1,027 research outputs found
D-brane Instantons in Type II String Theory
We review recent progress in determining the effects of D-brane instantons in
N=1 supersymmetric compactifications of Type II string theory to four
dimensions. We describe the abstract D-brane instanton calculus for holomorphic
couplings such as the superpotential, the gauge kinetic function and higher
fermionic F-terms. This includes a discussion of multi-instanton effects and
the implications of background fluxes for the instanton sector. Our
presentation also highlights, but is not restricted to the computation of
D-brane instanton effects in quiver gauge theories on D-branes at
singularities. We then summarize the concrete consequences of stringy D-brane
instantons for the construction of semi-realistic models of particle physics or
SUSY-breaking in compact and non-compact geometries.Comment: Invited review to appear in Annu.Rev.Nuc.Part.Sci 2009 59; 69 pages,
8 figures, 5 tables; v2: 1 reference adde
Simulation of some quantum gates, with decoherence
Methods and results for numerical simulations of one and two interacting
rf-Squid systems suitable for adiabatic quantum gates are presented. These are
based on high accuracy numerical solutions to the static and time dependent
Schroedinger equation for the full Squid Hamiltonian in one and two variables.
Among the points examined in the static analysis is the range of validity of
the effective two-state or ``spin 1/2'' picture. A range of parameters is
determined where the picture holds to good accuracy as the energy levels
undergo gate manipulations. Some general points are presented concerning the
relations between device parameters and ``good'' quantum mechanical state
spaces.
The time dependent simulations allow the examination of suitable conditions
for adiabatic behavior, and permits the introduction of a random noise to
simulate the effects of decoherence. A formula is derived and tested relating
the random noise to the decoherence rate. Sensitivity to device and operating
parameters for the logical gates NOT and CNOT are examined, with particular
attention to values of the tunnel parameter beta slightly above one. It appears
that with values of beta close to one, a quantum CNOT gate is possible even
with rather short decoherence times.
Many of the methods and results will apply to coupled double-potential well
systems in general.Comment: 26 pages, 15 figures, Some clarification added on decoherence
treatment, many small errors corrected, symbols on some figures enlarged,
refs added. No change in conten
Toward Realistic Intersecting D-Brane Models
We provide a pedagogical introduction to a recently studied class of
phenomenologically interesting string models, known as Intersecting D-Brane
Models. The gauge fields of the Standard-Model are localized on D-branes
wrapping certain compact cycles on an underlying geometry, whose intersections
can give rise to chiral fermions. We address the basic issues and also provide
an overview of the recent activity in this field. This article is intended to
serve non-experts with explanations of the fundamental aspects, and also to
provide some orientation for both experts and non-experts in this active field
of string phenomenology.Comment: 85 pages, 8 figures, Latex, Bibtex, v2: refs added, typos correcte
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
Coupling of hard dimers to dynamical lattices via random tensors
We study hard dimers on dynamical lattices in arbitrary dimensions using a
random tensor model. The set of lattices corresponds to triangulations of the
d-sphere and is selected by the large N limit. For small enough dimer
activities, the critical behavior of the continuum limit is the one of pure
random lattices. We find a negative critical activity where the universality
class is changed as dimers become critical, in a very similar way hard dimers
exhibit a Yang-Lee singularity on planar dynamical graphs. Critical exponents
are calculated exactly. An alternative description as a system of
`color-sensitive hard-core dimers' on random branched polymers is provided.Comment: 12 page
Potential effects of optical solar sail degredation on trajectory design
The optical properties of the thin metalized polymer films that are projected for solar sails are assumed to be affected by the erosive effects of the space environment. Their degradation behavior in the real space environment, however, is to a considerable degree indefinite, because initial ground test results are controversial and relevant inspace tests have not been made so far. The standard optical solar sail models that are currently used for trajectory design do not take optical degradation into account, hence its potential effects on trajectory design have not been investigated so far. Nevertheless, optical degradation is important for high-fidelity solar sail mission design, because it decreases both the magnitude of the solar radiation pressure force acting on the sail and also the sail control authority. Therefore, we propose a simple parametric optical solar sail degradation model that describes the variation of the sail film's optical coefficients with time, depending on the sail film's environmental history, i.e., the radiation dose. The primary intention of our model is not to describe the exact behavior of specific film-coating combinations in the real space environment, but to provide a more general parametric framework for describing the general optical degradation behavior of solar sails. Using our model, the effects of different optical degradation behaviors on trajectory design are investigated for various exemplary missions
