944 research outputs found
One-loop amplitudes for W+3 jet production in hadron collisions
We employ the recently developed method of generalized -dimensional
unitarity to compute one-loop virtual corrections to all scattering amplitudes
relevant for the production of a boson in association with three jets in
hadronic collisions, treating all quarks as massless.Comment: 26 pages, 5 figures, v2 to agree with published versio
Semi-Numerical Evaluation of One-Loop Corrections
We present a semi-numerical algorithm to calculate one-loop virtual
corrections to scattering amplitudes. The divergences of the loop amplitudes
are regulated using dimensional regularization. We treat in detail the case of
amplitudes with up to five external legs and massless internal lines, although
the method is more generally applicable. Tensor integrals are reduced to
generalized scalar integrals, which in turn are reduced to a set of known basis
integrals using recursion relations. The reduction algorithm is modified near
exceptional configurations to ensure numerical stability. To test the procedure
we apply these techniques to one-loop corrections to the Higgs to four quark
process for which analytic results have recently become available.Comment: 33 pages, 10 figures, some typos fixed and references added, to
appear in Phys. Rev. D. v3 corrects a typo in eq. A2
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
A Numerical Unitarity Formalism for Evaluating One-Loop Amplitudes
Recent progress in unitarity techniques for one-loop scattering amplitudes
makes a numerical implementation of this method possible. We present a
4-dimensional unitarity method for calculating the cut-constructible part of
amplitudes and implement the method in a numerical procedure. Our technique can
be applied to any one-loop scattering amplitude and offers the possibility that
one-loop calculations can be performed in an automatic fashion, as tree-level
amplitudes are currently done. Instead of individual Feynman diagrams, the
ingredients for our one-loop evaluation are tree-level amplitudes, which are
often already known. To study the practicality of this method we evaluate the
cut-constructible part of the 4, 5 and 6 gluon one-loop amplitudes numerically,
using the analytically known 4, 5 and 6 gluon tree-level amplitudes.
Comparisons with analytic answers are performed to ascertain the numerical
accuracy of the method.Comment: 29 pages with 8 figures; references updated in rsponse to readers'
suggestion
Generalized unitarity at work: first NLO QCD results for hadronic W+3jet production
We compute the leading color, next-to-leading order QCD corrections to the
dominant partonic channels for the production of a W boson in association with
three jets at the Tevatron and the LHC. This is the first application of
generalized unitarity for realistic one-loop calculations. The method performs
well in this non-trivial test and offers great promise for the future.Comment: 20 pages, 4 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
Space-time properties of the higher twist amplitudes
A consistent and intuitive description of the twist-4 corrections to the
hadron structure functions is presented in a QCD-improved parton model using
time-ordered perturbative theory, where the collinear singularities are
naturally eliminated. We identify the special propagators with the backward
propagators of partons in time order.Comment: 18 Pages, Latex, 8 Ps figures, To appear in Phys. Rev.
Search for Large Rapidity Gap Events in e^+ e^- Annihilation
We investigate the cross-section for the production of a low-mass
colour-singlet cluster in annihilation with a large rapidity gap
between the colour-singlet cluster and the other jets. It is argued that such
events are the cross-channel analogue of large-rapidity-gap events in
deep-inelastic scattering, and therefore could in principle be used to
investigate the analytic continuation of the BFKL pomeron to the positive-
kinematic regime, where one would expect the trajectory to pass through
glueball states. The cross section can be calculated in perturbative QCD, so
that the infrared scale arising from non-perturbative effects, which prevents
an exponential fall-off with rapidity gap in the case of deep-inelastic
scattering, is absent in annihilation. Correspondingly, the cross
section for such events decreases rapidly with increasing rapidity gap.Comment: LATEX file - 21 pages + 15 figure
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New results in perturbative QCD
Three topics in perturbative QCD important for Super-collider physics are reviewed. The topics are: (2 2) jet phenomena calculated in O( sT); new techniques for the calculation of tree graphs; and colour coherence in jet phenomena. 31 refs., 6 figs
Analytic Structure of Three-Mass Triangle Coefficients
``Three-mass triangles'' are a class of integral functions appearing in
one-loop gauge theory amplitudes. We discuss how the complex analytic
properties and singularity structures of these amplitudes can be combined with
generalised unitarity techniques to produce compact expressions for three-mass
triangle coefficients. We present formulae for the N=1 contributions to the
n-point NMHV amplitude.Comment: 22 pages; v3: NMHV n=point expression added. 7 point expression
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