378 research outputs found

### Four Dimensional Gravitational Backgrounds Based on N=4 c=4 Superconformal Systems

We propose two new realizations of the N=4, $\hat{c}=4$ superconformal system
based on the compact and non-compact versions of parafermionic algebras. The
target space interpretation of these systems is given in terms of
four-dimensional target spaces with non-trivial metric and topology different
from the previously known four-dimensional semi-wormhole realization. The
proposed $\hat{c}=4$ systems can be used as a building block to construct
perturbatively stable superstring solutions with covariantized target space
supersymmetry around non-trivial gravitational and dilaton backgrounds.Comment: LateX, 21p

### Construction of Superstrings in Wormhole-Like Backgrounds

We construct a class of superstring solutions in non trivial space-time. The
existence of an $N=4$ world-sheet superconformal symmetry stabilizes our
solutions under perturbative string loop corrections and implies in target
space some unbroken space-time supersymmetries.Comment: 8 pages, no figures, preprint (Talk given at the International
Europhysics Conference on High Energy Physics, Marseille, 22-28 July 1993)
CERN-TH.7169/94 (Missprints corrected and title changed

### Gravitational Phenomena from Superstrings in Curved Spacetime

The four-dimensional superstring solutions define at low energy effective
supergravity theories. A class of them extends successfully the validity of the
standard model up to the string scale (${\cal O}(10^{17})~TeV$). We stress the
importance of string corrections which are relevant for low energy (${\cal
O}(1)~TeV$) predictions of gauge and Yukawa couplings as well as the spectrum
of the supersymmetric particles. A class of exact string solutions are also
presented, providing non trivial space-time backgrounds, from which we can draw
some lessons concerning the regions of space-time where the notion of the
effective field theory prescription make sense. We show that the string
gravitational phenomena may induce during the cosmological evolution,
transitions from one effective field theory prescription to a different one
where the geometrical and topological data, as well as the relevant observable
states are drastically different. Talk presented by C. Kounnas in the XXIX
Moriond Meeting, M\'eribel, France.Comment: LateX, 11pp., CERN-TH.7332/9

### Dynamical Topology Change, Compactification and Waves in a Stringy Early Universe

Exact string solutions are presented, where moduli fields are varying with
time. They provide examples where a dynamical change of the topology of space
is occurring. Some other solutions give cosmological examples where some
dimensions are compactified dynamically or simulate pre-big bang type
scenarios. Some lessons are drawn concerning the region of validity of
effective theories and how they can be glued together, using stringy
information in the region where the geometry and topology are not well defined
from the low energy point of view. Other time dependent solutions are presented
where a hierarchy of scales is absent. Such solutions have dynamics which is
qualitatively different and resemble plane gravitational waves. Talk presented
by E. Kiritsis in the 2\`eme Journ\'ee Cosmologie, Observatoire de Paris, 2-4
June 1994.Comment: LaTeX, 28pp. CERN-TH.7328/9

### Perturbative and Non-Perturbative Partial Supersymmetry Breaking; N=4 -> N=2 -> N=1

We show the existence of a supersymmetry breaking mechanism in string theory,
where N=4 supersymmetry is broken spontaneously to N=2 and N=1 with moduli
dependent gravitino masses. The spectrum of the spontaneously broken theory
with lower supersymmetry is in one-to-one correspondence with the spectrum of
the heterotic N=4 string. The mass splitting of the N=4 spectrum depends on the
compactification moduli as well as the three R-symmetry charges. We also show
that, in string theory, chiral theories can be obtained after spontaneous
breaking of extended supersymmetry. This was impossible at the level of field
theory. In the large moduli limit a restoration of the N=4 supersymmetry is
obtained. As expected the graviphotons and some of the gauge bosons become
massive in N=1 vacua. At some special points of the moduli space some of the
N=4 states with non-zero winding numbers and with spin 0 and {1/2} become
massless chiral superfields of the unbroken N=1 supersymmetry. Such vaccua have
a dual type II description, in which there are magnetically charged states with
spin 0 and {1/2} that become massless. The heterotic-type II duality suggests
some novel non-perturbative transitions on the type II side. Such transitions
do not seem to have a geometric interpretation, since they relate type II
vaccua with symmetric worlsheet structure to assymetric ones. The heteroric
interpretation of such a transition is an ordinary Higgsing of an SU(2) factor.
In the case of N=4 --> N=2, the perturbative N=2 prepotential is determined by
the perturbative N=4 BPS states. This observation let us to suggest a method
which determines the exact non-perturbative prepotential of the effective N=2
supergravity using the shifted spectrum of the non-perturbative BPS states of
the underlying N=4 theory.Comment: 36 pages, Latex. An important part is added. We point out that there
are string theory groundstates with spontaneously broken extended
supersymmetry and chiral spectru

