49 research outputs found
Heavy baryon properties with NLO accuracy in perturbative QCD
We present an analysis of the static properties of heavy baryons at
next-to-leading order in the perurbative expansion of QCD. We obtain analytical
next-to-leading order three-loop results for the two-point correlators of
baryonic currents with one finite mass quark field for a variety of quantum
numbers of the baryonic currents. We consider both the massless limit and the
HQET limit of the correlator as special cases of the general finite mass
formula and find agreement with previous results. We present closed form
expressions for the moments of the spectral density. We determine the residues
of physical baryon states using sum rule techniques.Comment: 43 pages in LaTeX, including 3 figure
Analysis of and with QCD sum rules
In this article, we calculate the masses and the pole residues of the
heavy baryons and with the QCD
sum rules. The numerical values (or
) and (or ) are in good agreement
with the experimental data.Comment: 18 pages, 18 figures, slight revisio
Perturbative relations between annihilation and decay observables including resummation effects
By exploiting the analyticity properties of the two-point current-current
correlator we obtain numerical predictions for the moments in terms of
the decay rate. We perform a partial resummation of the pertinent
perturbative series expansion by solving the renormalization group equation for
Adler's function. Our predictions are renormalization scheme independent but
depend on the order of the perturbative -function expansion. The
analysis involves the unknown five-loop coefficient for which we give
some new estimates.Comment: 10 pages, LaTeX, 2 postscript figure
On the evaluation of a certain class of Feynman diagrams in x-space: Sunrise-type topologies at any loop order
We review recently developed new powerful techniques to compute a class of
Feynman diagrams at any loop order, known as sunrise-type diagrams. These
sunrise-type topologies have many important applications in many different
fields of physics and we believe it to be timely to discuss their evaluation
from a unified point of view. The method is based on the analysis of the
diagrams directly in configuration space which, in the case of the sunrise-type
diagrams and diagrams related to them, leads to enormous simplifications as
compared to the traditional evaluation of loops in momentum space. We present
explicit formulae for their analytical evaluation for arbitrary mass
configurations and arbitrary dimensions at any loop order. We discuss several
limiting cases of their kinematical regimes which are e.g. relevant for
applications in HQET and NRQCD.Comment: 100 pages, 16 eps-figures include
Strong coupling constant from decay within renormalization scheme invariant treatment
We extract a numerical value for the strong coupling constant \alpha_s from
the \tau-lepton decay rate into nonstrange particles. A new feature of our
procedure is the explicit use of renormalization scheme invariance in
analytical form in order to perform the actual analysis in a particular
renormalization scheme. For the reference coupling constant in the
\MSsch-scheme we obtain \alpha_s(M_\tau)= 0.3184 \pm 0.0060_{exp} which
corresponds to \al_s(M_Z)= 0.1184 \pm 0.0007_{exp} \pm 0.0006_{hq mass}. This
new numerical value is smaller than the standard value from -data quoted
in the literature and is closer to \al_s(M_Z)-values obtained from high energy
experiments.Comment: 8 page
Electroweak and finite width corrections to top quark decays into transverse and longitudinal -bosons
We calculate the electroweak and finite width corrections to the decay of an
unpolarized top quark into a bottom quark and a -gauge boson where the
helicities of the are specified as longitudinal, transverse-plus and
transverse-minus. Together with the corrections these
corrections may become relevant for the determination of the mass of the top
quark through angular decay measurements.Comment: 4 pages, 7 postscript figures adde
A new technique for computing the spectral density of sunset-type diagrams: integral transformation in configuration space
We present a new method to investigate a class of diagrams which generalizes
the sunset topology to any number of massive internal lines. Our attention is
focused on the computation of the spectral density of these diagrams which is
related to many-body phase space in dimensional space-time. The spectral
density is determined by the inverse -transform of the product of
propagators in configuration space. The inverse -transform reduces to the
inverse Laplace transform in any odd number of space-time dimensions for which
we present an explicit analytical result.Comment: 13 pages in LaTeX, one PostScript figure included in the tex
Analytical continuation and resummed perturbation theory
A pattern of partial resummation of perturbation theory series inspired by
analytical continuation is discussed for some physical observables.Comment: 9 pages, 2 tables, Late
Finite calculation of divergent selfenergy diagrams
Using dispersive techniques, it is possible to avoid ultraviolet divergences
in the calculation of Feynman diagrams, making subsequent regularization of
divergent diagrams unnecessary. We give a simple introduction to the most
important features of such dispersive techniques in the framework of the
so-called finite causal perturbation theory. The method is also applied to the
'divergent' general massive two-loop sunrise selfenergy diagram, where it leads
directly to an analytic expression for the imaginary part of the diagram in
accordance with the literature, whereas the real part can be obtained by a
single integral dispersion relation. It is pointed out that dispersive methods
have been known for decades and have been applied to several nontrivial Feynman
diagram calculations.Comment: 15 pages, Latex, one figure, added reference
On the evaluation of sunset-type Feynman diagrams
We introduce an efficient configuration space technique which allows one to
compute a class of Feynman diagrams which generalize the scalar sunset topology
to any number of massive internal lines. General tensor vertex structures and
modifications of the propagators due to particle emission with vanishing
momenta can be included with only a little change of the basic technique
described for the scalar case. We discuss applications to the computation of
-body phase space in -dimensional space-time. Substantial simplifications
occur for odd space-time dimensions where the final results can be expressed in
closed form through rational functions. We present explicit analytical formulas
for three-dimensional space-time.Comment: 45 pages in LaTeX, 3 PostScript figures included in the tex