155 research outputs found
t \bar{t} W production and decay at NLO
We present results for the production of a top pair in association with a
W-boson at next-to-leading order. We have implemented this process into the
parton-level integrator MCFM including the decays of both the top quarks and
the W-bosons with full spin correlations. Although the cross section for this
process is small, it is a Standard Model source of same-sign lepton events that
must be accounted for in many new physics searches. For a particular analysis
of same-sign lepton events in which b-quarks are also present, we investigate
the effect of the NLO corrections as a function of the signal region cuts.Comment: 10 pages, 7 figure
Counting loop diagrams: computational complexity of higher-order amplitude evaluation
We discuss the computational complexity of the perturbative evaluation of
scattering amplitudes, both by the Caravaglios-Moretti algorithm and by direct
evaluation of the individual diagrams. For a self-interacting scalar theory, we
determine the complexity as a function of the number of external legs. We
describe a method for obtaining the number of topologically inequivalent
Feynman graphs containing closed loops, and apply this to one- and two-loop
amplitudes. We also compute the number of graphs weighted by their symmetry
factors, thus arriving at exact and asymptotic estimates for the average
symmetry factor of diagrams. We present results for the asymptotic number of
diagrams up to 10 loops, and prove that the average symmetry factor approaches
unity as the number of external legs becomes large.Comment: 27 pages, 17 table
Automatic Integral Reduction for Higher Order Perturbative Calculations
We present a program for the reduction of large systems of integrals to
master integrals. The algorithm was first proposed by Laporta; in this paper,
we implement it in MAPLE. We also develop two new features which keep the size
of intermediate expressions relatively small throughout the calculation. The
program requires modest input information from the user and can be used for
generic calculations in perturbation theory.Comment: 23 page
Diagrammatic proof of the BCFW recursion relation for gluon amplitudes in QCD
We present a proof of the Britto-Cachazo-Feng-Witten tree-level recursion
relation for gluon amplitudes in QCD, based on a direct equivalence between
BCFW decompositions and Feynman diagrams. We demonstrate that this equivalence
can be made explicit when working in a convenient gauge. We exhibit that gauge
invariance and the particular structure of Yang-Mills vertices guarantees the
validity of the BCFW construction.Comment: 24 pages, 33 figure
Evaluating multi-loop Feynman diagrams with infrared and threshold singularities numerically
We present a method to evaluate numerically Feynman diagrams directly from
their Feynman parameters representation. We first disentangle overlapping
singularities using sector decomposition. Threshold singularities are treated
with an appropriate contour deformation. We have validated our technique
comparing with recent analytic results for the gg->h two-loop amplitudes with
heavy quarks and scalar quarks.Comment: 8 pages, 3 figures; references added, version to appear in JHE
On the Deformation of a Hyperelastic Tube Due to Steady Viscous Flow Within
In this chapter, we analyze the steady-state microscale fluid--structure
interaction (FSI) between a generalized Newtonian fluid and a hyperelastic
tube. Physiological flows, especially in hemodynamics, serve as primary
examples of such FSI phenomena. The small scale of the physical system renders
the flow field, under the power-law rheological model, amenable to a
closed-form solution using the lubrication approximation. On the other hand,
negligible shear stresses on the walls of a long vessel allow the structure to
be treated as a pressure vessel. The constitutive equation for the microtube is
prescribed via the strain energy functional for an incompressible, isotropic
Mooney--Rivlin material. We employ both the thin- and thick-walled formulations
of the pressure vessel theory, and derive the static relation between the
pressure load and the deformation of the structure. We harness the latter to
determine the flow rate--pressure drop relationship for non-Newtonian flow in
thin- and thick-walled soft hyperelastic microtubes. Through illustrative
examples, we discuss how a hyperelastic tube supports the same pressure load as
a linearly elastic tube with smaller deformation, thus requiring a higher
pressure drop across itself to maintain a fixed flow rate.Comment: 19 pages, 3 figures, Springer book class; v2: minor revisions, final
form of invited contribution to the Springer volume entitled "Dynamical
Processes in Generalized Continua and Structures" (in honour of Academician
D.I. Indeitsev), eds. H. Altenbach, A. Belyaev, V. A. Eremeyev, A. Krivtsov
and A. V. Porubo
Numerical evaluation of one-loop QCD amplitudes
We present the publicly available program NGluon allowing the numerical
evaluation of primitive amplitudes at one-loop order in massless QCD. The
program allows the computation of one-loop amplitudes for an arbitrary number
of gluons. The focus of the present article is the extension to one-loop
amplitudes including an arbitrary number of massless quark pairs. We discuss in
detail the algorithmic differences to the pure gluonic case and present cross
checks to validate our implementation. The numerical accuracy is investigated
in detail.Comment: Talk given at ACAT 2011 conference in London, 5-9 Septembe
Feynman Rules for the Rational Part of the QCD 1-loop amplitudes
We compute the complete set of Feynman Rules producing the Rational Terms of
kind R_2 needed to perform any QCD 1-loop calculation. We also explicitly check
that in order to account for the entire R_2 contribution, even in case of
processes with more than four external legs, only up to four-point vertices are
needed. Our results are expressed both in the 't Hooft Veltman regularization
scheme and in the Four Dimensional Helicity scheme, using explicit color
configurations as well as the color connection language.Comment: 18 pages, 11 figures. Misprints corrected in Appendix A. Version to
be published in JHE
NLO QCD corrections to pp -> ttbb+X via quark anti-quark annihilation
The process pp -> top anti-top bottom anti-bottom + X represents a very
important background reaction to searches at the LHC, in particular to top
anti-top H production where the Higgs boson decays into a bottom anti-bottom
pair. A successful analysis of top anti-top H at the LHC requires the knowledge
of direct top anti-top bottom anti-bottom production at NLO in QCD. We take the
first step in this direction upon calculating the NLO QCD corrections to the
subprocess initiated by quark anti-quark annihilation.Comment: 6 pages, LaTeX, 4 postscript figures, to appear in proceedings of the
"9th Workshop on Elementary Particle Theory: Loops and Legs in Quantum Field
Theory", Sondershausen, Germany, April 20-25, 200
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