### Classification of the N=2, Z2 X Z2-symmetric type II orbifolds and their type II asymmetric duals

Using free world-sheet fermions, we construct and classify all the N=2, Z2 X
Z2 four-dimensional orbifolds of the type IIA/B strings for which the orbifold
projections act symmetrically on the left and right movers. We study the
deformations of these models out of the fermionic point, deriving the partition
functions at a generic point in the moduli of the internal torus T6=T2 X T2 X
T2. We investigate some of their perturbative and non-perturbative dualities
and construct new dual pairs of type IIA/type II asymmetric orbifolds, which
are related non-perturbatively and allow us to gain insight into some of the
non-perturbative properties of the type IIA/B strings in four dimensions. In
particular, we consider some of the (non-)perturbative gravitational
corrections.Comment: Latex, 47 pages, no figure

### Mass formulae and natural hierarchy in string effective supergravities

We study some conditions for the hierarchy $m_{3/2} << M_P$ to occur
naturally in a generic effective supergravity theory. Absence of fine-tuning
and perturbative calculability require that the effective potential has a
sliding gravitino mass and vanishing cosmological constant, up to ${\cal
O}(m_{3/2}^4)$ corrections. In particular, cancellation of quadratically
divergent contributions to the one-loop effective potential should take place,
including the `hidden sector' of the theory. We show that these conditions can
be met in the effective supergravities derived from four-dimensional
superstrings, with supersymmetry broken either at the string tree level via
compactification, or by non-perturbative effects such as gaugino condensation.
A crucial role is played by some approximate scaling symmetries, which are
remnants of discrete target-space dualities in the large moduli limit. We
derive explicit formulae for the soft breaking terms arising from this class of
`large hierarchy compatible' (LHC) supergravities.Comment: 40 pages, no figures, CERN-TH.7192/94, LPTENS -94/12, UCLA/94/TEP13
(plain LATEX, to be run twice). Some trivial misprints and references have
been correcte

### A Large Class of New Gravitational and Axionic Backgrounds for Four-Dimensional Superstrings

A large class of new 4-D superstring vacua with non-trivial/singular
geometries, spacetime supersymmetry and other background fields (axion,
dilaton) are found. Killing symmetries are generic and are associated with
non-trivial dilaton and antisymmetric tensor fields. Duality symmetries
preserving N=2 superconformal invariance are employed to generate a large class
of explicit metrics for non-compact 4-D Calabi-Yau manifolds with Killing
symmetries.Comment: LateX file, 34pp., (v5) Some misprints corrected in sections 3.1, 3.2
as pointed out in the paper of Hori and Kapustin (hep-th/0203147). Some
comsetic changes also made in the same section

### Non-perturbative Supersymmetry Breaking and Finite Temperature Instabilities in N=4 Superstrings

We obtain the non-perturbative effective potential for the dual
five-dimensional N=4 strings in the context of finite-temperature regarded as a
breaking of supersymmetry into four space-time dimensions. Using the properties
of gauged N=4 supergravity we derive the universal thermal effective potential
describing all possible high-temperature instabilities of the known N=4
superstrings. These strings undergo a high-temperature transition to a new
phase in which five-branes condense. This phase is described in detail, using
both the effective supergravity and non-critical string theory in six
dimensions. In the new phase, supersymmetry is perturbatively restored but
broken at the non-perturbative level.Comment: 21 pages. Proceedings of the Corfu Summer Institute on Elementary
Particle Physics, Corfu, Greece, September 199

### On the Heterotic Effective Action at One-Loop, Gauge Couplings and the Gravitational Sector

We present in detail the procedure for calculating the heterotic one-loop
effective action. We focus on gravitational and gauge couplings. We show that
the two-derivative couplings of the gravitational sector are not renormalized
at one loop when the ground state is supersymmetric. Arguments are presented
that this non-renormalization theorem persists to all orders in perturbation
theory. We also derive the full one-loop correction to the gauge coupling. For
a class of $N=2$ ground states, namely those that are obtained by toroidal
compactification to four dimensions of generic six-dimensional $N=1$ models, we
give an explicit formula for the gauge-group independent thresholds, and show
that these are equal within the whole family.Comment: LateX, 17pp. A minor correction mad

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