240 research outputs found
D-particles on T^4/Z_n orbifolds and their resolutions
We formulate the effective field theory of a D-particle on orbifolds of
by a cyclic group as a gauge theory in a -bundle over the dual orbifold. We
argue that this theory admits Fayet-Iliopoulos terms analogous to those present
in the case of noncompact orbifolds. In the case, we present some
evidence that turning on such terms resolves the orbifold singularities and may
lead to a surface realized as a blow up of the fixed points of the cyclic
group action.Comment: 34 pages, 3 figures; very minor typos corrected, 2 references adde
C^2/Z_n Fractional branes and Monodromy
We construct geometric representatives for the C^2/Z_n fractional branes in
terms of branes wrapping certain exceptional cycles of the resolution. In the
process we use large radius and conifold-type monodromies, and also check some
of the orbifold quantum symmetries. We find the explicit Seiberg-duality which
connects our fractional branes to the ones given by the McKay correspondence.
We also comment on the Harvey-Moore BPS algebras.Comment: 34 pages, v1 identical to v2, v3: typos fixed, discussion of
Harvey-Moore BPS algebras update
Closed string tachyons, flips and conifolds
Following the analysis of tachyons and orbifold flips described in
hep-th/0412337, we study nonsupersymmetric analogs of the supersymmetric
conifold singularity and show using their toric geometry description that they
are nonsupersymmetric orbifolds of the latter. Using linear sigma models, we
see that these are unstable to localized closed string tachyon condensation and
exhibit flip transitions between their two small resolutions (involving
2-cycles), in the process mediating mild dynamical topology change. Our
analysis shows that the structure of these nonsupersymmetric conifolds as
quotients of the supersymmetric conifold obstructs the 3-cycle deformation of
such singularities, suggesting that these nonsupersymmetric conifolds decay by
evolving towards their stable small resolutions.Comment: Latex, 22 pgs, 2 figs. v4: matches JHEP version, 29 pgs, 3 figures,
more elaborate Introduction, various clarifications adde
Gauge Theory and the Excision of Repulson Singularities
We study brane configurations that give rise to large-N gauge theories with
eight supersymmetries and no hypermultiplets. These configurations include a
variety of wrapped, fractional, and stretched branes or strings. The
corresponding spacetime geometries which we study have a distinct kind of
singularity known as a repulson. We find that this singularity is removed by a
distinctive mechanism, leaving a smooth geometry with a core having an enhanced
gauge symmetry. The spacetime geometry can be related to large-N Seiberg-Witten
theory.Comment: 31 pages LaTeX, 2 figures (v3: references added
Geometric K-Homology of Flat D-Branes
We use the Baum-Douglas construction of K-homology to explicitly describe
various aspects of D-branes in Type II superstring theory in the absence of
background supergravity form fields. We rigorously derive various stability
criteria for states of D-branes and show how standard bound state constructions
are naturally realized directly in terms of topological K-cycles. We formulate
the mechanism of flux stabilization in terms of the K-homology of non-trivial
fibre bundles. Along the way we derive a number of new mathematical results in
topological K-homology of independent interest.Comment: 45 pages; v2: References added; v3: Some substantial revision and
corrections, main results unchanged but presentation improved, references
added; to be published in Communications in Mathematical Physic
Effective superpotentials for compact D5-brane Calabi-Yau geometries
For compact Calabi-Yau geometries with D5-branes we study N=1 effective
superpotentials depending on both open- and closed-string fields. We develop
methods to derive the open/closed Picard-Fuchs differential equations, which
control D5-brane deformations as well as complex structure deformations of the
compact Calabi-Yau space. Their solutions encode the flat open/closed
coordinates and the effective superpotential. For two explicit examples of
compact D5-brane Calabi-Yau hypersurface geometries we apply our techniques and
express the calculated superpotentials in terms of flat open/closed
coordinates. By evaluating these superpotentials at their critical points we
reproduce the domain wall tensions that have recently appeared in the
literature. Finally we extract orbifold disk invariants from the
superpotentials, which, up to overall numerical normalizations, correspond to
orbifold disk Gromov-Witten invariants in the mirror geometry.Comment: 55 pages; v2: references added, typos correcte
The statistics of string/M theory vacua
We discuss systematic approaches to the classification of string/M theory
vacua, and physical questions this might help us resolve. To this end, we
initiate the study of ensembles of effective Lagrangians, which can be used to
precisely study the predictive power of string theory, and in simple examples
can lead to universality results. Using these ideas, we outline an approach to
estimating the number of vacua of string/M theory which can realize the
Standard Model.Comment: harvmac, 72pp (v4: fixed error in discussion of quiver ensembles
Flux Compactifications on Calabi-Yau Threefolds
The presence of RR and NS three-form fluxes in type IIB string
compactification on a Calabi-Yau orientifold gives rise to a nontrivial
superpotential W for the dilaton and complex structure moduli. This
superpotential is computable in terms of the period integrals of the Calabi-Yau
manifold. In this paper, we present explicit examples of both supersymmetric
and nonsupersymmetric solutions to the resulting 4d N=1 supersymmetric no-scale
supergravity, including some nonsupersymmetric solutions with relatively small
values of W. Our examples arise on orientifolds of the hypersurfaces in
and . They serve as explicit
illustrations of several of the ingredients which have played a role in the
recent proposals for constructing de Sitter vacua of string theory.Comment: 30 pages, harvmac big; refs and minor comments adde
Mechanistic target of rapamycin (MTOR) protein expression in the tumor and its microenvironment correlates with more aggressive pathology at cystectomy
Background: The mechanistic target of rapamycin (mTOR) has been implicated in driving tumor biology in multiple malignancies, including urothelial carcinoma (UC). We investigate how mTOR and phosphorylated mTOR (pmTOR) protein expression correlate with chemoresponsiveness in the tumor and its microenvironment at final pathologic staging after neoadjuvant chemotherapy (NAC). Methods: A single-institution retrospective analysis was performed on 62 patients with cT2–4Nany UC undergoing NAC followed by radical cystectomy. Diagnostic (transurethral resection specimens, TURBT) and postchemotherapy radical cystectomy specimens were evaluated for mTOR and pmTOR protein expression using immunohistochemistry of the tumor, peritumoral stroma, and normal surrounding stroma. Protein expression levels were compared between clinical and pathologic stage. Whole transcriptome analysis was performed to evaluate mRNA expression relative to mTOR pathway activation. Results: Baseline levels of mTOR and pmTOR within TURBT specimens were not associated with clinical stage and response to chemotherapy overall. Nonresponders with advanced pathologic stage at cystectomy (ypT2–4/ypTanyN+) had significantly elevated mTOR tumor staining (P = 0.006) and a sustained mTOR and pmTOR staining in the peritumoral and surrounding normal stroma (NS). Several genes relevant to mTOR activity were found to be up-regulated in the tumors of nonresponders. Remarkably, complete responders at cystectomy (ypT0) had significant decreases in both mTOR and pmTOR protein expression in the peritumoral and normal stroma (P = 0.01–0.03). Conclusions: Our results suggest that mTOR pathway activity is increased in tumor and sustained in its microenvironment in patients with adverse pathologic findings at cystectomy. These findings suggest the relevance of targeting this pathway in bladder cancer
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