34 research outputs found
Generalization of the BLM procedure and its scales in any order of pQCD
The Brodsky--Lepage--Mackenzie procedure is sequentially and unambiguously
extended to any fixed order of perturbative QCD beyond the so called
``large--\beta_0 approximation''. As a result of this procedure, the obtained
perturbation series looks like a continued-fraction representation. A
subsequent generalization of this procedure is developed, in order to optimize
the convergence of the final series, along the lines of the Fastest Convergence
Prescription. This generalized BLM procedure is applied to the Adler D function
and also to R_{e^+e^-} in QCD at NLO. A further extension of the sequential
BLM is presented which makes use of additional parameters to optimize the
convergence of the power-series at any fixed order of expansion.Comment: 24 pages, JHEP3, 4 figures are enclosed as eps-file, final version to
be published in JHE
Strange quark mass from Finite Energy QCD sum rules to five loops
The strange quark mass is determined from a new QCD Finite Energy Sum Rule
(FESR) optimized to reduce considerably the systematic uncertainties arising
from the hadronic resonance sector. As a result, the main uncertainty in this
determination is due to the value of . The correlator of
axial-vector divergences is used in perturbative QCD to five-loop order,
including quark and gluon condensate contributions, in the framework of both
Fixed Order (FOPT), and Contour Improved Perturbation Theory (CIPT). The latter
exhibits very good convergence, leading to a remarkably stable result in the
very wide range , where is the radius of the
integration contour in the complex energy (squared) plane. The value of the
strange quark mass in this framework at a scale of 2 GeV is for , respectively.Comment: Additional comments added at the end of the Conclusions, and one
extra reference is given. A note added in proof uses the most recent
determination of Lambda_QCD from ALEPH to narrow down the predictio
Renormalization of the Fayet-Iliopoulos Term in Softly Broken SUSY Gauge Theories
It is shown that renormalization of the Fayet-Iliopoulos term in a softly
broken SUSY gauge theory, in full analogy with all the other soft terms
renormalizations, is completely defined in a rigid or an unbroken theory.
However, contrary to the other soft renormalizations, there is no simple
differential operator that acts on the renormalization functions of a rigid
theory and allows one to get the renormalization of the F-I term. One needs an
analysis of the superfield diagrams and some additional diagram calculations in
components. The method is illustrated by the four loop calculation of some part
of renormalization proportional to the soft scalar masses and the soft triple
couplings.Comment: Latex2e, 14 pages, uses axodraw.sty. References adde
1/N_c and 1/n preasymptotic corrections to Current-Current correlators
We obtain the corrections in and in ( is the principal
quantum number of the bound state) of the decay constants of scalar and
pseudoscalar currents in two and four dimensions in the large . We obtain
them from the operator product expansion provided a model for the large
mass spectrum is given. In the two-dimensional case the spectrum is known and
the corrections obtained in this paper are model independent. We confirm these
results by confronting them with the numerical solution of the 't Hooft model.
