2,187 research outputs found

### Electroweak Corrections to e+e- --> 4 fermions

The calculation of the full electroweak O(alpha) corrections to the
charged-current four-fermion production processes e+e- --> nu_tau tau+ mu-
anti-nu_mu, u anti-d mu- anti-nu_mu, and u anti-d s anti-c is briefly reviewed.
The calculation is performed using the complex-mass scheme for the gauge-boson
resonances. The evaluation of the occurring one-loop tensor integrals, which
include 5- and 6-point functions, requires new techniques. The effects of the
complete O(alpha) corrections to the total cross section and to the
production-angle distribution are discussed and compared to predictions based
on the double-pole approximation, revealing that the latter approximation is
not sufficient to fully exploit the potential of a future linear collider in an
analysis of W-boson pairs at high energies.Comment: 5 pages, LaTeX, 6 postscript figures, to appear in the proceedings of
the "7th International Symposium on Radiative Corrections (RADCOR05)", Shonan
Village, 200

### Four-fermion production at gamma gamma colliders: 2. Radiative corrections in double-pole approximation

The O(alpha) electroweak radiative corrections to gamma gamma --> WW --> 4f
within the electroweak Standard Model are calculated in double-pole
approximation (DPA). Virtual corrections are treated in DPA, leading to a
classification into factorizable and non-factorizable contributions, and
real-photonic corrections are based on complete lowest-order matrix elements
for gamma gamma --> 4f + gamma. Soft and collinear singularities appearing in
the virtual and real corrections are combined alternatively in two different
ways, namely by using the dipole subtraction method or by applying phase-space
slicing. The radiative corrections are implemented in a Monte Carlo generator
called COFFERgammagamma, which optionally includes anomalous triple and quartic
gauge-boson couplings in addition and performs a convolution over realistic
spectra of the photon beams. A detailed survey of numerical results comprises
O(alpha) corrections to integrated cross sections as well as to angular,
energy, and invariant-mass distributions. Particular attention is paid to the
issue of collinear-safety in the observables.Comment: 42 pages, latex, 34 postscript figure

### Radiative corrections to e+e- --> WW --> 4f with RACOONWW

RACOONWW is the first Monte Carlo generator for e+e- --> WW --> 4f(+gamma)
that includes the electroweak O(alpha) radiative corrections in the double-pole
approximation completely. Some numerical results for LEP2 energies are
discussed, and the predictions for the total W-pair cross section are
confronted with LEP2 data.Comment: 7 pages, latex, 5 postscript files, to appear in the Proceedings of
the 5th Zeuthen Workshop on Elementary Particle Theory ``Loops and Legs in
Quantum Field Theory'', Bastei/Koenigstein, Germany, April 9-14, 200

### Electroweak corrections and anomalous triple gauge-boson couplings in WW and WZ production at the LHC

We have analysed the production of WW and WZ vector-boson pairs at the LHC.
These processes give rise to four-fermion final states, and are particularly
sensitive to possible non-standard trilinear gauge-boson couplings. We have
studied the interplay between the influence of these anomalous couplings and
the effect of the complete logarithmic electroweak O(\alpha) corrections.
Radiative corrections to the Standard Model processes in double-pole
approximation and non-standard terms due to trilinear couplings are implemented
into a Monte Carlo program for p p -> 4f (+\gamma) with final states involving
four or two charged leptons. We numerically investigate purely leptonic final
states and find that electroweak corrections can fake new-physics signals,
modifying the observables by the same amount and shape, in kinematical regions
of statistical significance.Comment: 19 pages, LaTex, 12 eps figure

### The complex-mass scheme for perturbative calculations with unstable particles

Perturbative calculations with unstable particles require the inclusion of
their finite decay widths. A convenient, universal scheme for this purpose is
the complex-mass scheme. It fully respects gauge-invariance, is
straight-forward to apply, and has been successfully used for the calculation
of various tree-level processes and of the electroweak radiative corrections to
e+ e- -> 4f and H -> 4f.Comment: 5 pages, LaTeX, to appear in the proceedings of the "8th DESY
Workshop on Elementary Particle Theory, Loops and Legs in Quantum Field
Theory", Eisenach, 200

