178 research outputs found

### Top Quark Spin Polarization in ep Collision

We discuss the degree of spin polarization of single top quarks produced via
$Wg$ fusion process in $ep$ collision at TESLA+HERAp and CLIC+LHC energies
$\sqrt{s}=1.6$ and 5.3 TeV. For $eb \to t \bar{\nu}$ subprocess we show that
the top quark spin is completely polarized when the spin basis is chosen in the
direction of the incoming positron beam in the rest frame of top quark. A
description on how to combine the cross sections of $e^{+}b\to t\bar{\nu}$ and
$e^{+}g\to t\bar{b}\bar{\nu}$ processes is given. $e^{+}$-beam direction is
taken to be the favorite top quark spin decomposition axis in its rest frame
and it is found to be comparable with the ones in $pp$ collision. It is argued
that theoretical simplicity and experimental clearness are the advantage of
$ep$ collision.Comment: Revised version of Phys. Rev. D69 (2004)03401

### A compact representation of the 2 photon 3 gluon amplitude

A compact representation of the loop amplitude gamma gamma ggg -> 0 is
presented. The result has been obtained by using helicity methods and sorting
with respect to an irreducible function basis. We show how to convert spinor
representations into a field strength representation of the amplitude. The
amplitude defines a background contribution for Higgs boson searches at the LHC
in the channel H -> gamma gamma + jet which was earlier extracted indirectly
from the one-loop representation of the 5-gluon amplitude.Comment: 15 pages Latex, 6 eps files included, revised versio

### Recursive numerical calculus of one-loop tensor integrals

A numerical approach to compute tensor integrals in one-loop calculations is
presented. The algorithm is based on a recursion relation which allows to
express high rank tensor integrals as a function of lower rank ones. At each
level of iteration only inverse square roots of Gram determinants appear. For
the phase-space regions where Gram determinants are so small that numerical
problems are expected, we give general prescriptions on how to construct
reliable approximations to the exact result without performing Taylor
expansions. Working in 4+epsilon dimensions does not require an analytic
separation of ultraviolet and infrared/collinear divergences, and, apart from
trivial integrals that we compute explicitly, no additional ones besides the
standard set of scalar one-loop integrals are needed.Comment: Typo corrected in formula 79. 22 pages, Latex, 1 figure, uses
axodraw.st

### Numerical integration of one-loop Feynman diagrams for N-photon amplitudes

In the calculation of cross sections for infrared-safe observables in high
energy collisions at next-to-leading order, one approach is to perform all of
the integrations, including the virtual loop integration numerically. One would
use a subtraction scheme that removes infrared and collinear divergences from
the integrand in a style similar to that used for real emission graphs. Then
one would perform the loop integration by Monte Carlo integration along with
the integrations over final state momenta. In this paper, we have explored how
one can perform the numerical integration. We have studied the N-photon
scattering amplitude with a massless electron loop in order to have a case with
a singular integrand that is not, however, so singular as to require the
subtractions. We report results for N = 4, N = 5 with left-handed couplings,
and N=6.Comment: 30 pages including 5 figures. This is a revised version that is close
to the published versio

### The t->WZb decay in the Standard Model: A Critical Reanalysis

We compute the t->WZb decay rate, in the Standard Model, at the leading order
in perturbation theory, with special attention to the effects of the finite
widths of the W and Z bosons. These effects are extremely important, since the
t->WZb decay occurs near its kinematical threshold. They increase the value of
the decay rate by orders of magnitude near threshold or allow it below the
nominal threshold. We discuss a procedure to take into account the finite-width
effects and compare the results with previous studies of this decay. Within the
Standard Model, for a top quark mass in the range between 170 and 180 GeV, we
find BR(t->WZb) ~ 2 x 10^{-6}, which makes the observation at the LHC very
difficult if at all possible.Comment: 10 pages, 4 eps figures, LaTeX. Few references added and minor
changes in the text. Results unchanged. Final version to appear on PL

### Recursion Rules for Scattering Amplitudes in Non-Abelian Gauge Theories

We present a functional derivation of recursion rules for scattering
amplitudes in a non-Abelian gauge theory in a form valid to arbitrary loop
order. The tree-level and one-loop recursion rules are explicitly displayed.Comment: 18 pages, RevTeX, 2 postscript figures, a reference added, minor
typographical errors correcte

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