363 research outputs found
A Compact Codimension Two Braneworld with Precisely One Brane
Building on earlier work on football shaped extra dimensions, we construct a
compact codimension two braneworld with precisely one brane. The two extra
dimensions topologically represent a 2-torus which is stabilized by a bulk
cosmological constant and magnetic flux. The torus has positive constant
curvature almost everywhere, except for a single conical singularity at the
location of the brane. In contradistinction to the football shaped case, there
is no fine-tuning required for the brane tension. We also present some
plausibility arguments why the model should not suffer from serious stability
issues.Comment: 13 pages, 2 figures; references added, typos fixes; essentially the
version published in PR
Relative entropy as a measure of inhomogeneity in general relativity
We introduce the notion of relative volume entropy for two spacetimes with
preferred compact spacelike foliations. This is accomplished by applying the
notion of Kullback-Leibler divergence to the volume elements induced on
spacelike slices. The resulting quantity gives a lower bound on the number of
bits which are necessary to describe one metric given the other. For
illustration, we study some examples, in particular gravitational waves, and
conclude that the relative volume entropy is a suitable device for quantitative
comparison of the inhomogeneity of two spacetimes.Comment: 15 pages, 7 figure
Stringy Instantons and Cascading Quivers
D-brane instantons can perturb the quantum field theories on space-time
filling D-branes by interesting operators. In some cases, these D-brane
instantons are novel "stringy" effects (not interpretable directly as instanton
effects in the low-energy quantum field theory), while in others the D-brane
instantons can be directly interpreted as field theory effects. In this note,
we describe a situation where both perspectives are available, by studying
stringy instantons in quivers which arise at simple Calabi-Yau singularities.
We show that a stringy instanton which wraps an unoccupied node of the quiver,
and gives rise to a non-perturbative mass in the space-time field theory, can
be reinterpreted as a conventional gauge theory effect by going up in an
appropriate renormalization group cascade. Interestingly, in the cascade, the
contribution of the stringy instanton does not come from gauge theory
instantons but from strong coupling dynamics.Comment: 17 pages, 6 figures, harvma
Instanton effects in N=1 brane models and the Kahler metric of twisted matter
We consider locally consistent systems of magnetized D9 branes on an
orbifolded six-torus which support N=1 gauge theories. In such realizations,
the matter multiplets arise from "twisted" strings connecting different stacks
of branes. The introduction of Euclidean 5 branes (E5) wrapped on the
six-dimensional compact space leads to instanton effects. For instance, if the
system is engineered so as to yield SQCD, a single E5 brane may account for the
ADS/TVY superpotential. We discuss the subtle interplay that exists between the
annuli diagrams with an E5 boundary and the holomorphicity properties of the
effective low-energy action of the N=1 theory. The consistency of this picture
allows to obtain information on the Kahler metric of the chiral matter
multiplets arising from twisted strings.Comment: 33 pages, 4 figures. V2: improved discussion, clarifyng comments and
references added. Version to be published in JHE
Instantons in N=2 magnetized D-brane worlds
In a toroidal orbifold of type IIB string theory we study instanton effects
in N=2 super Yang-Mills theories engineered with systems of wrapped magnetized
D9 branes and Euclidean D5 branes. We analyze the various open string sectors
in this brane system and study the 1-loop amplitudes described by annulus
diagrams with mixed boundary conditions, explaining their role in the stringy
instanton calculus. We show in particular that the non-holomorphic terms in
these annulus amplitudes precisely reconstruct the appropriate Kahler metric
factors that are needed to write the instanton correlators in terms of purely
holomorphic variables. We also explicitly derive the correct holomorphic
structure of the instanton induced low energy effective action in the Coulomb
branch.Comment: 40 pages, 5 figures, JHEP class. Some footnotes added and typos
corrected. Version published in JHE
Non-intersecting leaf insertion algorithm for tree structure models
We present an algorithm and an implementation to insert broadleaves or needleleaves to a quantitative structure model according to an arbitrary distribution, and a data structure to store the required information efficiently. A structure model contains the geometry and branching structure of a tree. The purpose of the work is to offer a tool for making more realistic simulations with tree models with leaves, particularly for tree models developed from terrestrial laser scan (TLS) measurements. We demonstrate leaf insertion using cylinder-based structure models, but the associated software implementation is written in a way that enables the easy use of other types of structure models. Distributions controlling leaf location, size and angles as well as the shape of individual leaves are user-definable, allowing any type of distribution. The leaf generation process consist of two stages, the first of which generates individual leaf geometry following the input distributions, while in the other stage intersections are prevented by doing transformations when required. Initial testing was carried out on English oak trees to demonstrate the approach and to assess the required computational resources. Depending on the size and complexity of the tree, leaf generation takes between 6 and 18 minutes. Various leaf area density distributions were defined, and the resulting leaf covers were compared to manual leaf harvesting measurements. The results are not conclusive, but they show great potential for the method. In the future, if our method is demonstrated to work well for TLS data from multiple tree types, the approach is likely to be very useful for 3D structure and radiative transfer simulation applications, including remote sensing, ecology and forestry, among others
