435 research outputs found
On Susy Standard-like models from orbifolds of D=6 Gepner orientifolds
As a further elaboration of the proposal of Ref. [1] we address the
construction of Standard-like models from configurations of stacks of
orientifold planes and D-branes on an internal space with the structure
. As a first step, the construction of
D=6 Type II B orientifolds on Gepner points, in the diagonal invariant case and
for both, odd and even, affine levels is discussed. We build up the explicit
expressions for B-type boundary states and crosscaps and obtain the amplitudes
among them. From such amplitudes we read the corresponding spectra and the
tadpole cancellation equations. Further compactification on a T^2 torus, by
simultaneously orbifolding the Gepner and the torus internal sectors, is
performed. The embedding of the orbifold action in the brane sector breaks the
original gauge groups and leads to N=1 supersymmetric chiral spectra. Whenever
even orbifold action on the torus is considered, new branes, with worldvolume
transverse to torus coordinates, must be included. The detailed rules for
obtaining the D=4 model spectra and tadpole equations are shown. As an
illustration we present a 3 generations Left-Right symmetric model that can be
further broken to a MSSM model.Comment: 40 pages, 2 figures, added references, table 3 correcte
A class of non-supersymmetric orientifolds
We study type IIB orientifolds on T^{2d}/Z_N with supersymmetry broken by the
compactification. We determine tadpole cancellation conditions including
anti-branes and considering different actions for the parity Omega. Using these
conditions we then obtain the spectrum of tachyons and massless states. Various
examples with N even correspond to type 0B orientifolds.Comment: 49 pages, Late
A Class of Non-Supersymmetric Open String Vacua
We analyze non-supersymmetric four dimensional open string models of type IIB
string theory compactified on with Scherk-Schwarz deformation
acting on an of the torus. We find that there are always two
solutions to the tadpole conditions that are shown to be connected via Wilson
lines in an non-trivial way. These models although non-supersymmetric, are free
of R-R and NS-NS tadpoles.Comment: 9 pages, no figues, Latex file (v2
Anomalous U(1)s masses in non-supersymmetric open string vacua
Anomalous U(1)s are omnipresent in realizations of the Standard Model using
D-branes. Such models are typically non-supersymmetric, and the anomalous U(1)
masses are potentially relevant for experiment. In this paper, the string
calculation of anomalous U(1) masses (hep-th/0204153) is extended to
non-supersymmetric orientifolds.Comment: 12 pages, Latex2e. (v2) final version to be published in Phys. Lett.
Brane-Antibrane Inflation in Orbifold and Orientifold Models
We analyse the cosmological implications of brane-antibrane systems in
string-theoretic orbifold and orientifold models. In a class of realistic
models, consistency conditions require branes and antibranes to be stuck at
different fixed points, and so their mutual attraction generates a potential
for one of the radii of the underlying torus or the 4D string dilaton. Assuming
that all other moduli have been fixed by string effects, we find that this
potential leads naturally to a period of cosmic inflation with the radion or
dilaton field as the inflaton. The slow-roll conditions are satisfied more
generically than if the branes were free to move within the space. The
appearance of tachyon fields at certain points in moduli space indicates the
onset of phase transitions to different non-BPS brane systems, providing ways
of ending inflation and reheating the corresponding observable brane universe.
