2,316 research outputs found
D-brane gauge theories from toric singularities of the form and
We discuss examples of D-branes probing toric singularities, and the
computation of their world-volume gauge theories from the geometric data of the
singularities. We consider several such examples of D-branes on partial
resolutions of the orbifolds , and .Comment: 34+1 pages, References added, Version to appear in Nuclear Physics
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
Orbifold Resolution by D-Branes
We study topological properties of the D-brane resolution of
three-dimensional orbifold singularities, C^3/Gamma, for finite abelian groups
Gamma. The D-brane vacuum moduli space is shown to fill out the background
spacetime with Fayet--Iliopoulos parameters controlling the size of the
blow-ups. This D-brane vacuum moduli space can be classically described by a
gauged linear sigma model, which is shown to be non-generic in a manner that
projects out non-geometric regions in its phase diagram, as anticipated from a
number of perspectives.Comment: 26 pages, 2 figures (TeX, harvmac big, epsf
D-branes on Fourfolds with Discrete Torsion
We study D1-branes on the fourfold \C^4/(\Z_2\times\Z_2\times\Z_2), in the
presence of discrete torsion. Discrete torsion is incorporated in the gauge
theory of the D1-branes by considering a projective representation of the
finite group . The corresponding orbifold is then
deformed by perturbing the F-flatness condition of the gauge theory. The moduli
space of the resulting gauge theory retains a stable singularity of codimension
three.Comment: Two references adde
D-Brane Gauge Theories from Toric Singularities and Toric Duality
Via partial resolution of Abelian orbifolds we present an algorithm for
extracting a consistent set of gauge theory data for an arbitrary toric variety
whose singularity a D-brane probes. As illustrative examples, we tabulate the
matter content and superpotential for a D-brane living on the toric del Pezzo
surfaces as well as the zeroth Hirzebruch surface. Moreover, we discuss the
non-uniqueness of the general problem and present examples of vastly different
theories whose moduli spaces are described by the same toric data. Our methods
provide new tools for calculating gauge theories which flow to the same
universality class in the IR. We shall call it ``Toric Duality.''Comment: 38 pages, 6 figures, 2 references added and 1 equation correcte
Evidence for a gravitational Myers effect
An indication for the existence of a collective Myers solution in the
non-abelian D0-brane Born-Infeld action is the presence of a tachyonic mode in
fluctuations around the standard diagonal background. We show that this
computation for non-abelian D0-branes in curved space has the geometric
interpretation of computing the eigenvalues of the geodesic deviation operator
for U(N)-valued coordinates. On general grounds one therefore expects a
geometric Myers effect in regions of sufficiently negative curvature. We
confirm this by explicit computations for non-abelian D0-branes on a sphere and
a hyperboloid. For the former the diagonal solution is stable, but not so for
the latter. We conclude by showing that near the horizon of a Schwarzschild
black hole one also finds a tachyonic mode in the fluctuation spectrum,
signaling the possibility of a near-horizon gravitationally induced Myers
effect.Comment: LaTeX, 23 page
Supersymmetric Non-singular Fractional D2-branes and NS-NS 2-branes
We obtain regular deformed D2-brane solutions with fractional D2-branes
arising as wrapped D4-branes. The space transverse to the D2-brane is a
complete Ricci-flat 7-manifold of G_2 holonomy, which is asymptotically conical
with principal orbits that are topologically CP^3 or the flag manifold
SU(3)/(U(1) x U(1)). We obtain the solution by first constructing an L^2
normalisable harmonic 3-form. We also review a previously-obtained regular
deformed D2-brane whose transverse space is a different 7-manifold of G_2
holonomy, with principal orbits that are topologically S^3 x S^3. This
describes D2-branes with fractional NS-NS 2-branes coming from the wrapping of
5-branes, which is supported by a non-normalisable harmonic 3-form on the
7-manifold. We prove that both types of solutions are supersymmetric,
preserving 1/16 of the maximal supersymmetry and hence that they are dual to
{\cal N}=1 three-dimensional gauge theories. In each case, the spectrum for
minimally-coupled scalars is discrete, indicating confinement in the infrared
region of the dual gauge theories. We examine resolutions of other branes, and
obtain necessary conditions for their regularity. The resolution of many of
these seems to lie beyond supergravity. In the process of studying these
questions, we construct new explicit examples of complete Ricci-flat metrics.Comment: Latex, 30 page
Race/Ethnicity and Retention in Traumatic Brain Injury Outcomes Research: A Traumatic Brain Injury Model Systems National Database Study
Objective: To investigate the contribution of race/ethnicity to retention in traumatic brain injury (TBI) research at 1 to 2 years post-injury. Setting: Community. Participants: 5548 Whites, 1347 Blacks, and 790 Hispanics enrolled in the Traumatic Brain Injury Model Systems National Database with dates of injury between October 1, 2002 and March 31, 2013. Design:
Retrospective database analysis. Main Measure: Retention, defined as completion of at least one question on the follow-up interview by the person with TBI or a proxy. Results: Retention rates 1-2 years post-TBI were significantly lower for Hispanic (85.2%) than for White (91.8%) or Black participants (90.5%) and depended significantly on history of problem drug or alcohol use. Other variables associated with low retention included older age, lower education, violent cause of injury, and discharge to an institution versus private residence. Conclusions: The findings emphasize the importance of investigating retention rates separately for Blacks and Hispanics rather than combining them or grouping either with other races or ethnicities. The results also suggest the need for implementing procedures to increase retention of Hispanics in longitudinal TBI researc
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
Model Building and Phenomenology of Flux-Induced Supersymmetry Breaking on D3-branes
We study supersymmetry breaking effects induced on D3-branes at singularities
by the presence of NSNS and RR 3-form fluxes. First, we discuss some local
constructions of chiral models from D3-branes at singularities, as well as
their global embedding in flux compactifications. The low energy spectrum of
these constructions contains features of the supersymmetric Standard Model. In
these models, both the soft SUSY parameters and the mu-term are generated by
turning on the 3-form NSNS and RR fluxes. We then explore some
model-independent phenomenological features as, e.g., the fine-tuning problem
of electroweak symmetry breaking in flux compactifications. We also comment on
other phenomenological features of this scenario.Comment: 41 pages, 10 figure
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