186 research outputs found
Effective-Lagrangian approach to gamma gamma --> WW; II: Results and comparison with e+e- --> WW
We present a study of anomalous electroweak gauge-boson couplings which can
be measured in e+e- and gamma gamma collisions at a future linear collider like
ILC. We consider the gauge-boson sector of a locally SU(2) x U(1) invariant
effective Lagrangian with ten dimension-six operators added to the Lagrangian
of the Standard Model. These operators induce anomalous three- and
four-gauge-boson couplings and an anomalous gamma gamma H coupling. We
calculate the reachable sensitivity for the measurement of the anomalous
couplings in gamma gamma --> WW. We compare these results with the reachable
precision in the reaction e+e- --> WW on the one hand and with the bounds that
one can get from high-precision observables in Z decays on the other hand. We
show that one needs both the e+e- and the gamma gamma modes at an ILC to
constrain the largest possible number of anomalous couplings and that the
Giga-Z mode offers the best sensitivity for certain anomalous couplings.Comment: 25 pages, 1 figure, 7 tables, comments, references and a table added;
to appear in EPJ
Stability and Symmetry Breaking in the General Two-Higgs-Doublet Model
A method is presented for the analysis of the scalar potential in the general
Two-Higgs-Doublet Model. This allows us to give the conditions for the
stability of the potential and for electroweak symmetry breaking in this model
in a very concise way. These results are then applied to two different Higgs
potentials in the literature, namely the MSSM and the Two-Higgs-Doublet
potential proposed by Gunion at al. All known results for these models follow
easily as special cases from the general results. In particular, in the
potential of Gunion et al. we can clarify some interesting aspects of the model
with the help of the proposed method.Comment: 20 pages, 3 figures, extended version, typos corrected, references
adde
Effective-Lagrangian approach to gamma gamma --> WW; I: Couplings and amplitudes
We consider the gauge-boson sector of a locally SU(2) x U(1) invariant
effective Lagrangian with ten dimension-six operators added to the Lagrangian
of the Standard Model. These operators induce anomalous three- and
four-gauge-boson couplings and an anomalous gamma gamma H coupling. In the
framework of this effective Lagrangian we calculate the helicity amplitudes and
differential and total cross sections for the process gamma gamma --> WW at a
photon collider. We give relations between different parts of the amplitudes
that show which linear combinations of anomalous couplings are measurable in
this reaction. The transformation properties of the differential cross section
under CP are discussed. We find that three linear combinations of CP conserving
and of CP violating couplings can be measured independently of the photon
polarisation in gamma gamma --> WW.Comment: 29 pages, 11 figures, references added; to appear in EPJ
CP Violation and the Width
We discuss the effect of CP-violating , and
couplings on the width . The
presence of such couplings leads in a natural way to an increase of this width
relative to the prediction of the standard model. Various strategies of a
direct search for such CP-violating couplings by using CP-odd observables are
outlined. The number of bosons required to obtain significant information
on the couplings in this way is well within the reach of present LEP
experiments.Comment: 18 pages, LaTeX, no figure
Soft Photons in Hadron-Hadron Collisions: Synchrotron Radiation from the QCD Vacuum?
We discuss the production of soft photons in high energy hadron-hadron
collisions. We present a model where quarks and antiquarks in the hadrons emit
``synchrotron light'' when being deflected by the chromomagnetic fields of the
QCD vacuum, which we assume to have a nonperturbative structure. This gives a
source of prompt soft photons with frequencies in the c.m.