In each case we find relations between the inflationary parameters and the
string scale to get the correct spectrum of density perturbations. In some
examples the small numbers required as inputs are no smaller than 0.01, and are
the same small quantities which are required to explain the gauge hierarchy.Comment: 30 pages, 2 figures. Substantial changes on version 1. New
cosmological scenarios proposed including the dilaton as the inflaton. Main
conclusions unchange
A Classification of Toroidal Orientifold Models
We develop the general tools for model building with orientifolds, including
SS supersymmetry breaking. In this paper, we work out the general formulae of
the tadpole conditions for a class of non supersymmetric orientifold models of
type IIB string theory compactified on , based on the general properties
of the orientifold group elements. By solving the tadpoles we obtain the
general anomaly free massless spectrum.Comment: 35 pages, 3 eps figures, Latex2
Kahler Potentials of Chiral Matter Fields for Calabi-Yau String Compactifications
The Kahler potential is the least understood part of effective N=1
supersymmetric theories derived from string compactifications. Even at
tree-level, the Kahler potential for the physical matter fields, as a function
of the moduli fields, is unknown for generic Calabi-Yau compactifications and
has only been computed for simple toroidal orientifolds. In this paper we
describe how the modular dependence of matter metrics may be extracted in a
perturbative expansion in the Kahler moduli. Scaling arguments, locality and
knowledge of the structure of the physical Yukawa couplings are sufficient to
find the relevant Kahler potential. Using these techniques we compute the
`modular weights' for bifundamental matter on wrapped D7 branes for
large-volume IIB Calabi-Yau flux compactifications. We also apply our
techniques to the case of toroidal compactifications, obtaining results
consistent with those present in the literature. Our techniques do not provide
the complex structure moduli dependence of the Kahler potential, but are
sufficient to extract relevant information about the canonically normalised
matter fields and the soft supersymmetry breaking terms in gravity mediated
scenarios.Comment: JHEP style, 24 pages, 4 figures. v2: New section and reference adde
Low-Energy Supersymmetry Breaking from String Flux Compactifications: Benchmark Scenarios
Soft supersymmetry breaking terms were recently derived for type IIB string
flux compactifications with all moduli stabilised. Depending on the choice of
the discrete input parameters of the compactification such as fluxes and ranks
of hidden gauge groups, the string scale was found to have any value between
the TeV and GUT scales. We study the phenomenological implications of these
compactifications at low energy. Three realistic scenarios can be identified
depending on whether the Standard Model lies on D3 or D7 branes and on the
value of the string scale. For the MSSM on D7 branes and the string scale
between 10^12 GeV and 10^17 GeV we find that the LSP is a neutralino, while for
lower scales it is the stop. At the GUT scale the results of the fluxed MSSM
are reproduced, but now with all moduli stabilised. For the MSSM on D3 branes
we identify two realistic scenarios. The first one corresponds to an
intermediate string scale version of split supersymmetry. The second is a
stringy mSUGRA scenario. This requires tuning of the flux parameters to obtain
the GUT scale. Phenomenological constraints from dark matter, (g-2)_mu and
BR(b->s gamma) are considered for the three scenarios. We provide benchmark
points with the MSSM spectrum, making the models suitable for a detailed
phenomenological analysis.Comment: 29 pages, 12 figures, reference adde
Fluxes, Gaugings and Gaugino Condensates
Based on the correspondence between the N = 1 superstring compactifications
with fluxes and the N = 4 gauged supergravities, we study effective N = 1
four-dimensional supergravity potentials arising from fluxes and gaugino
condensates in the framework of orbifold limits of (generalized) Calabi-Yau
compactifications. We give examples in heterotic and type II orientifolds in
which combined fluxes and condensates lead to vacua with small supersymmetry
breaking scale. We clarify the respective roles of fluxes and condensates in
supersymmetry breaking, and analyze the scaling properties of the gravitino
mass.Comment: 17 pages, C
Type IIB orientifolds on Gepner points
We study various aspects of orientifold projections of Type IIB closed string
theory on Gepner points in different dimensions. The open string sector is
introduced, in the usual constructive way, in order to cancel RR charges
carried by orientifold planes. Moddings by cyclic permutations of the internal
N=2 superconformal blocks as well as by discrete phase symmetries are
implemented. Reduction in the number of generations, breaking or enhancements
of gauge symmetries and topology changes are shown to be induced by such
moddings. Antibranes sector is also considered; in particular we show how non
supersymmetric models with antibranes and free of closed and open tachyons do
appear in this context. A systematic study of consistent models in D=8
dimensions and some illustrative examples in D=6 and D=4 dimensions are
presented.Comment: 67 pages, no figures References added, typos correcte
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