47 research outputs found
Non-singlet splitting functions in QED
Iterative solution of QED evolution equations for non-singlet electron
structure functions is considered. Analytical expressions in the fourth and
fifth orders are presented in terms of splitting functions. Relation to the
existing exponentiated solution is discussed.Comment: 8 pages, LaTeX2
Analytical Results for Dimensionally Regularized Massless On-shell Double Boxes with Arbitrary Indices and Numerators
We present an algorithm for the analytical evaluation of dimensionally
regularized massless on-shell double box Feynman diagrams with arbitrary
polynomials in numerators and general integer powers of propagators. Recurrence
relations following from integration by parts are solved explicitly and any
given double box diagram is expressed as a linear combination of two master
double boxes and a family of simpler diagrams. The first master double box
corresponds to all powers of the propagators equal to one and no numerators,
and the second master double box differs from the first one by the second power
of the middle propagator. By use of differential relations, the second master
double box is expressed through the first one up to a similar linear
combination of simpler double boxes so that the analytical evaluation of the
first master double box provides explicit analytical results, in terms of
polylogarithms \Li{a}{-t/s}, up to , and generalized polylogarithms
, with and , dependent on the Mandelstam variables
and , for an arbitrary diagram under consideration.Comment: LaTeX, 16 pages; misprints in ff. (8), (24), (30) corrected; some
explanations adde
Analytical Result for Dimensionally Regularized Massless Master Non-planar Double Box with One Leg off Shell
The dimensionally regularized massless non-planar double box Feynman diagram
with powers of propagators equal to one, one leg off the mass shell, i.e. with
p_1^2=q^2\neq 0, and three legs on shell, p_i^2=0, i=2,3,4, is analytically
calculated for general values of q^2 and the Mandelstam variables s,t and u
(not necessarily restricted by the physical condition s+t+u=q^2). An explicit
result is expressed through (generalized) polylogarithms, up to the fourth
order, dependent on rational combinations of q^2,s,t and u, and simple finite
two- and three fold Mellin--Barnes integrals of products of gamma functions
which are easily numerically evaluated for arbitrary non-zero values of the
arguments.Comment: 9 pages, LaTeX with axodraw.sty, minor changes in references, to
appear in Physics Letters
Numerical evaluation of multiple polylogarithms
Multiple polylogarithms appear in analytic calculations of higher order
corrections in quantum field theory. In this article we study the numerical
evaluation of multiple polylogarithms. We provide algorithms, which allow the
evaluation for arbitrary complex arguments and without any restriction on the
weight. We have implemented these algorithms with arbitrary precision
arithmetic in C++ within the GiNaC framework.Comment: 23 page
Analytical Result for Dimensionally Regularized Massless On-Shell Planar Triple Box
The dimensionally regularized massless on-shell planar triple box Feynman
diagram with powers of propagators equal to one is analytically evaluated for
general values of the Mandelstam variables s and t in a Laurent expansion in
the parameter \ep=(4-d)/2 of dimensional regularization up to a finite part. An
explicit result is expressed in terms of harmonic polylogarithms, with
parameters 0 and 1, up to the sixth order. The evaluation is based on the
method of Feynman parameters and multiple Mellin-Barnes representation. The
same technique can be quite similarly applied to planar triple boxes with any
numerators and integer powers of the propagators.Comment: 8 pages, LaTeX with axodraw.st
A Comparative Study of Automatic Localization Algorithms for Spherical Markers within 3D MRI Data
Localization of features and structures in images is an important task in medical image-processing. Characteristic structures and features are used in diagnostics and surgery planning for spatial adjustments of the volumetric data, including image registration or localization of bone-anchors and fiducials. Since this task is highly recurrent, a fast, reliable and automated approach without human interaction and parameter adjustment is of high interest. In this paper we propose and compare four image processing pipelines, including algorithms for automatic detection and localization of spherical features within 3D MRI data. We developed a convolution based method as well as algorithms based on connected-components labeling and analysis and the circular Hough-transform. A blob detection related approach, analyzing the Hessian determinant, was examined. Furthermore, we introduce a novel spherical MRI-marker design. In combination with the proposed algorithms and pipelines, this allows the detection and spatial localization, including the direction, of fiducials and bone-anchors