### Techniques for one-loop tensor integrals in many-particle processes

We briefly sketch the methods for a numerically stable evaluation of tensor
one-loop integrals that have been used in the calculation of the complete
electroweak one-loop corrections to \Pep\Pem\to4 fermions. In particular, the
improvement of the new methods over the conventional Passarino--Veltman
reduction is illustrated for some 4-point integrals in the delicate limits of
small Gram (and other kinematical) determinants.Comment: 5 pages, latex, 2 postscript figures, to appear in the proceedings of
the "7th International Symposium on Radiative Corrections (RADCOR2005)",
Shonan Village, 200

### Gauge-Boson Production in Electron-Photon Collisions

We summarize results for the processes $e^-\gamma \to \nu W^-, e^-Z,
e^-\gamma$ within the electroweak Standard Model. We discuss the essential
features of the corresponding lowest-order cross-sections and present numerical
results for the unpolarized cross-sections including the complete $O(\alpha)$
virtual, soft-photonic, and hard-photonic corrections. While at low energies
the weak corrections are dominated by the leading universal corrections, at
high energies we find large, non-universal corrections, which arise from vertex
and box diagrams involving non-Abelian gauge couplings.Comment: 7 pages + 5 figures appended as ps-files (uuencoded), latex, CERN-TH.
6980/9

### Approximations for W-Pair Production at Linear-Collider Energies

We determine the accuracy of various approximations to the O(alpha)
corrections for on-shell W-pair production. While an approximation based on the
universal corrections arising from initial-state radiation, from the running of
alpha, and from corrections proportional to m_t^2 fails in the Linear-Collider
energy range, a high-energy approximation improved by the exact universal
corrections is sufficiently good above about 500GeV. These results indicate
that in Monte Carlo event generators for off-shell W-pair production the
incorporation of the universal corrections is not sufficient and more
corrections should be included.Comment: 7 pages, late

### Reduction schemes for one-loop tensor integrals

We present new methods for the evaluation of one-loop tensor integrals which
have been used in the calculation of the complete electroweak one-loop
corrections to e+ e- -> 4 fermions. The described methods for 3-point and
4-point integrals are, in particular, applicable in the case where the
conventional Passarino-Veltman reduction breaks down owing to the appearance of
Gram determinants in the denominator. One method consists of different variants
for expanding tensor coefficients about limits of vanishing Gram determinants
or other kinematical determinants, thereby reducing all tensor coefficients to
the usual scalar integrals. In a second method a specific tensor coefficient
with a logarithmic integrand is evaluated numerically, and the remaining
coefficients as well as the standard scalar integral are algebraically derived
from this coefficient. For 5-point tensor integrals, we give explicit formulas
that reduce the corresponding tensor coefficients to coefficients of 4-point
integrals with tensor rank reduced by one. Similar formulas are provided for
6-point functions, and the generalization to functions with more internal
propagators is straightforward. All the presented methods are also applicable
if infrared (soft or collinear) divergences are treated in dimensional
regularization or if mass parameters (for unstable particles) become complex.Comment: 55 pages, latex, some references updated and few comments added,
version to appear in Nucl. Phys.

### Electroweak corrections to e+ e- -> f anti-f H

Some of the most interesting Higgs-production processes at future e+ e-
colliders are of the type e+ e- -> f anti-f H. We present a calculation of the
complete O(alpha) corrections to these processes in the Standard Model for
final-state neutrinos and top quarks. Initial-state radiation beyond O(alpha)
at the leading-logarithmic level as well as QCD corrections are also included.
The electroweak corrections turn out to be sizable and reach the order of
+/-10% and will thus be an important part of precise theoretical predictions
for future e+ e- colliders.Comment: 4 pages, LaTeX, 4 postscript figures, talk given at Loops and Legs
2004, 7th DESY Workshop on Elementary Particle Theory, Zinnowitz, German

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