59 research outputs found
NNLL soft and Coulomb resummation for squark and gluino production at the LHC
We present predictions for the total cross sections for pair production of
squarks and gluinos at the LHC including a combined NNLL resummation of soft
and Coulomb gluon effects. We derive all terms in the NNLO cross section that
are enhanced near the production threshold, which include contributions from
spin-dependent potentials and so-called annihilation corrections. The NNLL
corrections at TeV range from up to for squark-squark
production to for gluino pair production relative to the NLO results and
reduce the theoretical uncertainties of the perturbative calculation to the
level. Grid files with our numerical results are publicly available.Comment: 42 pages, 17 figures. v2: published version; corrected fig. 6;
generalized eq.(A.5) to arbitrary SU(N) gauge group
Higgs effects in top anti-top production near threshold in e+ e- annihilation
The completion of the third-order QCD corrections to the inclusive top-pair
production cross section near threshold demonstrates that the strong dynamics
is under control at the few percent level. In this paper we consider the
effects of the Higgs boson on the cross section and, for the first time,
combine the third-order QCD result with the third-order P-wave, the leading QED
and the leading non-resonant contributions. We study the size of the different
effects and investigate the sensitivity of the cross section to variations of
the top-quark Yukawa coupling due to possible new physics effects.Comment: LaTeX, 17 page
Analytic structure in the coupling constant plane in perturbative QCD
We investigate the analytic structure of the Borel-summed perturbative QCD
amplitudes in the complex plane of the coupling constant. Using the method of
inverse Mellin transform, we show that the prescription dependent Borel-Laplace
integral can be cast, under some conditions, into the form of a dispersion
relation in the a-plane. We also discuss some recent works relating resummation
prescriptions, renormalons and nonperturbative effects, and show that a method
proposed recently for obtaining QCD nonperturbative condensates from
perturbation theory is based on special assumptions about the analytic
structure in the coupling plane that are not valid in QCD.Comment: 14 pages, revtex4, 1 eps-figur
Leptonic decay of the Upsilon(1S) meson at third order in QCD
We present the complete next-to-next-to-next-to-leading order short-distance
and bound-state QCD correction to the leptonic decay rate
Gamma(Upsilon(1S)->l+l-) of the lowest-lying spin-1 bottomonium state. The
perturbative QCD prediction is compared to the measurement
Gamma(Upsilon(1S)->e+e-)=1.340(18) keV.Comment: 4 pages, 2 figure
Convergence of the expansion of the Laplace-Borel integral in perturbative QCD improved by conformal mapping
The optimal conformal mapping of the Borel plane was recently used to
accelerate the convergence of the perturbation expansions in QCD. In this work
we discuss the relevance of the method for the calculation of the Laplace-Borel
integral expressing formally the QCD Green functions. We define an optimal
expansion of the Laplace-Borel integral in the principal value prescription and
establish conditions under which the expansion is convergent.Comment: 10 pages, no figure
from decays: contour-improved versus fixed-order summation in a new QCD perturbation expansion
We consider the determination of from hadronic decays, by
investigating the contour-improved (CI) and the fixed-order (FO)
renormalization group summations in the frame of a new perturbation expansion
of QCD, which incorporates in a systematic way the available information about
the divergent character of the series. The new expansion functions, which
replace the powers of the coupling, are defined by the analytic continuation in
the Borel complex plane, achieved through an optimal conformal mapping. Using a
physical model recently discussed by Beneke and Jamin, we show that the new
CIPT approaches the true results with great precision when the perturbative
order is increased, while the new FOPT gives a less accurate description in the
regions where the imaginary logarithms present in the expansion of the running
coupling are large. With the new expansions, the discrepancy of 0.024 in
between the standard CI and FO summations is reduced to
only 0.009. From the new CIPT we predict , which practically coincides with the result of the
standard FOPT, but has a more solid theoretical basis
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