69 research outputs found
Gauge and Matter Condensates in Realistic String Models
We examine the inter-relationship of the superpotential containing hidden and
observable matter fields and the ensuing condensates in free fermionic string
models. These gauge and matter condensates of the strongly interacting hidden
gauge groups play a crucial role in the determination of the physical
parameters of the observable sector. Supplementing the above information with
the requirement of modular invariance, we find that a generic model with only
trilinear superpotential allows for a degenerate (and sometimes pathological)
set of vacua. This degeneracy may be lifted by higher order terms in the
superpotential. We also point out some other subtle points that may arise in
calculations of this nature. We exemplify our observations by computing
explicitly the modular invariant gaugino and matter condensates in the flipped
SU(5) string model with hidden gauge group .Comment: 11 pages (7 if reduced
The dilaton-dominated supersymmetry breaking scenario in the context of the non-minimal supersymmetric model
The phenomenological consequences of the dilaton-type soft supersymmetry
breaking terms in the context of the next to minimal supersymmetric standard
model are investigated. We always find a very low top quark mass. As a
consequence such string vacua are excluded by recent experimental results. The
viability of the solution of the term through the introduction of a gauge
singlet field is also briefly discussed.Comment: 10 pages,LATE
Couplings in Asymmetric Orbifolds and Grand Unified String Models
Using the bosonic supercurrent (or covariant lattice) formalism, we review
how to compute scattering amplitudes in asymmetric orbifold string models. This
method is particularly useful for calculating scattering of multiple
asymmetrically twisted string states, where the twisted states are rewritten as
ordinary momentum states. We show how to reconstruct some of the 3-family grand
unified string models in this formalism, and identify the quantum numbers of
the massless states in their spectra. The discrete symmetries of these models
are rather intricate. The superpotentials for the 3-family E_6 model and a
closely related SO(10) model are discussed in some detail. The forms of the
superpotentials of the two 3-family SU(6) models (with asymptotically-free
hidden sectors SU(3) and SU(2) \otimes SU(2)) are also presented.Comment: 54 pages, Revtex 3.0 (to appear in Nucl. Phys. B
Physics Implications of Flat Directions in Free Fermionic Superstring Models II: Renormalization Group Analysis
We continue the investigation of the physics implications of a class of flat
directions for a prototype quasi-realistic free fermionic string model (CHL5),
building upon the results of the previous paper in which the complete mass
spectrum and effective trilinear couplings of the observable sector were
calculated to all orders in the superpotential. We introduce soft supersymmetry
breaking mass parameters into the model, and investigate the gauge symmetry
breaking patterns and the renormalization group analysis for two representative
flat directions, which leave an additional as well as the SM gauge
group unbroken at the string scale. We study symmetry breaking patterns that
lead to a phenomenologically acceptable hierarchy, and for electroweak and intermediate
scale symmetry breaking, respectively, and the associated mass
spectra after electroweak symmetry breaking. The fermion mass spectrum exhibits
unrealistic features, including massless exotic fermions, but has an
interesting -quark hierarchy and associated CKM matrix in one case. There
are (some) non-canonical effective terms, which lead to a non-minimal
Higgs sector with more than two Higgs doublets involved in the symmetry
breaking, and a rich structure of Higgs particles, charginos, and neutralinos,
some of which, however, are massless or ultralight. In the electroweak scale
cases the scale of supersymmetry breaking is set by the mass, with the
sparticle masses in the several TeV range.Comment: 38 pages, 5 figures, LaTex. Minor correction
A new scenario for string unification
We present a new scenario for gauge coupling unification in flipped SU(5)
string models, which identifies the scale of SU(3) and SU(2)
unification with the empirical ~GeV scale, and the
scale of SU(5) and U(1) unification with the theoretical ~GeV string unification scale. The vacuum shift
necessary for the cancellation of the anomalous and an SU(4)
hidden sector with fractionally-charged particles, play a crucial role in the
dynamical determination of all intermediate mass scales in this scenario.Comment: 8 pages, LaTeX, 2 figures (uuencoded
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
Searching singlet extensions of the supersymmetric standard model in orbifold compactification of heterotic string
We search for supersymmetric standard model realizations with extra singlets
and extra using the heterotic string compactification on the orbifold with two Wilson lines. We analyze the vacuum restabilization
mechanism for three representative Pati-Salam string models obtained in the
literature and present detailed results for the effective superpotential
compatible with the string selection rules. An automated selection of
semi-realistic vacua along flat directions in the non-Abelian singlet modes
field space is performed by requiring the presence of massless pairs of
electroweak Higgs bosons having trilinear superpotential couplings with
massless singlet modes and the decoupling of color triplet exotic modes needed
to suppress and number violating processes.Comment: revtex4 format, 21 pages, 7 tables, shortened version added
reference
Singularities In Scalar-Tensor Cosmologies
In this article, we examine the possibility that there exist special
scalar-tensor theories of gravity with completely nonsingular FRW solutions.
Our investigation in fact shows that while most probes living in such a
Universe never see the singularity, gravity waves always do. This is because
they couple to both the metric and the scalar field, in a way which effectively
forces them to move along null geodesics of the Einstein conformal frame. Since
the metric of the Einstein conformal frame is always singular for
configurations where matter satisfies the energy conditions, the gravity wave
world lines are past inextendable beyond the Einstein frame singularity, and
hence the geometry is still incomplete, and thus singular. We conclude that the
singularity cannot be entirely removed, but only be made invisible to most, but
not all, probes in the theory.Comment: 23 pages, latex, no figure
Cosmological viability of f(R)-gravity as an ideal fluid and its compatibility with a matter dominated phase
We show that f(R)-gravity can, in general, give rise to cosmological viable
models compatible with a matter-dominated epoch evolving into a late
accelerated phase. We discuss the various representations of f(R)-gravity as an
ideal fluid or a scalar-tensor gravity theory, taking into account conformal
transformations. We point out that mathematical equivalence does not
correspond, in several cases, to the physical equivalence of Jordan frame and
Einstein frame. Finally, we show that wide classes of f(R)-gravity models,
including matter and accelerated phases, can be phenomenologically
reconstructed by means of observational data. In principle, any popular
quintessence models could be "reframed" as an f(R)-gravity model.Comment: 11 pages, 1 figur
- …