Phospholipid Scramblase 4 (PLSCR4) Regulates Adipocyte Differentiation via PIP3-Mediated AKT Activation
Phospholipid scramblase 4 (PLSCR4) is a member of a conserved enzyme family with high relevance for the remodeling of phospholipid distribution in the plasma membrane and the regulation of cellular signaling. While PLSCR1 and -3 are involved in the regulation of adipose-tissue expansion, the role of PLSCR4 is so far unknown. PLSCR4 is significantly downregulated in an adipose-progenitor-cell model of deficiency for phosphatase and tensin homolog (PTEN). PTEN acts as a tumor suppressor and antagonist of the growth and survival signaling phosphoinositide 3-kinase (PI3K)/AKT cascade by dephosphorylating phosphatidylinositol-3,4,5-trisphosphate (PIP3). Patients with PTEN germline deletion frequently develop lipomas. The underlying mechanism for this aberrant adipose-tissue growth is incompletely understood. PLSCR4 is most highly expressed in human adipose tissue, compared with other phospholipid scramblases, suggesting a specific role of PLSCR4 in adipose-tissue biology. In cell and mouse models of lipid accumulation, we found PLSCR4 to be downregulated. We observed increased adipogenesis in PLSCR4-knockdown adipose progenitor cells, while PLSCR4 overexpression attenuated lipid accumulation. PLSCR4 knockdown was associated with increased PIP3 levels and the activation of AKT. Our results indicated that PLSCR4 is a regulator of PI3K/AKT signaling and adipogenesis and may play a role in PTEN-associated adipose-tissue overgrowth and lipoma formation
DGLAP and BFKL evolution equations in the N=4 supersymmetric gauge theory
We derive the DGLAP and BFKL evolution equations in the N=4 supersymmetric
gauge theory in the next-to-leading approximation. The eigenvalue of the BFKL
kernel in this model turns out to be an analytic function of the conformal spin
|n|. Its analytic continuation to negative |n| in the leading logarithmic
approximation allows us to obtain residues of anomalous dimensions \gamma of
twist-2 operators in the non-physical points j=0,-1,... from the BFKL equation
in an agreement with their direct calculation from the DGLAP equation.
Moreover, in the multi-color limit of the N=4 model the BFKL and DGLAP dynamics
in the leading logarithmic approximation is integrable for an arbitrary number
of particles. In the next-to-leading approximation the holomorphic separability
of the Pomeron hamiltonian is violated, but the corresponding Bethe-Salpeter
kernel has the property of a hermitian separability. The main singularities of
anomalous dimensions \gamma at j=-r obtained from the BFKL and DGLAP equations
in the next-to-leading approximation coincide but our accuracy is not enough to
verify an agreement for residues of subleading poles.Comment: 45 pages, latex. In the last version the expression (16) for the
t-channel partial wave of the process e+e- --> \mu+\mu- in the
double-logarithmic approximation at QED is corrected and its derivation is
given in the Appendix
Two-Loop Vertices in Quantum Field Theory: Infrared and Collinear Divergent Configurations
A comprehensive study is performed of two-loop Feynman diagrams with three
external legs which, due to the exchange of massless gauge-bosons, give raise
to infrared and collinear divergencies. Their relevance in assembling realistic
computations of next-to-next-to-leading corrections to physical observables is
emphasised. A classification of infrared singular configurations, based on
solutions of Landau equations, is introduced. Algorithms for the numerical
evaluation of the residues of the infrared poles and of the infrared finite
parts of diagrams are introduced and discussed within the scheme of dimensional
regularization. Integral representations of Feynman diagrams which form a
generalization of Nielsen - Goncharov polylogarithms are introduced and their
numerical evaluation discussed. Numerical results are shown for all different
families of multi-scale, two-loop, three-point infrared divergent diagrams and
successful comparisons with analytical results, whenever available, are
performed. Part of these results has already been included in a recent
evaluation of electroweak pseudo-observables at the two-loop level.Comment: 62 pages, 15 figures, 16 table
NLO corrections to the BFKL equation in QCD and in supersymmetric gauge theories
We study next-to-leading corrections to the integral kernel of the BFKL
equation for high energy cross-sections in QCD and in supersymmetric gauge
theories. The eigenvalue of the BFKL kernel is calculated in an analytic form
as a function of the anomalous dimension \gamma of the local gauge-invariant
operators and their conformal spin n. For the case of an extended N=4 SUSY the
kernel is significantly simplified. In particular, the terms non-analytic in n
are canceled. We discuss the relation between the DGLAP and BFKL equations in
the N=4 model.Comment: Latex, 26 pages, typos corrected, to be published in Nucl.Phys.