810 research outputs found
Reverse geometric engineering of singularities
One can geometrically engineer supersymmetric field theories theories by
placing D-branes at or near singularities. The opposite process is described,
where one can reconstruct the singularities from quiver theories. The
description is in terms of a noncommutative quiver algebra which is constructed
from the quiver diagram and the superpotential. The center of this
noncommutative algebra is a commutative algebra, which is the ring of
holomorphic functions on a variety V. If certain algebraic conditions are met,
then the reverse geometric engineering produces V as the geometry that D-branes
probe. It is also argued that the identification of V is invariant under
Seiberg dualities.Comment: 17 pages, Latex. v2: updates reference
Nonabelian D-branes and Noncommutative Geometry
We discuss the nonabelian world-volume action which governs the dynamics of N
coincident Dp-branes. In this theory, the branes' transverse displacements are
described by matrix-valued scalar fields, and so this is a natural physical
framework for the appearance of noncommutative geometry. One example is the
dielectric effect by which Dp-branes may be polarized into a noncommutative
geometry by external fields. Another example is the appearance of
noncommutative geometries in the description of intersecting D-branes of
differing dimensions, such as D-strings ending on a D3- or D5-brane. We also
describe the related physics of giant gravitons.Comment: 21 pages, Latex, ref. adde
Gauge theory, topological strings, and S-duality
We offer a derivation of the duality between the topological U(1) gauge
theory on a Calabi-Yau 3-fold and the topological A-model on the same manifold.
This duality was conjectured recently by Iqbal, Nekrasov, Okounkov, and Vafa.
We deduce it from the S-duality of the IIB superstring. We also argue that the
mirror version of this duality relates the topological B-model on a Calabi-Yau
3-fold and a topological sector of the Type IIA Little String Theory on the
same manifold.Comment: 9 pages, latex. v2: a footnote has been added. The footnote corrects
an inaccuracy in the original argument; the results are unchanged. v3:
exposition improve
World-sheet Instantons via the Myers Effect and N=1^* Quiver Superpotentials
In this note we explore the stringy interpretation of non-perturbative
effects in N=1^* deformations of the A_{k-1} quiver models. For certain types
of deformations we argue that the massive vacua are described by Nk fractional
D3-branes at the orbifold polarizing into k concentric 5-brane spheres each
carrying fractional brane charge. The polarization of the D3-branes induces a
polarization of D-instantons into string world-sheets wrapped on the Myers
spheres. We show that the superpotentials in these models are indeed generated
by these world-sheet instantons. We point out that for certain parameter values
the condensates yield the exact superpotential for a relevant deformation of
the Klebanov-Witten conifold theory.Comment: 24 pages, JHEP, some small errors and typos correcte
Generally Covariant Actions for Multiple D-branes
We develop a formalism that allows us to write actions for multiple D-branes
with manifest general covariance. While the matrix coordinates of the D-branes
have a complicated transformation law under coordinate transformations, we find
that these may be promoted to (redundant) matrix fields on the transverse space
with a simple covariant transformation law. Using these fields, we define a
covariant distribution function (a matrix generalization of the delta function
which describes the location of a single brane). The final actions take the
form of an integral over the curved space of a scalar single-trace action built
from the covariant matrix fields, tensors involving the metric, and the
covariant distribution function. For diagonal matrices, the integral localizes
to the positions of the individual branes, giving N copies of the single-brane
action.Comment: 34 pages, LaTeX. v2: comments and refs adde
Boundary States for Supertubes in Flat Spacetime and Godel Universe
We construct boundary states for supertubes in the flat spacetime. The T-dual
objects of supertubes are moving spiral D1-branes (D-helices). Since we can
obtain these D-helices from the usual D1-branes via null deformation, we can
construct the boundary states for these moving D-helices in the covariant
formalism. Using these boundary states, we calculate the vacuum amplitude
between two supertubes in the closed string channel and read the open string
spectrum via the open closed duality. We find there are critical values of the
energy for on-shell open strings on the supertubes due to the non-trivial
