113 research outputs found
Mearsurement of and the function with the DELPHI detector at LEP
Event shape distributions in annihilation are determined from the
DELPHI data taken between 183 and 207\gev. From these the strong coupling
is extracted with several techniques. Together with the results from
other LEP2 energies and at about this allows both, a combined measurement
of and a test of the scale dependence of the strong interaction.
Alternatively the renormalisation group invariant (RGI) perturbation theory is
applied to measure the function of strong interaction. The results are
good agreement with the QCD expectation and allow to exclude the existence of
light gluinos with a mass below 30\gev in a model independent way.Comment: Talk presented at the QCD02 conference, Montpellier 200
A nonperturbative model for the strong running coupling within potential approach
A nonperturbative model for the QCD invariant charge, which contains no
low-energy unphysical singularities and possesses an elevated higher loop
corrections stability, is developed in the framework of potential approach. The
static quark-antiquark potential is constructed by making use of the proposed
model for the strong running coupling. The obtained result coincides with the
perturbative potential at small distances and agrees with relevant lattice
simulation data in the nonperturbative physically-relevant region. The
developed model yields a reasonable value of the QCD scale parameter, which is
consistent with its previous estimations obtained within potential approach.Comment: 14 pages, 4 figure
Hadronization effects in event shape moments
We study the moments of hadronic event shapes in annihilation within
the context of next-to-next-to-leading order (NNLO) perturbative QCD
predictions combined with non-perturbative power corrections in the dispersive
model. This model is extended to match upon the NNLO perturbative prediction.
The resulting theoretical expression has been compared to experimental data
from JADE and OPAL, and a new value for has been determined, as
well as of the average coupling in the non-perturbative region below
GeV within the dispersive model:
\alpha_s(M_Z)&=0.1153\pm0.0017(\mathrm{exp})\pm0.0023(\mathrm{th}),\alpha_0&=0.5132\pm0.0115(\mathrm{exp})\pm0.0381(\mathrm{th}),
The precision of the value has been improved in comparison to
the previously available next-to-leading order analysis. We observe that the
resulting power corrections are considerably larger than those estimated from
hadronization models in multi-purpose event generator programs.Comment: 23 pages, 5 figures, 15 tables. Few minor changes. Version accepted
for publication in European Physical Journal C
Approximate NNLO Threshold Resummation in Heavy Flavour Decays
We present an approximate NNLO evaluation of the QCD form factor resumming
large logarithmic perturbative contributions in semi-inclusive heavy flavour
decays.Comment: 16 pages, 3 figures, Latex; minor changes; 2 figures adde
Factorization Properties of Soft Graviton Amplitudes
We apply recently developed path integral resummation methods to perturbative
quantum gravity. In particular, we provide supporting evidence that eikonal
graviton amplitudes factorize into hard and soft parts, and confirm a recent
hypothesis that soft gravitons are modelled by vacuum expectation values of
products of certain Wilson line operators, which differ for massless and
massive particles. We also investigate terms which break this factorization,
and find that they are subleading with respect to the eikonal amplitude. The
results may help in understanding the connections between gravity and gauge
theories in more detail, as well as in studying gravitational radiation beyond
the eikonal approximation.Comment: 35 pages, 5 figure
A novel series solution to the renormalization group equation in QCD
Recently, the QCD renormalization group (RG) equation at higher orders in
MS-like renormalization schemes has been solved for the running coupling as a
series expansion in powers of the exact 2-loop order coupling. In this work, we
prove that the power series converges to all orders in perturbation theory.
