5,166 research outputs found
On the Construction of Scattering Amplitudes for Spinning Massless Particles
In this paper the general form of scattering amplitudes for massless
particles with equal spins s () or unequal spins () are derived. The imposed conditions are that the amplitudes should have
the lowest possible dimension, have propagators of dimension , and obey
gauge invariance. It is shown that the number of momenta required for
amplitudes involving particles with s > 2 is higher than the number implied by
3-vertices for higher spin particles derived in the literature. Therefore, the
dimension of the coupling constants following from the latter 3-vertices has a
smaller power of an inverse mass than our results imply. Consequently, the
3-vertices in the literature cannot be the first interaction terms of a
gauge-invariant theory. When no spins s > 2 are present in the process the
known QCD, QED or (super) gravity amplitudes are obtained from the above
general amplitudes.Comment: 19 pages, Late
Navigating Around the Algebraic Jungle of QCD: Efficient Evaluation of Loop Helicity Amplitudes
A method is developed whereby spinor helicity techniques can be used to
simplify the calculation of loop amplitudes. This is achieved by using the
Feynman-parameter representation where the offending off-shell loop momenta do
not appear. Background Feynman gauge also helps to simplify the calculations.
This method is applicable to any Feynman diagram with any number of loops as
long as the external masses can be ignored, and it is at least as efficient as
the string technique in the special circumstances when the latter can be used.
In order to minimize the very considerable algebra encountered in non-abelian
gauge theories, graphical methods are developed for most of the calculations.
This enables the large number of terms encountered to be organized visually in
the Feynman diagram without the necessity of having to write down any of them
algebraically. A one-loop four-gluon amplitude in a particular helicity
configuration is computed explicity to illustrate the method.Comment: 22 pages of text in PLAIN TEX, plus 14 figures in 10 pages stored in
a POSTSCRIPT file (attached to the end of the PLAIN TEX file); McGill
preprint no. McGill/92-3
BHAGEN-1PH: A Monte Carlo event generator for radiative Bhabha scattering
BHAGEN-1PH is a FORTRAN program providing fast Monte Carlo event generation
of the process , within electroweak theory, for
both unpolarized beams and also for the longitudinally polarized electron beam.
The program is designed for final leptons outside a small cone around the
initial leptons direction and has a new algorithm allowing also for a fast
generation of non collinear initial and final emission, as well as for
asymmetric and different angular cuts for final leptons.Comment: 23 pages, plain Tex, no figure
Emission of Two Hard Photons in Large-Angle Bhabha Scattering
A closed expression for the differential cross section of the large-angle
Bhabha scattering which explicitly takes into account the leading and
next-to-leading contributions due to the emission of two hard photons is
presented. Both collinear and semi-collinear kinematical regions are
considered. The results are illustrated by numerical calculations.Comment: 13 pages, LaTeX, 1 PostScript figure, submitted to Nucl. Phys.
Simulation of the process within electroweak theory with longitudinally polarized initial electrons
We present simple analytic expressions for the distributions of the Bhabha
scattering process with emission of one hard photon, including weak boson
exchanges, and with longitudinal polarization of the initial electron. The
results from the Monte Carlo generator BHAGEN-1PH, based on these expressions,
are presented and compared, for the unpolarized case, with those existing in
literature.Comment: 9 pages, plain Tex, no figures, small change in Table
All electroweak four fermion processes in electron-positron collisions
This paper studies the electroweak production of all possible four fermion
states in e+ e- collisions. Since the methods employed to evaluate the complete
matrix elements and phase space are very general, all four fermion final states
in which the charged particles are detected can be considered. Also all kinds
of experimental cuts can be imposed. With the help of the constructed event
generator a large number of illustrative results is obtained, which show the
relevance of backgrounds to a number of signals. For LEP 200 the W-pair signal
and its background are discussed, for higher energies also Z-pair and single W
and Z signals and backgrounds are presented.Comment: INLO-PUB-1/94,NIKHEF-H/94-08, LaTeX, uses axodraw.sty (appended to
end of TeX file), 45 page
Massive Spin-5/2 Fields Coupled to Gravity: Tree-Level Unitarity vs. the Equivalence Principle
I show that the gravitational scattering amplitudes of a spin-5/2 field with
mass violate tree-level unitarity at energies
if the coupling to gravity is minimal.
Unitarity up to energies is restored by adding a
suitable non-minimal term, which gives rise to interactions violating the
(strong) equivalence principle. These interactions are only relevant at
distances d\lequiv 1/m.Comment: 6 pages, NYU-TH.93/01/01, (a misprint corrected, minor modifications
of the text
Higgs production in at LEP and NLC
Predictions for Higgs production with processes of the type at LEP~2 and the NLC are calculated. Short
analytical formulae describe the double differential distribution in the
invariant masses of the and pairs. The total cross
section may be got with two numerical integrations. The various
Higgs-background interferences vanish either identically or are small. The
background contributions depend strongly on cuts applied on the invariant
masses.Comment: 8 pages (LaTeX), 5 figures ( 3 of them not included, a uuencoded file
containing the LaTeX and all postscript files is available via anonymous ftp
at ftp://ftp.ifh.de/pub/preprint/desy95-057.uu
Cross section of the processes , , , in the energy region 200 MeV 3 GeV
The cross section for different processes induced by annihilation,
in the kinematical limit
, is
calculated taking into account first order corrections to the amplitudes and
the corrections due to soft emitted photons, with energy in the center of mass of the colliding beams. The results
are given separately for charge--odd and charge--even terms in the final
channels and . In case of pions, form
factors are taken into account. The differential cross sections for the
processes: , , have been calculated and the
corresponding formula are given in the ultrarelativistic limit . For a quantitative evaluation of the
contribution of higher order of the perturbation theory, the production of
, including radiative corrections, is calculated in the approach of
the lepton structure functions. This allows to estimate the precision of the
obtained results as better than 0.5% outside the energy region corresponding to
narrow resonances. A method to integrate the cross section, avoiding the
difficulties which arise from singularities is also described.Comment: 25 pages 3 firgur
Multiloop String-Like Formulas for QED
Multiloop gauge-theory amplitudes written in the Feynman-parameter
representation are poised to take advantage of two important developments of
the last decade: the spinor-helicity technique and the superstring
reorganization. The former has been considered in a previous article; the
latter will be elaborated in this paper. We show here how to write multiloop
string-like formulas in the Feynman-parameter representation for any process in
QED, including those involving other non-electromagnetic interactions. The
general connection between the Feynman-parameter approach and the
superstring/first-quantized approach is discussed. In the special case of a
one-loop multi-photon amplitude, these formulas reduce to the ones obtained by
the superstring and the first quantized methods. The string-like formulas
exhibits a simple gauge structure which makes the Ward-Takahashi identity
apparent, and enables the integration-by-parts technique of Bern and Kosower to
be applied, so that gauge-invariant parts can be extracted diagram-by-diagram
with the seagull vertex neglected.Comment: 25 pages in Plain Tex, plus four figures in a postscript file;
McGill/92-5
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