691 research outputs found
Recursion relations, Helicity Amplitudes and Dimensional Regularization
Using the method of on-shell recursion relations we compute tree level
amplitudes including D-dimensional scalars and fermions. These tree level
amplitudes are needed for calculations of one-loop amplitudes in QCD involving
external quarks and gluons.Comment: 28 pages, 6 figures, clarifications adde
Multigluon tree amplitudes with a pair of massive fermions
We consider the calculation of n-point multigluon tree amplitudes with a pair
of massive fermions in QCD. We give the explicit transformation rules of this
kind of massive fermion-pair amplitudes with respect to different reference
momenta and check the correctness of them by SUSY Ward identities. Using these
rules and onshell BCFW recursion relation, we calculate the analytic results of
several n-point multigluon amplitudes.Comment: 15page
MHV Techniques for QED Processes
Significant progress has been made in the past year in developing new `MHV'
techniques for calculating multiparticle scattering amplitudes in Yang-Mills
gauge theories. Most of the work so far has focussed on applications to Quantum
Chromodynamics, both at tree and one-loop level. We show how such techniques
can also be applied to abelian theories such as QED, by studying the simplest
tree-level multiparticle process, e^+e^- to n \gamma. We compare explicit
results for up to n=5 photons using both the Cachazo, Svrcek and Witten `MHV
rules' and the related Britto-Cachazo-Feng `recursion relation' approaches with
those using traditional spinor techniques.Comment: 19 pages, 10 figures. References adde
On-shell recursion relations for all Born QCD amplitudes
We consider on-shell recursion relations for all Born QCD amplitudes. This
includes amplitudes with several pairs of quarks and massive quarks. We give a
detailed description on how to shift the external particles in spinor space and
clarify the allowed helicities of the shifted legs. We proof that the
corresponding meromorphic functions vanish at z --> infinity. As an application
we obtain compact expressions for helicity amplitudes including a pair of
massive quarks, one negative helicity gluon and an arbitrary number of positive
helicity gluons.Comment: 30 pages, minor change
Seven parton amplitudes from recursion relations
We present the first calculation of two-quark and five-gluon tree amplitudes
using on-shell recursion relations. These amplitudes are needed for tree level
5-jet cross-section and an essential ingredient for next-to-leading order 4-jet
and next-to-next-to-leading order 3-jet production at hadronic colliders. Very
compact expressions for all possible helicity configurations are provided,
allowing for direct implementation in Monte-Carlo codes.Comment: 11 page
A direct proof of the CSW rules
Using recursion methods similar to those of Britto, Cachazo, Feng and Witten
(BCFW) a direct proof of the CSW rules for computing tree-level gluon
amplitudes is given.Comment: 11 pages, uses axodraw.st
Scalar diagrammatic rules for Born amplitudes in QCD
We show that all Born amplitudes in QCD can be calculated from scalar
propagators and a set of three- and four-valent vertices. In particular, our
approach includes amplitudes with any number of quark pairs. The quarks may be
massless or massive. The proof of the formalism is given entirely within
quantum field theory.Comment: 20 pages, references adde
Recursion Relations for One-Loop Gravity Amplitudes
We study the application of recursion relations to the calculation of finite
one-loop gravity amplitudes. It is shown explicitly that the known four, five,
and six graviton one-loop amplitudes for which the external legs have identical
outgoing helicities, and the four graviton amplitude with helicities (-,+,+,+)
can be derived from simple recursion relations. The latter amplitude is derived
by introducing a one-loop three-point vertex of gravitons of positive helicity,
which is the counterpart in gravity of the one-loop three-plus vertex in
Yang-Mills. We show that new issues arise for the five point amplitude with
helicities (-,+,+,+,+), where the application of known methods does not appear
to work, and we discuss possible resolutions.Comment: 28 pages, LaTeX, 12 figures. v2:typos and references correcte
Scattering amplitudes with massive fermions using BCFW recursion
We study the QCD scattering amplitudes for \bar{q}q \to gg and \bar{q}q \to
ggg where q is a massive fermion. Using a particular choice of massive fermion
spinor we are able to derive very compact expressions for the partial spin
amplitudes for the 2 \to 2 process. We then investigate the corresponding 2 \to
3 amplitudes using the BCFW recursion technique. For the helicity conserving
partial amplitudes we again derive very compact expressions, but were unable to
treat the helicity-flip amplitudes recursively, except for the case where all
the gluon helicities are the same. We therefore evaluate the remaining partial
amplitudes using standard Feynman diagram techniques.Comment: 21 page
Color-dressed recursive relations for multi-parton amplitudes
Remarkable progress inspired by twistors has lead to very simple analytic
expressions and to new recursive relations for multi-parton color-ordered
amplitudes. We show how such relations can be extended to include color and
present the corresponding color-dressed formulation for the Berends-Giele, BCF
and a new kind of CSW recursive relations. A detailed comparison of the
numerical efficiency of the different approaches to the calculation of
multi-parton cross sections is performed.Comment: 31 pages, 4 figures, 6 table
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