Solving the RG equation at higher orders, we determine the running coupling as
an implicit function of the 2-loop order running coupling. Then we analyze the
singularity structure of the higher order coupling in the complex 2-loop
coupling plane. This enables us to calculate the radii of convergence of the
series solutions at the 3- and 4-loop orders as a function of the number of
quark flavours . In parallel, we discuss in some detail the
singularity structure of the coupling at the 3- and 4-loops in
the complex momentum squared plane for . The
correspondence between the singularity structure of the running coupling in the
complex momentum squared plane and the convergence radius of the series
solution is established. For sufficiently large values, we find
that the series converges for all values of the momentum squared variable
. For lower values of , in the scheme,
we determine the minimal value of the momentum squared above
which the series converges. We study properties of the non-power series
corresponding to the presented power series solution in the QCD Analytic
Perturbation Theory approach of Shirkov and Solovtsov. The Euclidean and
Minkowskian versions of the non-power series are found to be uniformly
convergent over whole ranges of the corresponding momentum squared variables.Comment: 29 pages,LateX file, uses IOP LateX class file, 2 figures, 13 Tables.
Formulas (4)-(7) and Table 1 were relegated to Appendix 1, some notations
changed, 2 footnotes added. Clarifying discussion added at the end of Sect.
3, more references and acknowledgments added. Accepted for publication in
Few-Body System
Toward a Systematic Holographic QCD: A Braneless Approach
Recently a holographic model of hadrons motivated by AdS/CFT has been
proposed to fit the low energy data of mesons. We point out that the infrared
physics can be developed in a more systematic manner by exploiting backreaction
of the nonperturbative condensates. We show that these condensates can
naturally provide the IR cutoff corresponding to confinement, thus removing
some of the ambiguities from the original formulation of the model. We also
show how asymptotic freedom can be incorporated into the theory, and the
substantial effect it has on the glueball spectrum and gluon condensate of the
theory. A simple reinterpretation of the holographic scale results in a
non-perturbative running for alpha_s which remains finite for all energies. We
also find the leading effects of adding the higher condensate into the theory.
The difficulties for such models to reproduce the proper Regge physics lead us
to speculate about extensions of our model incorporating tachyon condensation.Comment: 27 pages, LaTe
Analogs of noninteger powers in general analytic QCD
In contrast to the coupling parameter in the usual perturbative QCD (pQCD),
the coupling parameter in the analytic QCD models has cuts only on the negative
semiaxis of the Q^2-plane (where q^2 = -Q^2 is the momentum squared), thus
reflecting correctly the analytic structure of the spacelike observables. The
Minimal Analytic model (MA, named also APT) of Shirkov and Solovtsov removes
the nonphysical cut (at positive Q^2) of the usual pQCD coupling and keeps the
pQCD cut discontinuity of the coupling at negative Q^2 unchanged. In order to
evaluate in MA the physical QCD quantities whose perturbation expansion
involves noninteger powers of the pQCD coupling, a specific method of
construction of MA analogs of noninteger pQCD powers was developed by Bakulev,
Mikhailov and Stefanis (BMS). We present a construction, applicable now in any
analytic QCD model, of analytic analogs of noninteger pQCD powers; this method
generalizes the BMS approach obtained in the framework of MA. We need to know
only the discontinuity function of the analytic coupling (the analog of the
pQCD coupling) along its cut in order to obtain the analytic analogs of the
noninteger powers of the pQCD coupling, as well as their timelike (Minkowskian)
counterparts. As an illustration, we apply the method to the evaluation of the
width for the Higgs decay into b+(bar b) pair.Comment: 29 pages, 5 figures; sections II and III extended, appendix B is ne
The Adler Function for Light Quarks in Analytic Perturbation Theory
The method of analytic perturbation theory, which avoids the problem of
ghost-pole type singularities and gives a self-consistent description of both
spacelike and timelike regions, is applied to describe the "light" Adler
function corresponding to the non-strange vector channel of the inclusive decay
of the lepton. The role of threshold effects is investigated. The
behavior of the quark-antiquark system near threshold is described by using a
new relativistic resummation factor. It is shown that the method proposed leads
to good agreement with the ``experimental'' Adler function down to the lowest
energy scale.Comment: 13 pages, one ps figure, REVTe
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