23 research outputs found
Moduli and K\"ahler potential in fermionic strings
We study the problem of identifying the moduli fields in fermionic
four-dimensional string models. We deform a free-fermionic model by introducing
exactly marginal operators in the form of Abelian Thirring interactions on the
world-sheet, and show that their couplings correspond to the untwisted moduli
fields. We study the consequences of this method for simple free-fermionic
models which correspond to orbifolds and obtain their moduli
space and K\"ahler potential by symmetry arguments and by direct calculation of
string scattering amplitudes. We then generalize our analysis to more
complicated fermionic structures which arise in constructions of realistic
models corresponding to asymmetric orbifolds, and obtain the moduli space and
K\"ahler potential for this case. Finally we extend our analysis to the
untwisted matter sector and derive expressions for the full K\"ahler potential
to be used in phenomenological applications, and the target space duality
transformations of the corresponding untwisted matter fields.Comment: 27pp Latex text, no figs, CERN-TH.7259/94, CTP-TAMU-14/94 and
ACT-06/9
Squarks Below the Z
We investigate the possibility that the difference between the measurements
of from the hadronic branching ratio of the and the world
average of other measurements is due to the decay of the into quark,
anti-squark, and gluino. Consequences for supersymmetry breaking models are
discussed.Comment: 9 pages, 4 figures not include
Number of Generations in Free Fermionic String Models,
In string theory there seems to be an intimate connection between spacetime
and world-sheet physics. Following this line of thought we investigate the
family problem in a particular class of string solutions, namely the free
fermionic string models. We find that the number of generations is
related to the index of the supersymmetry generator of the underlying
internal superconformal field theory which is always present in any
spacetime supersymmetric string vacuum. We also derive a formula for the index
and thus for the number of generations which is sensitive to the boundary
condition assignments of the internal fermions and to certain coefficients
which determine the weight with which each spin-structure of the model
contributes to the one-loop partition function. Finally we apply our formula to
several realistic string models in order to derive and we verify our
results by constructing explicitly the massless spectrum of these string
models.Comment: 17 pages, Plain Tex, no figures
The Search for a realistic flipped SU(5) string model
We present an extensive search for a general class of flipped models
built within the free fermionic formulation of the heterotic string. We
describe a set of algorithms which constitute the basis for a computer program
capable of generating systematically the massless spectrum and the
superpotential of all possible models within the class we consider. Our search
through the huge parameter space to be explored is simplified considerably by
the constraint of spacetime supersymmetry and the need for extra representations beyond the standard ones in order to possibly achieve string
gauge coupling unification at scales of {\cal O}(10^{18}\GeV). Our results
are remarkably simple and evidence the large degree of redundancy in this kind
of constructions. We find one model with gauge group SU(5)\times
U(1)_\ty\times SO(10)_h\times SU(4)_h\times U(1)^5 and fairly acceptable
phenomenological properties. We study the - and -flatness constraints and
the symmetry breaking pattern in this model and conclude that string gauge
coupling unification is quite possible.Comment: 44 page
Symmetry of Gravity
Two-dimensional gravity in the light-cone gauge was shown by Polyakov to
exhibit an underlying Kac-Moody symmetry, which may be used to
express the energy-momentum tensor for the metric component in terms
of the currents {\it via}\ the Sugawara construction. We review some
recent results which show that in a similar manner, and
gravities have underlying and
Kac-Moody symmetries respectively.Comment: 10 pages (Talk presented at the Trieste Summer School in High-Energy
Physics, August 1991
New Constraints on Neutralino Dark Matter in the Supersymmetric Standard Model
We investigate the prospects for neutralino dark matter within the
Supersymmetric Standard Model (SSM) including the constraints from universal
soft supersymmetry breaking and radiative breaking of the electroweak symmetry.
The latter is enforced by using the one-loop Higgs effective potential which
automatically gives the one-loop corrected Higgs boson masses. We perform an
exhaustive search of the allowed five-dimensional parameter space and find that
the neutralino relic abundance depends most strongly on the
ratio . For the relic abundance is almost
always much too large, whereas for the opposite occurs. For
there are wide ranges of the remaining parameters for which
. We also determine that m_{\tilde q}\gsim250\GeV and
m_{\tilde l}\gsim100\GeV are necessary in order to possibly achieve
. These lower bounds are much weaker than the corresponding
ones derived previously when radiative breaking was {\it not} enforced.Comment: 12 pages plus 6 figures (not included), CERN-TH.6584/92,
CTP-TAMU-56/92, UAHEP921
Accurate Neutralino Relic Density Computations in Supergravity Models
We investigate the question of the proper thermal averaging of neutralino
annihilation amplitudes which possess poles and thresholds, as they impact on
the calculated neutralino relic density and therefore on the cosmological
viability of supersymmetric models. We focus on two typical resonances, namely
the boson and the lightest Higgs boson (). In the context of
supergravity models with radiative electroweak symmetry breaking, an
exploration of the whole parameter space of the model is possible and the
overall relevance of these sophisticated analyses can be ascertained. As an
example we chose the minimal supergravity model since the presence of
such poles is essential to obtain a cosmologically acceptable model. We find
that the proper thermal averaging is important for individual points in
parameter space and that the fraction of cosmologically acceptable points is
increased somewhat by the accurate procedure. However, qualitatively the new
set of points is very similar to that obtained previously using the usual
series approximations to the thermal average. We conclude that all
phenomenological analyses based on the previously determined cosmologically
allowed set remain valid.Comment: 15 pages, 9 figures (available upon request as uuencoded file or
separate ps files), tex (harvmac) CTP-TAMU-14/9