843 research outputs found
Electroweak-correction effects in gauge-boson pair production at the LHC
We have studied the effect of one-loop logarithmic electroweak radiative
corrections on WZ and production processes at the LHC. We present
analytical results for the leading-logarithmic electroweak corrections to the
corresponding partonic processes du -> WZ, Wgamma. Using the leading-pole
approximation we implement these corrections into Monte Carlo programs for
. We find that electroweak corrections
lower the predictions by 5-20% in the physically interesting region of large
transverse momentum and small rapidity separation of the gauge bosons.Comment: 28 pages, LaTex, 13 eps figures included; references added and
corrected typo
Electroweak corrections to W-boson pair production at the LHC
Vector-boson pair production ranks among the most important Standard-Model
benchmark processes at the LHC, not only in view of on-going Higgs analyses.
These processes may also help to gain a deeper understanding of the electroweak
interaction in general, and to test the validity of the Standard Model at
highest energies. In this work, the first calculation of the full one-loop
electroweak corrections to on-shell W-boson pair production at hadron colliders
is presented. We discuss the impact of the corrections on the total cross
section as well as on relevant differential distributions. We observe that
corrections due to photon-induced channels can be amazingly large at energies
accessible at the LHC, while radiation of additional massive vector bosons does
not influence the results significantly.Comment: 29 pages, 15 figures, 4 tables; some references and comments on
\gamma\gamma -> WW added; matches version published in JHE
Radiative Corrections to in the Electroweak Standard Model
The cross-section for with arbitrary polarized
photons is calculated within the electroweak Standard Model including the
complete virtual and soft-photonic corrections. We present a
detailed numerical discussion of the radiative corrections with particular
emphasis on the purely weak corrections. These are usually of the order of
1--10\% for energies up to 1 TeV. For unpolarized or equally polarized photons
they reach almost 10\% close to threshold. The large corrections cannot be
traced back to a universal origin like the running of or the
-parameter. Apart from the energy region around the Higgs resonance
the weak corrections are widely independent
of the Higgs-boson mass.Comment: 22 pages, LaTeX (compressed, uuencoded), 20 figures as compressed
uuencoded ps-files, complete ps-file available via anonymous ftp from
ftp://ftp.physik.uni-wuerzburg.de/pub/preprint/WUE-ITP-95-017.p
QCD Corrections to Electroweak Annihilation Decays of Superheavy Quarkonia
QCD corrections to all the allowed decays of superheavy groundstate quarkonia
into electroweak gauge and Higgs bosons are presented. For quick estimates,
approximations that reproduce the exact results within less than at worst two
percent are also given.Comment: 20 pages RevTeX, 9 figures. The complete paper, including figures, is
also available via anonymous ftp at (129.13.102.139) as
ftp://ttpux2.physik.uni-karlsruhe.de/ttp95-05/ttp95-05.ps, or via www at
http://ttpux2.physik.uni-karlsruhe.de/cgi-bin/preprints
Of Higgs, Unitarity and other Questions
On the verge of conclusive checks on the Standard Model by the LHC, we
discuss some of the basic assumptions. The reason for this analysis stems from
a recent proposal of an Electroweak Model based on a nonlinearly realized gauge
group SU(2) X U(1), where, in the perturbative approximation, there is no Higgs
boson. The model enjoys the Slavnov-Taylor identities and therefore the
perturbative unitarity. On the other hand, it is commonly believed that the
existence of the Higgs boson is entangled with the property of unitarity, when
high energy processes are considered. The argument is based mostly on the
Froissart bound and on the Equivalence Theorem. In this talk we briefly review
some of our objections on the validity of such arguments. Some open questions
are pointed out, in particular on the limit of zero mass for the vector mesons
and on the fate of the longitudinal polarizations.Comment: 23 pages, 1 figure, presented by Ruggero Ferrari at the International
Conference "Gauge Fields. Yesterday, Today, Tomorrow" in honor of A.A.
