the conditions that the observable should satisfy for the
approach to be valid, in particular continuous globalness and recursive
infrared and collinear safety. The resulting resummation formula is expressed
in terms of certain well-defined characteristics of the observable. We have
written a computer program, CAESAR, which, given a subroutine for an arbitrary
observable, determines those characteristics, enabling full automation of a
large class of final-state resummations, in a range of processes.Comment: 111 pages. 6 figures, JHEP class included. Section 1 contains a guide
to reading the article; results obtained with CAESAR are available at
http://qcd-caesar.org; v2 includes substantial new explanatory material
(expansion of section 2, new appendices D & E), additional references, and
corrects misprint
Higgs and Z-boson production with a jet veto
We derive first next-to-next-to-leading logarithmic resummations for jet-veto
efficiencies in Higgs and Z-boson production at hadron colliders. Matching with
next-to-next-to-leading order results allows us to provide a range of
phenomenological predictions for the LHC, including cross-section results,
detailed uncertainty estimates and comparisons to current widely-used tools.Comment: 6 pages and 2 figures, plus 8 pages and 2 figures of supplemental
material. v2 contains additional references and small textual change
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
Resummation
We review the work discussed and developed under the topic ``Resummation'' at
Working Group 2 ``Multijet final states and energy flow'', of the HERA-LHC
Workshop. We emphasise the role played by HERA observables in the development
of resummation tools via, for instance, the discovery and resummation of
non-global logarithms. We describe the event-shapes subsequently developed for
hadron colliders and present resummed predictions for the same using the
automated resummation program CAESAR. We also point to ongoing studies at HERA
which can be of benefit for future measurements at hadron colliders such as the
LHC, specifically dijet and angular spectra and the transverse momentum
of the Breit current hemisphere.Comment: 15 pages, 6 figures. Submitted to the HERA-LHC workshop proceeding
Non-global logarithms and jet algorithms in high-pT jet shapes
We consider jet-shape observables of the type proposed recently, where the
shapes of one or more high-pT jets, produced in a multi-jet event with definite
jet multiplicity, may be measured leaving other jets in the event unmeasured.
We point out the structure of the full next-to-leading logarithmic resummation
specifically including resummation of non-global logarithms in the leading-Nc
limit and emphasising their properties. We also point out differences between
jet algorithms in the context of soft gluon resummation for such observables.Comment: 22 pages, 4 figures. Title and a few words changed. Several typos
corrected. Version accepted by JHE
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Infrared safe definition of jet flavor.
It is common, in both theoretical and experimental studies, to separately discuss
quark and gluon jets. However, even at parton level, widely-used jet algorithms fail
to provide an infrared safe way of making this distinction. We examine the origin
of the problem, and propose a solution in terms of a new ‘flavour-kt’ algorithm.
As well as being of conceptual interest this can be a powerful tool when combining
fixed-order calculations with multi-jet resummations and parton showers. It also has
applications to studies of heavy-quark jets
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
Quark masses in Higgs production with a jet veto
We study the impact of finite mass effects due to top and bottom loops in the jet-veto distribution for Higgs production. We discuss the appearance of non-factorizing logarithms in the region p t,veto ≳ m b . We study their numerical impact and argue that these terms can be treated as a finite remainder. We therefore detail our prescription for resumming the jet-vetoed cross section and for assessing its uncertainty in the presence of finite mass effects. Resummation for the jet-veto, including mass effects, has been implemented in the public code JetVHeto
From Long to Short Distances in Perturbative QCD
Infrared safe differential cross sections, such as event shape distributions,
can be measured over wide kinematic ranges, from regions where fixed order
calculations are adequate to regions where nonperturbative dynamics dominate.
Such observables provide an ideal laboratory for the study of the transition
between weak and strong coupling in quantum field theory. This talk begins with
some of the fundamentals of the perturbative description of QCD and the basis
of resummation techniques, followed by a brief discussion of selected topics
from recent fixed-order and resummed calculations. It focuses on how resummed
perturbation theory has been used to deduce the structure of nonperturbative
corrections, and to provide a framework with which to address the transition
from short- to long-distance dynamics in QCD.Comment: 24 pages, eight eps figures. Based on talks presented at the
International Conference on Theoretical Physics, TH2002, Paris, UNESCO, July
22-27, 2002, and the 26th Johns Hopkins Workshop on Current Problems in
Particle Theory, Heidelberg, Aug. 1-3, 200
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