We also consider a model at finite and obtain the associated decay
constants that are consistent with perturbation theory. This example shows that
that the inclusion of perturbative corrections, or finite effects, to the
OPE does not constrain the slope of the Regge trajectories, which remain a free
parameter for each different channel.Comment: 29 pages, 11 figures. Two references adde
Standard Model Higgs-Boson Branching Ratios with Uncertainties
We present an update of the branching ratios for Higgs-boson decays in the
Standard Model. We list results for all relevant branching ratios together with
corresponding uncertainties resulting from input parameters and missing
higher-order corrections. As sources of parametric uncertainties we include the
masses of the charm, bottom, and top quarks as well as the QCD coupling
constant. We compare our results with other predictions in the literature.Comment: 32 pages, 4 figures, contribution to LHC Higgs Cross Section Working
Group https://twiki.cern.ch/twiki/bin/view/LHCPhysics/CrossSections,
theoretical uncertainties for H->\mu\mu{} added, version to appear in
European Physical Journal
Theoretical uncertainties for measurements of alpha_s from electroweak observables
One of the most precise measurements of the strong coupling constant
alpha_s(MZ) is obtained in the context of global analyses of precision
electroweak data. This article reviews the sensitivity of different electroweak
observables to alpha_s and describes the perturbative uncertainties related to
missing higher orders. The complete renormalisation scale dependence for the
relevant observables is calculated at next-to-next-to-leading order and a new
method is presented to determine the corresponding perturbative uncertainty for
measurements of alpha_s based on these observables.Comment: v4: Revised version with new tables and figure
Scale setting for alpha_s beyond leading order
We present a general procedure for incorporating higher-order information
into the scale-setting prescription of Brodsky, Lepage and Mackenzie. In
particular, we show how to apply this prescription when the leading coefficient
or coefficients in a series in the strong coupling alpha_s are anomalously
small and the original prescription can give an unphysical scale. We give a
general method for computing an optimum scale numerically, within dimensional
regularization, and in cases when the coefficients of a series are known. We
apply it to the heavy quark mass and energy renormalization in lattice NRQCD,
and to a variety of known series. Among the latter, we find significant
corrections to the scales for the ratio of e+e- to hadrons over muons, the
ratio of the quark pole to MSbar mass, the semi-leptonic B-meson decay width,
and the top decay width. Scales for the latter two decay widths, expressed in
terms of MSbar masses, increase by factors of five and thirteen, respectively,
substantially reducing the size of radiative corrections.Comment: 39 pages, 15 figures, 5 tables, LaTeX2
Nonperturbative Effects from the Resummation of Perturbation Theory
Using the general argument in Borel resummation of perturbation theory that
links the divergent perturbation theory to the nonperturbative effect we argue
that the nonperturbative effect associated with the perturbation theory should
have a branch cut only along the positive real axis in the complex coupling
plane. The component in the weak coupling expansion of the nonperturbative
amplitude, which usually includes the leading term in the weak coupling
expansion, that gives rise to the branch cut can be calculated in principle
from the perturbation theory combined with some exactly calculable properties
of the nonperturbative effect. The realization of this mechanism is
demonstrated in the double well potential and the two-dimensional O(N)
nonlinear sigma model. In these models the leading term in weak coupling of the
nonperturbative effect can be obtained with good accuracy from the first terms
of the perturbation theory. Applying this mechanism to the infrared renormalon
induced nonperturbative effect in QCD, we suggest some of the QCD condensate
effects can be calculated in principle from the perturbation theory.Comment: 21 Pages, 1 Figure; To appear in Phys Rev
Contour-improved versus fixed-order perturbation theory in hadronic tau decays
The hadronic decay rate of the tau lepton serves as one of the most precise
determinations of the QCD coupling alpha_s. The dominant theoretical source of
uncertainty at present resides in the seeming disparity of two approaches to
improving the perturbative expansion with the help of the renormalisation
group, namely fixed-order and contour-improved perturbation theory. In this
work it is demonstrated that in fact both approaches yield compatible results.
However, the fixed-order series is found to oscillate around the
contour-improved result with an oscillation frequency of approximately six
perturbative orders, approaching it until about the 30th order, after which the
expansion reveals its asymptotic nature. Additionally, the renormalisation
scale and scheme dependencies of the perturbative series for the tau hadronic
width are investigated in detail.Comment: 20 pages, 5 eps-figures; discussion on scale and scheme dependence
added as compared to published journal version JHEP 09 (2005) 05
Associated production of charged Higgs bosons and top quarks with POWHEG
The associated production of charged Higgs bosons and top quarks at hadron
colliders is an important discovery channel to establish the existence of a
non-minimal Higgs sector. Here, we present details of a next-to-leading order
(NLO) calculation of this process using the Catani-Seymour dipole formalism and
describe its implementation in POWHEG, which allows to match NLO calculations
to parton showers. Numerical predictions are presented using the PYTHIA parton
shower and are compared to those obtained previously at fixed order, to a
leading order calculation matched to the PYTHIA parton shower, and to a
different NLO calculation matched to the HERWIG parton shower with MC@NLO. We
also present numerical predictions and theoretical uncertainties for various
Two Higgs Doublet Models at the Tevatron and LHC.Comment: 36 page