976 research outputs found
Decays of W Bosons to Charginos and Neutralinos
The region of the supersymmetry parameter space, in which charginos decay
predominantly into sneutrinos and leptons: chi+ -> \tilde{\nu} + l+, is not
excluded experimentally for small mass differences between charginos and
sneutrinos. The decay sneutrinos are invisible in R-parity conserving theories
since they are either the lightest supersymmetric particles or they decay
primarily into the channel: neutrino + lightest neutralino. If the energy of
the decay leptons is so small that they escape detection, chargino events e+e-
-> chi+chi- in e+e- collisions remain invisible, eroding the excluded chargino
mass range at LEP. This region of the supersymmetry parameter space can partly
be covered by searching for single W events in e+e- -> W+W-$, with one W boson
decaying to leptons or quark jets, but the second W boson decaying to
(undetected) charginos and neutralinos.Comment: 11 pages, 3 figures, uses epsfig.st
Stability of Circular Orbits in General Relativity: A Phase Space Analysis
Phase space method provides a novel way for deducing qualitative features of
nonlinear differential equations without actually solving them. The method is
applied here for analyzing stability of circular orbits of test particles in
various physically interesting environments. The approach is shown to work in a
revealing way in Schwarzschild spacetime. All relevant conclusions about
circular orbits in the Schwarzschild-de Sitter spacetime are shown to be
remarkably encoded in a single parameter. The analysis in the rotating Kerr
black hole readily exposes information as to how stability depends on the ratio
of source rotation to particle angular momentum. As a wider application, it is
exemplified how the analysis reveals useful information when applied to motion
in a refractive medium, for instance, that of optical black holes.Comment: 20 pages. Accepted for publication in Int. J. theor. Phy
Measuring the Spin of the Higgs Boson
By studying the threshold dependence of the excitation curve and the angular
distribution in Higgs-strahlung at e+e- colliders, e+e- -> HZ, the spin of the
Higgs boson in the Standard Model and related extensions can be determined
unambiguously in a model-independent way.Comment: 10 pages, 1 Postscript figure, sign typo correcte
Two-Photon Decay Widths of Higgs Particles
Two--photon decays of Higgs bosons are important channels for the search of
these particles in the intermediate mass range at the colliders LHC and
SSC. Dynamical aspects of the Higgs coupling to two photons can also be studied
by means of the fusion of Higgs particles at high--energy
ee linear colliders. Extending earlier analyses which had been
restricted to the Standard Model, we present in this note the QCD radiative
corrections to the decay widths of scalar and pseudoscalar
Higgs particles in multi--doublet extensions of the Higgs sector, as realized
for instance in supersymmetric theories.Comment: 8pages + 3figures (not included, a hard copy is available from A.D.
upon request), Prep. DESY 92-170 (to appear in Phys. Lett. B
Spiral Multi-component Structure in Pade - Approximant QCD
We present a graphical method of analyzing the infra-red fixed point
structure of Pade approximant QCD. The analysis shows a spiral multi-component
couplant structure as well as an infra-red attractor behavior of PQCD couplant
for all flavors .Comment: 78 pages, 4 tables, 44 graph
Classical and quantum properties of a 2-sphere singularity
Recently Boehmer and Lobo have shown that a metric due to Florides, which has
been used as an interior Schwarzschild solution, can be extended to reveal a
classical singularity that has the form of a two-sphere. Here the singularity
is shown to be a scalar curvature singularity that is both timelike and
gravitationally weak. It is also shown to be a quantum singularity because the
Klein-Gordon operator associated with quantum mechanical particles approaching
the singularity is not essentially self-adjoint.Comment: 10 pages, 1 figure, minor corrections, final versio
Ricci flows and expansion in axion-dilaton cosmology
We study renormalization-group flows by deforming a class of conformal
sigma-models. We consider overall scale factor perturbation of Einstein spaces
as well as more general anisotropic deformations of three-spheres. At leading
order in alpha, renormalization-group equations turn out to be Ricci flows. In
the three-sphere background, the latter is the Halphen system, which is exactly
solvable in terms of modular forms. We also analyze time-dependent deformations
of these systems supplemented with an extra time coordinate and time-dependent
dilaton. In some regimes time evolution is identified with
renormalization-group flow and time coordinate can appear as Liouville field.
The resulting space-time interpretation is that of a homogeneous isotropic
Friedmann-Robertson-Walker universe in axion-dilaton cosmology. We find as
general behaviour the superposition of a big-bang (polynomial) expansion with a
finite number of oscillations at early times. Any initial anisotropy disappears
during the evolution.Comment: 22 page
Gluon Radiation Off Scalar Stop Particles
We present the distributions for gluon radiation off stop-antistop particles
produced in annihilation: . For
high energies the splitting functions of the fragmentation processes and are derived; they are
universal and apply also to high-energy stop particles produced at hadron
colliders.Comment: 9 pages, 2 figures as uuencoded ps files, Latex, uses epsfig,
complete postscript version at
ftp://x4u2.desy.de/pub/preprints/desy/1994/desy94-235.p
Quantum Corrections to Lorentz Invariance Violating Theories: Fine-Tuning Problem
It is of general agreement that a quantum gravity theory will most probably
mean a breakdown of the standard structure of space-time at the Planck scale.
This has motivated the study of Planck-scale Lorentz Invariance Violating (LIV)
theories and the search for its observational signals. Yet, it has been
recently shown that, in a simple scalar-spinor Yukawa theory, radiative
corrections to tree-level Planck-scale LIV theories can induce large Lorentz
violations at low energies, in strong contradiction with experiment, unless an
unnatural fine-tuning mechanism is present. In this letter, we show the
calculation of the electron self-energy in the framework given by the
Myers-Pospelov model for a Lorentz Invariance Violating QED. We find a
contribution that depends on the prefered's frame four-velocity which is not
Planck-scale suppressed, showing that this model suffers from the same disease.
Comparison with Hughes-Drever experiments requires a fine-tuning of 21 orders
of magnitude for this model not to disagree with experiment.Comment: 10 pages, no figures. Version acceptd in Physics Letters
Universal moduli-dependent thresholds in Z(2)xZ(2) orbifolds
In the context of a recently proposed method for computing exactly string
loop corrections regularized in the infra-red, we determine and calculate the
universal moduli-dependent part of the threshold corrections to the gauge
couplings for the symmetric orbifold model. We show that these
corrections decrease the unification scale of the underlying effective field
theory. We also comment on the relation between this infra-red regularization
scheme and other proposed methods.Comment: 12 pages, Latex, contains two figures, final version, typos correcte
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