Slavnov. Moscow, January 19-24 201
Radiative Corrections to ZZ->ZZ in the Electroweak Standard Model
The cross-section for ZZ->ZZ with arbitrarily polarized Z bosons is
calculated within the electroweak Standard Model including the complete
O(alpha) corrections. We show the numerical importance of the radiative
corrections and elaborate its characteristic features. The treatment of the
Higgs-boson resonance is discussed in different schemes including the
S-matrix-motivated pole scheme and the background-field method. The numerical
accuracy of the equivalence theorem is investigated by comparing the
cross-sections for purely longitudinal Z bosons obtained from the equivalence
theorem and from the complete calculation. In this context the full O(alpha)
corrections are also confronted with the enhanced corrections of O(alpha
M_H^2/s_w^2 M_W^2), which were frequently used in the literature.Comment: 35 pages LaTeX, 19 postscript figures include
The Phenomenology of a Top Quark Seesaw Model
The top quark seesaw mechanism offers a method for constructing a composite
Higgs field without the usual difficulties that accompany traditional
technicolor or topcolor theories. The focus of this article is to study the
phenomenology of the new physics required by this mechanism. After establishing
a set of criteria for a plausible top quark seesaw theory, we develop two
models, the first of which has a heavy weak singlet fermion with hypercharge
4/3 while the second has, in addition, a heavy weak singlet hypercharge -2/3
fermion. At low energies, these theories contain one or two Higgs doublets
respectively. We then derive the low energy effective Higgs potential in detail
for the two-doublet theory as well as study the likely experimental signatures
for both theories. A strong constraint on the one-doublet model is the measured
value of the rho parameter which permits the new heavy fermion to have a mass
of about 5-7 TeV, when the Higgs has a mass greater than 300 GeV. In the
two-doublet model, mixing of the new heavy Y=-2/3 fermion and the b quark
affects the prediction for R_b. In order to agree with the current limits on
R_b, the mass of this fermion should be at least 12 TeV. The mass of the heavy
Y=4/3 fermion in the two-doublet model is not as sharply constrained by
experiments and can be as light as 2.5 TeV.Comment: 33 pages, 12 figures, uses harvmac and picte
Dyson summation without violating Ward identities and the Goldstone-boson equivalence theorem
In contrast to the conventional treatment of gauge theories, in the
background-field method the Ward identities for connected Green functions are
not violated by Dyson summation of self-energies in finite orders of
perturbation theory. Thus, Dyson summation does not spoil gauge cancelations at
high energies which are ruled by the Goldstone-boson equivalence theorem.
Moreover, in the background-field method the precise formulation of the
equivalence theorem in higher orders (including questions of renormalization)
is simplified rendering actual calculations easier. Finally, the equivalence
theorem is also formulated for the Standard Model with a non-linearly realized
scalar sector and for the gauged non-linear -model.Comment: 26 pages, latex, complete ps-file available via anonymous ftp from
ftp://ftp.physik.uni-wuerzburg.de/pub/preprint/1996/WUE-ITP-96-002.ps,
reference added and some comments on Ref.[6] modifie
NLO QCD corrections to off-shell top-antitop production with leptonic decays at hadron colliders
We present details of a calculation of the cross section for hadronic
top-antitop production in next-to-leading order (NLO) QCD, including the decays
of the top and antitop into bottom quarks and leptons. This calculation is
based on matrix elements for \nu e e+ \mu- \bar{\nu}_{\mu}b\bar{b} production
and includes all non-resonant diagrams, interferences, and off-shell effects of
the top quarks. Such contributions are formally suppressed by the top-quark
width and turn out to be small in the inclusive cross section. However, they
can be strongly enhanced in exclusive observables that play an important role
in Higgs and new-physics searches. Also non-resonant and off-shell effects due
to the finite W-boson width are investigated in detail, but their impact is
much smaller than naively expected. We also introduce a matching approach to
improve NLO calculations involving intermediate unstable particles. Using a
fixed QCD scale leads to perturbative instabilities in the high-energy tails of
distributions, but an appropriate dynamical scale stabilises NLO predictions.
Numerical results for the total cross section, several distributions, and
asymmetries are presented for Tevatron and the LHC at 7 TeV, 8 TeV, and 14 TeV.Comment: 61 pp. Matches version published in JHEP; one more reference adde
Electroweak Sudakov Logarithms and Real Gauge-Boson Radiation in the TeV Region
Electroweak radiative corrections give rise to large negative,
double-logarithmically enhanced corrections in the TeV region. These are partly
compensated by real radiation and, moreover, affected by selecting
isospin-noninvariant external states. We investigate the impact of real gauge
boson radiation more quantitatively by considering different restricted final
state configurations. We consider successively a massive abelian gauge theory,
a spontaneously broken SU(2) theory and the electroweak Standard Model. We find
that details of the choice of the phase space cuts, in particular whether a
fraction of collinear and soft radiation is included, have a strong impact on
the relative amount of real and virtual corrections.Comment: 20 pages, 4 figure
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