510 research outputs found
Complete (O_7,O_8) contribution to B -> X_s gamma at order alpha_s^2
We calculate the set of O(alpha_s^2) corrections to the branching ratio and
to the photon energy spectrum of the decay process B -> X_s gamma originating
from the interference of diagrams involving the electromagnetic dipole operator
O_7 with diagrams involving the chromomagnetic dipole operator O_8. The
corrections evaluated here are one of the elements needed to complete the
calculations of the B -> X_s gamma branching ratio at next-to-next-to-leading
order in QCD. We conclude that this set of corrections does not change the
central value of the Standard Model prediction for Br(B -> X_s gamma) by more
than 1 %.Comment: 19 pages, 8 figure
On the Numerical Evaluation of One-Loop Amplitudes: the Gluonic Case
We develop an algorithm of polynomial complexity for evaluating one-loop
amplitudes with an arbitrary number of external particles. The algorithm is
implemented in the Rocket program. Starting from particle vertices given by
Feynman rules, tree amplitudes are constructed using recursive relations. The
tree amplitudes are then used to build one-loop amplitudes using an integer
dimension on-shell cut method. As a first application we considered only three
and four gluon vertices calculating the pure gluonic one-loop amplitudes for
arbitrary external helicity or polarization states. We compare our numerical
results to analytical results in the literature, analyze the time behavior of
the algorithm and the accuracy of the results, and give explicit results for
fixed phase space points for up to twenty external gluons.Comment: 22 pages, 9 figures; v2: references added, version accepted for
publicatio
On the Rational Terms of the one-loop amplitudes
The various sources of Rational Terms contributing to the one-loop amplitudes
are critically discussed. We show that the terms originating from the generic
(n-4)-dimensional structure of the numerator of the one-loop amplitude can be
derived by using appropriate Feynman rules within a tree-like computation. For
the terms that originate from the reduction of the 4-dimensional part of the
numerator, we present two different strategies and explicit algorithms to
compute them.Comment: 14 pages, 3 figures, uses axodraw.st
Streptococcus pneumoniae as an UncommonCause of Superinfected Pancreatic Pseudocysts
Abstract.: We report a patient with pancreatic pseudocysts that were superinfected with Streptococcus pneumoniae. The literature on the prevalence of superinfection of pancreatic tissue by S. pneumoniae, as well as on its prophylaxis and treatment, is reviewed. In addition, a possible pathophysiologic pathway is discusse
MR- arthrography: anatomic variant from link between lateral meniscus and anterior and posterior cruciate ligaments. A case report and review of the literature
Menisci congenital anomalies are rare morphologic abnormalities including accessory meniscus, discoid meniscus, double-layered meniscus, hypoplastic meniscus and ring-shaped meniscus (RSM). In a 35 year-old male patient, MR arthrography showed a bridging accessory bundle connecting the anterior cruciate ligament and posterior cruciate ligament with the posterior horn of the lateral meniscus. Arthroscopic examination showed a fan-like obstacle embracing the posterior horn of the lateral meniscus. It would be important to correctly identify this anatomical variant, because the bundle connecting the external meniscus to the ligaments of the central pivot can be misinterpreted as a meniscal fragment
Optimizing the Reduction of One-Loop Amplitudes
We present an optimization of the reduction algorithm of one-loop amplitudes
in terms of master integrals. It is based on the exploitation of the polynomial
structure of the integrand when evaluated at values of the loop-momentum
fulfilling multiple cut-conditions, as emerged in the OPP-method. The
reconstruction of the polynomials, needed for the complete reduction, is rended
very versatile by using a projection-technique based on the Discrete Fourier
Transform. The novel implementation is applied in the context of the NLO QCD
corrections to u d-bar --> W+ W- W+
CutTools: a program implementing the OPP reduction method to compute one-loop amplitudes
We present a program that implements the OPP reduction method to extract the
coefficients of the one-loop scalar integrals from a user defined
(sub)-amplitude or Feynman Diagram, as well as the rational terms coming from
the 4-dimensional part of the numerator. The rational pieces coming from the
epsilon-dimensional part of the numerator are treated as an external input, and
can be computed with the help of dedicated tree-level like Feynman rules.
Possible numerical instabilities are dealt with the help of arbitrary
precision routines, that activate only when needed.Comment: Version published in JHE
Full one-loop amplitudes from tree amplitudes
We establish an efficient polynomial-complexity algorithm for one-loop
calculations, based on generalized -dimensional unitarity. It allows
automated computations of both cut-constructible {\it and} rational parts of
one-loop scattering amplitudes from on-shell tree amplitudes. We illustrate the
method by (re)-computing all four-, five- and six-gluon scattering amplitudes
in QCD at one-loop.Comment: 27 pages, revte
Feynman rules for the rational part of the Electroweak 1-loop amplitudes
We present the complete set of Feynman rules producing the rational terms of
kind R_2 needed to perform any 1-loop calculation in the Electroweak Standard
Model. Our results are given both in the 't Hooft-Veltman and in the Four
Dimensional Helicity regularization schemes. We also verified, by using both
the 't Hooft-Feynman gauge and the Background Field Method, a huge set of Ward
identities -up to 4-points- for the complete rational part of the Electroweak
amplitudes. This provides a stringent check of our results and, as a
by-product, an explicit test of the gauge invariance of the Four Dimensional
Helicity regularization scheme in the complete Standard Model at 1-loop. The
formulae presented in this paper provide the last missing piece for completely
automatizing, in the framework of the OPP method, the 1-loop calculations in
the SU(3) X SU(2) X U(1) Standard Model.Comment: Many thanks to Huasheng Shao for having recomputed, independently of
us, all of the effective vertices. Thanks to his help and by
comparing with an independent computation we performed in a general
gauge, we could fix, in the present version, the following formulae: the
vertex in Eq. (3.6), the vertex in Eq. (3.8),
Eqs (3.16), (3.17) and (3.18
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