87 research outputs found
Deformations on Tilted Tori and Moduli Stabilisation at the Orbifold Point
We discuss deformations of orbifold singularities on tilted tori in the
context of Type IIA orientifold model building with D6-branes on special
Lagrangian cycles. Starting from , we
mod out an additional symmetry to describe phenomenologically
appealing backgrounds and reduce to and
invariant orbits of deformations. While D6-branes carrying SO(2N) or USp(2N)
gauge groups do not constrain deformations, D6-branes with U(N) gauge groups
develop non-vanishing D-terms if they couple to previously singular, now
deformed cycles. We present examples for both types of D6-branes, and in a
three-generation Pati-Salam model on
we find that ten out of 15 twisted complex structure moduli are indeed
stabilised at the orbifold point by the existence of the brane stacks.Comment: 49 pages; v2: minor text-typo corrections, references adde
Calabi-Yau compactifications of non-supersymmetric heterotic string theory
Phenomenological explorations of heterotic strings have conventionally
focused primarily on the E8xE8 theory. We consider smooth compactifications of
all three ten-dimensional heterotic theories to exhibit the many similarities
between the non-supersymmetric SO(16)xSO(16) theory and the related
supersymmetric E8xE8 and SO(32) theories. In particular, we exploit these
similarities to determine the bosonic and fermionic spectra of Calabi-Yau
compactifications with line bundles of the non-supersymmetric string. We use
elements of four-dimensional supersymmetric effective field theory to
characterize the non-supersymmetric action at leading order and determine the
Green-Schwarz induced axion-couplings. Using these methods we construct a
non-supersymmetric Standard Model(SM)-like theory. In addition, we show that it
is possible to obtain SM-like models from the standard embedding using at least
an order four Wilson line. Finally, we make a proposal of the states that live
on five branes in the SO(16)xSO(16) theory and find under certain assumptions
the surprising result that anomaly factorization only admits at most a single
brane solution.Comment: 1+40 pages LaTeX, 3 figures, 8 tables. v2: Flux quantization
condition for the non-supersymmetric theory correcte
Heterotic Particle Models from various Perspectives
We consider the compactification of heterotic string theory on toroidal orbifolds and their resolutions. In the framework of gauged linear sigma models we develop realizations of such spaces, allowing to continously vary the moduli and thus smoothly interpolate between differrent corners of the theory. This way all factorizable orbifold resolutions as well as some non-factorizable ones can be obtained. We find that for a given geometry there are many model which realize it as a target space, differing in their complexity. We explore regions of moduli space which otherwise would not be accessible. In particular we are interested in the orbifold regime, where exact string calculations are possible, and the large volume regime, where techniques of supergravity compactification can be applied. By comparing these two theories and matching the spectra we find evidence for non-perturbative effects which interpolate between these regimes
The Hilbert basis method for D-flat directions and the superpotential
We discuss, using the Hilbert basis method, how to efficiently construct a
complete basis for D-flat directions in supersymmetric Abelian and non-Abelian
gauge theories. We extend the method to discrete (R and non-R) symmetries. This
facilitates the construction of a basis of all superpotential terms in a theory
with given symmetries.Comment: 11 pages; a related mathematica code can be found at
http://einrichtungen.ph.tum.de/T30e/codes/NonAbelianHilbert
A unique Z_4^R symmetry for the MSSM
We consider the possible anomaly free Abelian discrete symmetries of the MSSM
that forbid the mu-term at perturbative order. Allowing for anomaly
cancellation via the Green-Schwarz mechanism we identify discrete R-symmetries
as the only possibility and prove that there is a unique Z_4^R symmetry that
commutes with SO(10). We argue that non-perturbative effects will generate a
mu-term of electroweak order thus solving the mu-problem. The non-perturbative
effects break the Z_4^R symmetry leaving an exact Z_2 matter parity. As a
result dimension four baryon- and lepton-number violating operators are absent
while, at the non-perturbative level, dimension five baryon- and lepton-number
violating operators get induced but are highly suppressed so that the nucleon
decay rate is well within present bounds.Comment: 6 page
Non-supersymmetric heterotic model building
We investigate orbifold and smooth Calabi-Yau compactifications of the
non-supersymmetric heterotic SO(16)xSO(16) string. We focus on such Calabi-Yau
backgrounds in order to recycle commonly employed techniques, like index
theorems and cohomology theory, to determine both the fermionic and bosonic 4D
spectra. We argue that the N=0 theory never leads to tachyons on smooth
Calabi-Yaus in the large volume approximation. As twisted tachyons may arise on
certain singular orbifolds, we conjecture that such tachyonic states are lifted
in the full blow-up. We perform model searches on selected orbifold geometries.
In particular, we construct an explicit example of a Standard Model-like theory
with three generations and a single Higgs field.Comment: 1+30 pages latex, 11 tables; v2: references and minor revisions
added, matches version published in JHE
No-go theorems for R symmetries in four-dimensional GUTs
We prove that it is impossible to construct a grand unified model, based on a
simple gauge group, in four dimensions that leads to the exact MSSM, nor to a
singlet extension, and possesses an unbroken R symmetry. This implies that no
MSSM model with either a Z_{M>=3}^R or U(1)_R symmetry can be completed by a
four-dimensional GUT in the ultraviolet. However, our no-go theorem does not
apply to GUT models with extra dimensions. We also show that it is impossible
to construct a 4D GUT that leads to the MSSM plus an additional anomaly-free
symmetry that forbids the mu term.Comment: 11+1 page
- …