stringy correction. We also consider supertubes in the type IIA Godel universe
in order to use them as probes of closed timelike curves. This universe is the
T-dual of the maximally supersymmetric type IIB PP-wave background. Since the
null deformations of D-branes are also allowed in this PP-wave, we can
construct the boundary states for supertubes in the type IIA Godel universe in
the same way. We obtain the open string spectrum on the supertube from the
vacuum amplitude between supertubes. As a consequence, we find that the
tachyonic instability of open strings on the supertube, which is the signal of
closed time like curves, disappears due to the stringy correction.Comment: 26 pages, 3 figures, v2: explanations added, references added, v3:
explanations adde
Fuzzy Sphere Dynamics and Non-Abelian DBI in Curved Backgrounds
We consider the non-Abelian action for the dynamics of -branes in the
background of -branes, which parameterises a fuzzy sphere using the SU(2)
algebra. We find that the curved background leads to collapsing solutions for
the fuzzy sphere except when we have branes in the background, which
is a realisation of the gravitational Myers effect. Furthermore we find the
equations of motion in the Abelian and non-Abelian theories are identical in
the large limit. By picking a specific ansatz we find that we can
incorporate angular momentum into the action, although this imposes restriction
upon the dimensionality of the background solutions. We also consider the case
of non-Abelian non-BPS branes, and examine the resultant dynamics using
world-volume symmetry transformations. We find that the fuzzy sphere always
collapses but the solutions are sensitive to the combination of the two
conserved charges and we can find expanding solutions with turning points. We
go on to consider the coincident 5-brane background, and again construct
the non-Abelian theory for both BPS and non-BPS branes. In the latter case we
must use symmetry arguments to find additional conserved charges on the
world-volumes to solve the equations of motion. We find that in the Non-BPS
case there is a turning solution for specific regions of the tachyon and radion
fields. Finally we investigate the more general dynamics of fuzzy
in the -brane background, and find collapsing solutions
in all cases.Comment: 49 pages, 3 figures, Latex; Version to appear in JHE
Statefinder diagnostic and stability of modified gravity consistent with holographic and new agegraphic dark energy
Recently one of us derived the action of modified gravity consistent with the
holographic and new-agegraphic dark energy. In this paper, we investigate the
stability of the Lagrangians of the modified gravity as discussed in [M. R.
Setare, Int. J. Mod. Phys. D 17 (2008) 2219; M. R. Setare, Astrophys. Space
Sci. 326 (2010) 27]. We also calculate the statefinder parameters which
classify our dark energy model.Comment: 12 pages, 2 figures, accepted by Gen. Relativ. Gravi
Deconstructing Noncommutativity with a Giant Fuzzy Moose
We argue that the worldvolume theories of D-branes probing orbifolds with
discrete torsion develop, in the large quiver limit, new non-commutative
directions. This provides an explicit `deconstruction' of a wide class of
noncommutative theories. This also provides insight into the physical meaning
of discrete torsion and its relation to the T-dual B field. We demonstrate that
the strict large quiver limit reproduces the matrix theory construction of
higher-dimensional D-branes, and argue that finite `fuzzy moose' theories
provide novel regularizations of non-commutative theories and explicit string
theory realizations of gauge theories on fuzzy tori. We also comment briefly on
the relation to NCOS, (2,0) and little string theories.Comment: 22 pages, 3 figures, typos caught and refs added; expanded
interpretation of discrete torsio
Non Abelian Tachyon Kinks
Starting from the action of two coincident non-BPS D9-branes, we investigate
kink configurations of the U(2) matrix tachyon field. We consider both Str and
Tr prescriptions for the trace over gauge indices of the non-BPS action.
Non-abelian tachyon condensation in the theory with Tr prescription, and the
resulting fluctuations about the kink profile, are shown to give rise to a
theory of two coincident BPS D8-branes. This is a natural non-abelian
generalization of Sen's mechanism of tachyon condensation on a single non-BPS
Dp-brane yielding a single BPS brane of codimesion one. By contrast, starting
with the Str gauge trace prescription of the coincident non-BPS D9-brane
action, such a generalization of Sen's mechanism appears problematic.Comment: 18 pages, references added, version to appear in JHE
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