system of the collision in addition to hadronic bremsstrahlung. In comparing
the frequency spectrum and rate of ``synchrotron'' photons to experimental
results we find some supporting evidence for their existence. We make an
exclusive--inclusive connection argument to deduce from the ``synchrotron''
effect a behaviour of the neutron electric formfactor proportional
to for . We find this to be consistent with
available data. In our view, soft photon production in high energy
hadron-hadron and lepton-hadron collisions as well as the behaviour of
electromagnetic hadron formfactors for low are thus sensitive probes of
the nonperturbative structure of the QCD vacuum.Comment: Heidelberg preprint HD-THEP-94-36, 31 pages, LaTeX + ZJCITE.sty
(included), 12 figures appended as uuencoded compressed ps-fil
Anomalous gauge-boson couplings and the Higgs-boson mass
We study anomalous gauge-boson couplings induced by a locally SU(2) x U(1)
invariant effective Lagrangian containing ten operators of dimension six built
from the boson fields of the Standard Model (SM) before spontaneous symmetry
breaking (SSB). After SSB some operators lead to new three- and
four-gauge-boson interactions, some contribute to the diagonal and off-diagonal
kinetic terms of the gauge bosons and to the mass terms of the W and Z bosons.
This requires a renormalisation of the gauge-boson fields, which, in turn,
modifies the charged- and neutral-current interactions, although none of the
additional operators contain fermion fields. Bounds on the anomalous couplings
from electroweak precision measurements at LEP and SLD are correlated with the
Higgs-boson mass m_H. Rather moderate values of anomalous couplings allow m_H
up to 500 GeV. At a future linear collider the triple-gauge-boson couplings
gammaWW and ZWW can be measured in the reaction e+e- --> WW. We compare three
approaches to anomalous gauge-boson couplings: the form-factor approach, the
addition of anomalous coupling terms to the SM Lagrangian after and, as
outlined above, before SSB. The translation of the bounds on the couplings from
one approach to another is not straightforward. We show that it can be done for
the process e+e- --> WW by defining new effective ZWW couplings.Comment: 50 pages, 4 figures; version to appear in EPJ
Odd C-P contributions to diffractive processes
We investigate contributions to diffractive scattering, which are odd under
C- and P-parity. Comparison of p- and p-p scattering indicates that
these odderon contributions are very small and we show how a diquark clustering
in the proton can explain this effect. A good probe for the odderon exchange is
the photo- and electroproduction of pseudo-scalar mesons. We concentrate on the
pi^0 and show that the quasi elastic pi^0-production is again strongly
suppressed for a diquark structure of the proton whereas the cross sections for
diffractive proton dissociation are larger by orders of magnitude and rather
independent of the proton structure.Comment: 18 pages, LaTex2e, graphicx package, 14 eps figures include
Measuring transversity densities in singly polarized hadron-hadron and lepton-hadron collisions
We show how the transverse polarization of a quark initiating a jet can be
probed by the azimuthal distribution of two hadrons (of large ) in the jet.
This permits a twist 2 asymmetry in hard processes when only one of the initial
particles is polarized transversely. Applications to hadron-hadron and
lepton-hadron scattering are discussed.Comment: 16 pages, LaTeX + EPSF, 2 postscript figures. PSU/TH/10
Transverse QCD Dynamics Near the Light Cone
Starting from the QCD Hamiltonian in near-light cone coordinates, we study
the dynamics of the gluonic zero modes. Euclidean 2+1 dimensional lattice
simulations show that the gap at strong coupling vanishes at intermediate
coupling. This result opens the possibility to synchronize the continuum limit
with the approach to the light cone.Comment: 15 pages, LaTeX, 3 figures (7 PS files
Two-loop QCD corrections of the massive fermion propagator
The off-shell two-loop correction to the massive quark propagator in an
arbitrary covariant gauge is calculated and results for the bare and
renormalized propagator are presented. The calculations were performed by means
of a set of new generalized recurrence relations proposed recently by one of
the authors. From the position of the pole of the renormalized propagator we
obtain the relationship between the pole mass and the \bar{MS} mass. This
relation confirms the known result by Gray et al.. The bare amplitudes are
given for an arbitrary gauge group and for arbitrary space-time dimensions.Comment: 18 pages LaTeX, misprints in formula (12) are correcte
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