60 research outputs found
Moduli stabilization in (string) model building: gauge fluxes and loops
We discuss the moduli stabilization arising in the presence of gauge fluxes,
R-symmetry twists and non-perturbative effects in the context of 6-dimensional
supergravity models. We show how the presence of D-terms, due to the gauge
fluxes, is compatible with gaugino condensation, and that the two effects,
combined with the R-symmetry twist, do stabilize all the Kaehler moduli present
in the model, in the spirit of KKLT. We also calculate the flux-induced
one-loop correction to the scalar potential coming from charged
hypermultiplets, and find that it does not destabilize the minimum.Comment: Submitted for the SUSY07 proceedings, 4 pages, LaTe
Multiple anomalous U(1)s in heterotic blow-ups
The existence of multiple anomalous U(1)s is demonstrated explicitly in a
blow-up version of a heterotic Z_3 orbifold. Another blow-up of the same
orbifold supports further evidence for the type-I/heterotic duality in four
dimensions. It has a single anomalous U(1) which does not factorize
universally. As multiple anomalous U(1)s as well as non-universal factorization
have never been established on heterotic orbifolds explicitely, these findings
might appear contradictory at first sight. Possible inconsistencies are avoided
by reinterpreting a charged twisted state as a second non-universal localized
axion. The mismatch between the charges of the orbifold and blow-up spectra is
resolved by suitable field redefinitions. The anomaly of the field
redefinitions corresponds to the difference of blow-up and heterotic orbifold
anomalies.Comment: 7 pages LaTe
IFNN Manual: Integrated Framework for Neural Network and Conventional Modelling
Series: Research Reports of the Institute for Economic Geography and GIScienc
SIM User's Manual. A Flexible Toolbox for Spatial lnteraction Modelling.
Series: Research Reports of the Institute for Economic Geography and GIScienc
On the unfolding of the fundamental region in integrals of modular invariant amplitudes
We study generic one-loop (string) amplitudes where an integration over the
fundamental region F of the modular group is needed. We show how the known
lattice-reduction technique used to unfold F to a more suitable region S can be
modified to rearrange generic modular invariant amplitudes. The main aim is to
unfold F to the strip and, at the same time, to simplify the form of the
integrand when it is a sum over a finite number of terms, like in one-loop
amplitudes for closed strings compactified on orbifolds. We give a general
formula and a recipe to compute modular invariant amplitudes. As an application
of the technique we compute the one-loop vacuum energy \rho_n for a generic
\Z_n freely acting orbifold, generalizing the result that this energy is less
than zero and drives the system to a tachyonic divergence, and that
\rho_nm.Comment: 10 pages, 2 figure
On the quantum stability of IIB orbifolds and orientifolds with Scherk-Schwarz SUSY breaking
We study the quantum stability of Type IIB orbifold and orientifold string
models in various dimensions, including Melvin backgrounds, where supersymmetry
(SUSY) is broken {\it \`a la} Scherk-Schwarz (SS) by twisting periodicity
conditions along a circle of radius R. In particular, we compute the
R-dependence of the one-loop induced vacuum energy density , or
cosmological constant. For SS twists different from Z2 we always find, for both
orbifolds and orientifolds, a monotonic , eventually driving the
system to a tachyonic instability. For Z2 twists, orientifold models can have a
different behavior, leading either to a runaway decompactification limit or to
a negative minimum at a finite value R_0. The last possibility is obtained for
a 4D chiral orientifold model where a more accurate but yet preliminary
analysis seems to indicate that or towards the tachyonic
instability, as the dependence on the other geometric moduli is included.Comment: 26 pages, 5 figures; v2: version to appear in Nucl.Phys.
String vacua with flux from freely-acting obifolds
A precise correspondence between freely-acting orbifolds (Scherk-Schwarz
compactifications) and string vacua with NSNS flux turned on is established
using T-duality.
We focus our attention to a certain non-compact Z_2 heterotic freely-acting
orbifold with N=2 supersymmetry (SUSY). The geometric properties of the T-dual
background are studied. As expected, the space is non-Kahler with the most
generic torsion compatible with SUSY. All equations of motion are satisfied,
except the Bianchi identity for the NSNS field, that is satisfied only at
leading order in derivatives, i.e. without the curvature term. We point out
that this is due to unknown corrections to the standard heterotic T-duality
rules.Comment: 13 pages, no figures; v2: references added and rearranged, version to
appear in JHE
Heterotic SO(32) model building in four dimensions
Four dimensional heterotic SO(32) orbifold models are classified
systematically with model building applications in mind. We obtain all Z3, Z7
and Z2N models based on vectorial gauge shifts. The resulting gauge groups are
reminiscent of those of type-I model building, as they always take the form
SO(2n_0)xU(n_1)x...xU(n_{N-1})xSO(2n_N). The complete twisted spectrum is
determined simultaneously for all orbifold models in a parametric way depending
on n_0,...,n_N, rather than on a model by model basis. This reveals interesting
patterns in the twisted states: They are always built out of vectors and
anti--symmetric tensors of the U(n) groups, and either vectors or spinors of
the SO(2n) groups. Our results may shed additional light on the S-duality
between heterotic and type-I strings in four dimensions. As a spin-off we
obtain an SO(10) GUT model with four generations from the Z4 orbifold.Comment: 1+37 pages LaTeX, some typos in table 4 corrected, and we have
included some discussion on exceptional shift vectors which ignored in the
previous version
Gauge unification, non-local breaking, open strings
The issue of non-local GUT symmetry breaking is addressed in the context of
open string model building. We study ZNxZM' orbifolds with all the GUT-breaking
orbifold elements acting freely, as rotations accompanied by translations in
the internal space.We consider open strings quantized on these backgrounds,
distinguishing whether the translational action is parallel or perpendicular to
the D-branes. GUT breaking is impossible in the purely perpendicular case,
non-local GUT breaking is instead allowed in the purely parallel case. In the
latter, the scale of breaking is set by the compactification moduli, and there
are no fixed points with reduced gauge symmetry, where dangerous explicit
GUT-breaking terms could be located. We investigate the mixed
parallel+perpendicular case in a Z2xZ2' example, having also a simplified field
theory realization.It is a new S1/Z2xZ2' orbifold-GUT model, with bulk gauge
symmetry SU(5)xSU(5) broken locally to the Standard Model gauge group. In spite
of the locality of the GUT symmetry breaking, there is no localized
contribution to the running of the coupling constants, and the unification
scale is completely set by the length of S1.Comment: 16 pages, 6 figure
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