1,745 research outputs found
Calabi-Yau 3-folds from 2-folds
We consider type IIA string theory on a Calabi-Yau 2-fold with D6-branes
wrapping 2-cycles in the 2-fold. We find a complete set of conditions on the
supergravity solution for any given wrapped brane configuration in terms of
SU(2) structures. We reduce the problem of finding a supergravity solution for
the wrapped branes to finding a harmonic function on RCY. We then
lift this solution to 11-dimensions as a product of R and a
Calabi-Yau 3-fold. We show how the metric on the 3-fold is determined in terms
of the wrapped brane solution. We write down the distinguished (3,0) form and
the K{\"a}hler form of the 3-fold in terms of structures defined on the base
2-d complex manifold. We discuss the topology of the 3-fold in terms of the
D6-branes and the underlying 2-fold. We show that in addition to the
non-trivial cycles inherited from the underlying 2-fold there are new
2-cycles. We construct closed (1,1) forms corresponding to these new cycles. We
also display some explicit examples. One of our examples is that of D6-branes
wrapping the 2-cycle in an A ALE space, the resulting 3-fold has
, where is the number of D6-branes.Comment: 30 page
Could and Should America Have Made an Ottoman Republic in 1919?
Numerous Americans, perhaps especially American lawyers, have since the 1780s presumed to tell other peoples how to govern themselves. In 2006, that persistent impulse was once again echoed in an address to the American Bar Association by a Justice of the Supreme Court. The purpose of this essay is to question the wisdom of this evangelical ambition, especially when the form of instruction includes military force. It is draws on Spreading America\u27s Word (2005) and directs attention to the hopes of American Protestant Zionists to make a democratic republic in Ottoman Palestine. It suggests that chances were better in 1919 than they are in 2008, but were none to good at that time. It rejects the appeal of the militant neo-conservatives who expressed their hopes and expectations in The Project for A New American Century, an instrument that should be read and remembered for centuries to come
Splitting of macroscopic fundamental strings in flat space and holographic hadron decays
In this review article we present the calculation of the splitting rate in
flat space of a macroscopic fundamental string either intersecting at a generic
angle a Dp-brane or lying on it. The result is then applied, in the context of
the string/gauge theory correspondence, to the study of exclusive decay rates
of large spin mesons into mesons. As examples, we discuss the cases of N=4 SYM
with a small number of flavors, and of QCD-like theories in the quenched
approximation. In the latter context, explicit analytic formulas are given for
decay rates of mesons formed either by heavy quarks or by massless quarks.Comment: 17 pages, 3 figures. Invited review for Modern Physics Letters
String splitting and strong coupling meson decay
We study the decay of high spin mesons using the gauge/string theory
correspondence. The rate of the process is calculated by studying the splitting
of a macroscopic string intersecting a D-brane. The result is applied to the
decay of mesons in N=4 SYM with a small number of flavors and in a gravity dual
of large N QCD. In QCD the decay of high spin mesons is found to be heavily
suppressed in the regime of validity of the supergravity description.Comment: 17 pages, 2 figures. V2: References added. V3: Minor correction
The energy and stability of D-term strings
Cosmic strings derived from string theory, supergravity or any theory of
choice should be stable if we hope to observe them. In this paper we consider
D-term strings in D=4, N=1 supergravity with a constant Fayet-Iliopoulos term.
We show that the positive deficit angle supersymmetric D-term string is
non-perturbatively stable by using standard Witten-Nester techniques to prove a
positive energy theorem. Particular attention is paid to the negative deficit
angle D-term string, which is known to violate the dominant energy condition.
Within the class of string solutions we consider, this violation implies that
the negative deficit angle D-term string must have a naked pathology and
therefore the positive energy theorem we prove does not apply to it. As an
interesting aside, we show that the Witten-Nester charge calculates the total
gravitational energy of the D-term string without the need for a cut-off, which
may not have been expected.Comment: 18 pages. v2: minor changes and references adde
Flavor brane on the baryonic branch of moduli space
We study an extra flavor in the cascading SU((k+1)M)xSU(k M) gauge theory by
adding probe D7-brane to the geometry. By finding a solution to the
kappa-symmetry equation we establish that the D7-brane is mutually
supersymmetric with the background everywhere on the baryonic branch of moduli
space. We also discuss possible applications of this result.Comment: 15 pages; v2 typo corrected, references adde
From ten to four and back again: how to generalize the geometry
We discuss the four-dimensional N=1 effective approach in the study of warped
type II flux compactifications with SU(3)x SU(3)-structure to AdS_4 or flat
Minkowski space-time. The non-trivial warping makes it natural to use a
supergravity formulation invariant under local complexified Weyl
transformations. We obtain the classical superpotential from a standard
argument involving domain walls and generalized calibrations and show how the
resulting F-flatness and D-flatness equations exactly reproduce the full
ten-dimensional supersymmetry equations. Furthermore, we consider the effect of
non-perturbative corrections to this superpotential arising from gaugino
condensation or Euclidean D-brane instantons. For the latter we derive the
supersymmetry conditions in N=1 flux vacua in full generality. We find that the
non-perturbative corrections induce a quantum deformation of the internal
generalized geometry. Smeared instantons allow to understand KKLT-like AdS
vacua from a ten-dimensional point of view. On the other hand, non-smeared
instantons in IIB warped Calabi-Yau compactifications 'destabilize' the
Calabi-Yau complex structure into a genuine generalized complex one. This
deformation gives a geometrical explanation of the non-trivial superpotential
for mobile D3-branes induced by the non-perturbative corrections.Comment: LaTeX, 47 pages, v2, references, hyperref added, v3, correcting small
inaccuracies in eqs. (2.6a) and (5.16
Deformations of calibrated D-branes in flux generalized complex manifolds
We study massless deformations of generalized calibrated cycles, which
describe, in the language of generalized complex geometry, supersymmetric
D-branes in N=1 supersymmetric compactifications with fluxes. We find that the
deformations are classified by the first cohomology group of a Lie algebroid
canonically associated to the generalized calibrated cycle, seen as a
generalized complex submanifold with respect to the integrable generalized
complex structure of the bulk. We provide examples in the SU(3) structure case
and in a `genuine' generalized complex structure case. We discuss cases of
lifting of massless modes due to world-volume fluxes, background fluxes and a
generalized complex structure that changes type.Comment: 52 pages, added references, added comment on ellipticity in appendix
B, made minor changes according to instructions referee JHE
Synergic strategies to improve the PBF-LB/M processability of a cracking-sensitive alloy
The Powder Bed Fusion-Laser Beam\Metals (PBF-LB\M) is a promising additive manufacturing process
that can be used to directly produce functional components with a complex shape for a wide variety
of applications. However, the layer-by-layer scanning and high cooling rates result in a high thermal gra-
dient and thus, in thermally induced stresses. The stresses developed during the additive process could
lead to undesirable cracking and delamination phenomena that can seriously affect the performance of
the final component. The alloy composition can exacerbate crack and delamination formation, however,
the need to expand the portfolio of high-strength materials processable for PBF-LB\M makes the resolu-
tion of these undesirable phenomena a primary challenge in the additive manufacturing field. This works
aims to systematically investigate some strategies to make processable non-standard compositions. As no
standard compositions, the promising pre-alloyed AlSi10Cu8Mg composition was chosen for the present
work. Based on the results obtained from a condition of severe delamination, the synergetic use of appro-
priate process parameters and support structures can lead to crack-free and fully dense specimens also
when platform heating is not allowed. The developed approach could also be applied to adapt other
cracking-sensitive alloys for PBF-LB\M production
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