1,137 research outputs found
Dynamical Generation of Yukawa Interactions in Intersecting D-brane Models
We construct a supersymmetric composite model in type IIA T^6/(Z_2 x Z_2)
orientifold with intersecting D6-branes. Four generations of quarks and leptons
are naturally emerged as composite fields at low energies. Two pairs of light
electroweak Higgs doublets are also naturally obtained. The hierarchical Yukawa
couplings for the quark-lepton masses can be generated by the interplay between
the string-level higher dimensional interactions among "preons" and the
dynamics of the confinement of "preons". Besides having four generations of
quarks and leptons, the model is not realistic in some points: some exotic
particles, one additional U(1) gauge symmetry, no explicit mechanism for
supersymmetry breaking, and so on. This model is a toy model to illustrate a
new mechanism of dynamical generation of Yukawa couplings for the masses and
mixings of quarks and leptons.Comment: 18 pages, 6 figure
Intersecting D-Branes on Shift Z2 x Z2 Orientifolds
We investigate Z2 x Z2 orientifolds with group actions involving shifts. A
complete classification of possible geometries is presented where also previous
work by other authors is included in a unified framework from an intersecting
D-brane perspective. In particular, we show that the additional shifts not only
determine the topology of the orbifold but also independently the presence of
orientifold planes. In the second part, we work out in detail a basis of
homological three cycles on shift Z2 x Z2 orientifolds and construct all
possible fractional D-branes including rigid ones. A Pati-Salam type model with
no open-string moduli in the visible sector is presented.Comment: 36 pages, 4 figures, refs. adde
String GUT Scenarios with Stabilised Moduli
Taking into account the recently proposed poly-instanton corrections to the
superpotential and combining the race-track with a KKLT respectively LARGE
Volume Scenario in an intricate manner, we show that we gain exponential
control over the parameters in an effective superpotential. This allows us to
dynamically stabilise moduli such that a conventional MSSM scenario with the
string scale lowered to the GUT scale is realised. Depending on the cycles
wrapped by the MSSM branes, two different scenarios for the hierarchy of soft
masses arise. The first one is a supergravity mediated model with M_3/2=1TeV
while the second one features mixed anomaly-supergravity mediation with
M_3/2=10^10GeV and split supersymmetry. We also comment on dynamically lowering
the scales such that the tree-level cosmological constant is of the order
\Lambda=(10^-3eV)^4.Comment: 22 pages, 13 figures; v2: refs. and explanation adde
A Note on Representations of N=2 SW-Algebras
We investigate the representation theory of some recently constructed N=2
super W-algebras with two generators. Except for the central charges in the
unitary minimal series of the N=2 super Virasoro algebra we find no new
rational models. However, from our results it is possible to arrange all known
N=2 super W-algebras with two generators and vanishing self-coupling constant
into four classes. For the algebras existing for c >= 3 which can be understood
by the spectral flow of the N=2 super Virasoro algebra we find that the
representations have quantized U(1) charge.Comment: 11 pages + 7 figures (postscript); plain TeX, psfig.tex;
BONN-TH-94-0
Non-Abelian Brane Worlds: The Heterotic String Story
We discuss chiral supersymmetric compactifications of the SO(32) heterotic
string on Calabi-Yau manifolds equipped with direct sums of stable bundles with
structure group U(n). In addition we allow for non-perturbative heterotic
five-branes. These models are S-dual to Type I compactifications with D9- and
D5-branes, which by themselves are mirror symmetric to general intersecting
D6-brane models. For the construction of concrete examples we consider
elliptically fibered Calabi-Yau manifolds with SU(n) bundles given by the
spectral cover construction. The U(n) bundles are obtained via twisting by line
bundles. We present a four-generation Pati-Salam and a three-generation
Standard-like model.Comment: 29 pages, 7 tables, LATEX; v2: refs adde
Chiral Supersymmetric Gepner Model Orientifolds
We explicitly construct A-type orientifolds of supersymmetric Gepner models.
In order to reduce the tadpole cancellation conditions to a treatable number we
explicitly work out the generic form of the one-loop Klein bottle, annulus and
Moebius strip amplitudes for simple current extensions of Gepner models.
Equipped with these formulas, we discuss two examples in detail to provide
evidence that in this setting certain features of the MSSM like unitary gauge
groups with large enough rank, chirality and family replication can be
achieved.Comment: 37 pages, TeX (harvmac), minor changes, typos corrected, to appear in
JHE
Chiral N=1 4d Orientifolds with D-branes at Angles
D6-branes intersecting at angles allow for phenomenologically appealing
constructions of four dimensional string theory vacua. While it is
straightforward to obtain non-supersymmetric realizations of the standard
model, supersymmetric and stable models with three generations and no exotic
chiral matter require more involved orbifold constructions. The T^6/(Z_4 x Z_2
x OmegaR) case is discussed in detail. Other orbifolds including fractional
D6-branes are treated briefly.Comment: Invited brief review, 16 pages, 2 figures; references adde
Type IIA Orientifolds on General Supersymmetric Z_N Orbifolds
We construct Type IIA orientifolds for general supersymmetric Z_N orbifolds.
In particular, we provide the methods to deal with the non-factorisable
six-dimensional tori for the cases Z7, Z8, Z8', Z12 and Z12'. As an application
of these methods we explicitly construct many new orientifold models.Comment: 48 pages, LaTeX, 14 figures, refs. added, closed string spectra
slightly change
Neutrino Majorana Masses from String Theory Instanton Effects
Finding a plausible origin for right-handed neutrino Majorana masses in
semirealistic compactifications of string theory remains one of the most
difficult problems in string phenomenology. We argue that right-handed neutrino
Majorana masses are induced by non-perturbative instanton effects in certain
classes of string compactifications in which the gauge boson has a
St\"uckelberg mass. The induced operators are of the form
where is a closed string modulus whose imaginary part transforms
appropriately under . This mass term may be quite large since this is not
a gauge instanton and is not directly related to SM gauge couplings.
Thus the size of the induced right-handed neutrino masses could be a few orders
of magnitude below the string scale, as phenomenologically required. It is also
argued that this origin for neutrino masses would predict the existence of
R-parity in SUSY versions of the SM. Finally we comment on other
phenomenological applications of similar instanton effects, like the generation
of a -term, or of Yukawa couplings forbidden in perturbation theory.Comment: 40 pages, 4 figures (v2: added references, small corrections)(v3:
minor corrections
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