Lifting D-Instanton Zero Modes by Recombination and Background Fluxes
We study the conditions under which D-brane instantons in Type II orientifold
compactifications generate a non-perturbative superpotential. If the instanton
is non-invariant under the orientifold action, it carries four instead of the
two Goldstone fermions required for superpotential contributions. Unless these
are lifted, the instanton can at best generate higher fermionic F-terms of
Beasley-Witten type. We analyse two strategies to lift the additional zero
modes. First we discuss the process of instantonic brane recombination in Type
IIA orientifolds. We show that in some cases charge invariance of the measure
enforces the presence of further zero modes which, unlike the Goldstinos,
cannot be absorbed. In other cases, the instanton exhibits reparameterisation
zero modes after recombination and a superpotential is generated if these are
lifted by suitable closed or open string couplings. In the second part of the
paper we address lifting the extra Goldstinos of D3-brane instantons by
supersymmetric three-form background fluxes in Type IIB orientifolds. This
requires non-trivial gauge flux on the instanton. Only if the part of the
fermionic action linear in the gauge flux survives the orientifold projection
can the extra Goldstinos be lifted.Comment: 38 pages, 3 figures, 5 tables; v2: Appendix B slightly expanded,
minor rewordin
Infrared stability of ABJ-like theories
We consider marginal deformations of the superconformal ABJM/ABJ models which
preserve N=2 supersymmetry. We determine perturbatively the spectrum of fixed
points and study their infrared stability. We find a closed line of fixed
points which is IR stable. The fixed point corresponding to the ABJM/ABJ models
is stable under marginal deformations which respect the original SU(2)xSU(2)
invariance, while deformations which break this group destabilize the theory
which then flows to a less symmetric fixed point. We discuss the addition of
flavor degrees of freedom. We prove that in general a flavor marginal
superpotential does not destabilize the system in the IR. An exception is
represented by a marginal coupling which mixes matter charged under different
gauge sectors. Finally, we consider the case of relevant deformations which
should drive the system to a strongly coupled IR fixed point recently
investigated in arXiv:0909.2036 [hep-th].Comment: 1+11 pages, 4 figures; v2: minor correction
Dynamical supersymmetry breaking from unoriented D-brane instantons
We study the non-perturbative dynamics of an unoriented Z_5-quiver theory of
GUT kind with gauge group U(5) and chiral matter. At strong coupling the
non-perturbative dynamics is described in terms of set of baryon/meson
variables satisfying a quantum deformed constraint. We compute the effective
superpotential of the theory and show that it admits a line of supersymmetric
vacua and a phase where supersymmetry is dynamically broken via gaugino
condensation.Comment: 24 pages, 1 figur
Instanton Induced Neutrino Majorana Masses in CFT Orientifolds with MSSM-like spectra
Recently it has been shown that string instanton effects may give rise to
neutrino Majorana masses in certain classes of semi-realistic string
compactifications. In this paper we make a systematic search for supersymmetric
MSSM-like Type II Gepner orientifold constructions admitting boundary states
associated with instantons giving rise to neutrino Majorana masses and other L-
and/or B-violating operators. We analyze the zero mode structure of D-brane
instantons on general type II orientifold compactifications, and show that only
instantons with O(1) symmetry can have just the two zero modes required to
contribute to the 4d superpotential. We however discuss how the addition of
fluxes and/or possible non-perturbative extensions of the orientifold
compactifications would allow also instantons with and U(1) symmetries
to generate such superpotentials. In the context of Gepner orientifolds with
MSSM-like spectra, we find no models with O(1) instantons with just the
required zero modes to generate a neutrino mass superpotential. On the other
hand we find a number of models in one particular orientifold of the Gepner
model with instantons with a few extra uncharged
non-chiral zero modes which could be easily lifted by the mentioned effects. A
few more orientifold examples are also found under less stringent constraints
on the zero modes. This class of instantons have the interesting
property that R-parity conservation is automatic and the flavour structure of
the neutrino Majorana mass matrices has a simple factorized form.Comment: 68 pages, 2 figures; v2. typos corrected, refs adde
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