D3 instantons in Calabi-Yau orientifolds with(out) fluxes
We investigate the instanton effect due to D3 branes wrapping a four-cycle in
a Calabi-Yau orientifold with D7 branes. We study the condition for the nonzero
superpotentials from the D3 instantons. For that matter we work out the zero
mode structures of D3 branes wrapping a four-cycle both in the presence of the
fluxes and in the absence of the fluxes. In the presence of the fluxes, the
condition for the nonzero superpotential could be different from that without
the fluxes. We explicitly work out a simple example of the orientifold of with a suitable flux to show such behavior. The effects of
D3-D7 sectors are interesting and give further constraints for the nonzero
superpotential. In a special configuration where D3 branes and D7 branes wrap
the same four-cycle, multi-instanton calculus of D3 branes could be reduced to
that of a suitable field theory. The structure of D5 instantons in Type I
theory is briefly discussed.Comment: 17 pages; Typos corrected, arguments improved and references adde
Racetrack Inflation
We develop a model of eternal topological inflation using a racetrack
potential within the context of type IIB string theory with KKLT volume
stabilization. The inflaton field is the imaginary part of the K\"ahler
structure modulus, which is an axion-like field in the 4D effective field
theory. This model does not require moving branes, and in this sense it is
simpler than other models of string theory inflation. Contrary to
single-exponential models, the structure of the potential in this example
allows for the existence of saddle points between two degenerate local minima
for which the slow-roll conditions can be satisfied in a particular range of
parameter space. We conjecture that this type of inflation should be present in
more general realizations of the modular landscape. We also consider
`irrational' models having a dense set of minima, and discuss their possible
relevance for the cosmological constant problem.Comment: 23 pages 7 figures. The final version with minor modifications, to
appear in JHE
Interaction of eukaryotic translation initiation factor 4G with the nuclear cap-binding complex provides a link between nuclear and cytoplasmic functions of the m7 guanosine cap
In eukaryotes the majority of mRNAs have an m7G cap that is added cotranscriptionally and that plays an important role in many aspects of mRNA metabolism. The nuclear cap-binding complex (CBC; consisting of CBP20 and CBP80) mediates the stimulatory functions of the cap in pre-mRNA splicing, 3' end formation, and U snRNA export. As little is known about how nuclear CBC mediates the effects of the cap in higher eukaryotes, we have characterized proteins that interact with CBC in HeLa cell nuclear extracts as potential mediators of its function. Using cross-linking and coimmunoprecipitation, we show that eukaryotic translation initiation factor 4G (eIF4G), in addition to its function in the cytoplasm, is a nuclear CBC-interacting protein. We demonstrate that eIF4G interacts with CBC in vitro and that, in addition to its cytoplasmic localization, there is a significant nuclear pool of eIF4G in mammalian cells in vivo. Immunoprecipitation experiments suggest that, in contrast to the cytoplasmic pool, much of the nuclear eIF4G is not associated with eIF4E (translation cap binding protein of eIF4F) but is associated with CBC. While eIF4G stably associates with spliceosomes in vitro and shows close association with spliceosomal snRNPs and splicing factors in vivo, depletion studies show that it does not participate directly in the splicing reaction. Taken together the data indicate that nuclear eIF4G may be recruited to pre-mRNAs via its interaction with CBC and accompanies the mRNA to the cytoplasm, facilitating the switching of CBC for eIF4F. This may provide a mechanism to couple nuclear and cytoplasmic functions of the mRNA cap structure
The Influence of Huntingtin Protein Size on Nuclear Localization and Cellular Toxicity
Huntington disease is an autosomal dominant neurodegenerative disorder caused by the pathological expansion of a polyglutamine tract. In this study we directly assess the influence of protein size on the formation and subcellular localization of huntingtin aggregates. We have created numerous deletion constructs expressing successively smaller fragments of huntingtin and show that these smaller proteins containing 128 glutamines form both intranuclear and perinuclear aggregates. In contrast, larger NH2-terminal fragments of huntingtin proteins with 128 glutamines form exclusively perinuclear aggregates. These aggregates can form in the absence of endogenous huntingtin. Furthermore, expression of mutant huntingtin results in increased susceptibility to apoptotic stress that is greater with decreasing protein length and increasing polyglutamine size. As both intranuclear and perinuclear aggregates are clearly associated with increased cellular toxicity, this supports an important role for toxic polyglutamine-containing fragments forming aggregates and playing a key role in the pathogenesis of Huntington disease
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