475 research outputs found
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
Response of microbial activity to labile C addition in sandy soil from semi-arid woodland is influenced by vegetation patch and wildfire
Nutrient cycling in semi-arid woodlands is likely to be influenced by patchy vegetation, wildfire and the supply of easily available organic C, e.g. root exudates. The study assessed the effect of wildfire and vegetation patch on response of microbial activity to labile C addition in soil from a semi-arid Eucalyptus woodland. Two sites were studied: one unburnt and the other exposed to wildfire four-month before sampling. Top soil (0 – 30 cm) from under trees, under shrubs or in open areas from each site was air-dried and sieved to < 2 mm. The soils were incubated at 80% of maximum water holding capacity for 24 days without or with addition of 5 g C kg-1 as glucose. Soil organic carbon (TOC), microbial biomass C, N and P availability and cumulative respiration were greater under trees than in open areas. Fire decreased TOC and cumulative respiration only under trees and had little effect on available N, microbial biomass C and P concentrations. The greater increase in cumulative respiration by glucose addition under shrubs and in open areas compared to under trees and, in a given patch, greater in burnt than unburnt soils, indicate lower availability of native organic carbon.Qiaoqi Sun, Wayne S. Meyer, Georgia R. Koerber, Petra Marschne
D-branes on AdS flux compactifications
We study D-branes in N=1 flux compactifications to AdS_4. We derive their
supersymmetry conditions and express them in terms of background generalized
calibrations. Basically because AdS has a boundary, the analysis of stability
is more subtle and qualitatively different from the usual case of Minkowski
compactifications. For instance, stable D-branes filling AdS_4 may wrap trivial
internal cycles. Our analysis gives a geometric realization of the
four-dimensional field theory approach of Freedman and collaborators.
Furthermore, the one-to-one correspondence between the supersymmetry conditions
of the background and the existence of generalized calibrations for D-branes is
clarified and extended to any supersymmetric flux background that admits a
time-like Killing vector and for which all fields are time-independent with
respect to the associated time. As explicit examples, we discuss supersymmetric
D-branes on IIA nearly Kaehler AdS_4 flux compactifications.Comment: 43 pages, 2 pictures, 1 table; v2: added references, color to figure
and corrected typo in (6.21b
AdS4 flux vacua in type II superstrings and their domain-wall solutions
We investigate the emergence of supersymmetric negative-vacuum-energy ground
states in four dimensions. First, we rely on the analysis of the effective
superpotential, which depends on the background fluxes of the internal
manifold, or equivalently has its origin in the underlying gauged supergravity.
Four-dimensional, supersymmetric anti-de Sitter vacua with all moduli
stabilized appear when appropriate Ramond and Neveu--Schwarz fluxes are
introduced in IIA. Geometric fluxes are not necessary. Then the whole setup is
analyzed from the perspective of the sources, namely D/NS-branes or
Kaluza--Klein monopoles. Orientifold planes are also required for tadpole
cancellation. The solutions found in four dimensions correspond to domain walls
interpolating between AdS4 and flat spacetime. The various consistency
conditions (equations of motion, Bianchi identities and tadpole cancellation
conditions) are always satisfied, albeit with source terms. We also speculate
on the possibility of assigning (formal) entropies to AdS4 flux vacua via the
corresponding dual brane systems.Comment: Acknowledgment replace
A second look at N=1 supersymmetric AdS_4 vacua of type IIA supergravity
We show that a class of type IIA vacua recently found within the N=4
effective approach corresponds to compactification on Ads_4 \times S^3 \times
S^3/Z_2^3. The results obtained using the effective method completely match the
general ten-dimensional analysis for the existence of N=1 warped
compactifications on Ads_4 \times M_6. In particular, we verify that the
internal metric is nearly-Kahler and that for specific values of the parameters
the Bianchi identity of the RR 2-form is fulfilled without sources. For another
range of parameters, including the massless case, the Bianchi identity is
satisfied when D6-branes are introduced. Solving the tadpole cancellation
conditions in D=4 we are able to find examples of appropriate sets of branes.
In the second part of this paper we describe how an example with internal space
CP^3 but with non nearly-Kahler metric fits into the general analysis of flux
vacua.Comment: Latex file, 35 pages, no figures. Reference added, minor corrections
adde
Universal de Sitter solutions at tree-level
Type IIA string theory compactified on SU(3)-structure manifolds with
orientifolds allows for classical de Sitter solutions in four dimensions. In
this paper we investigate these solutions from a ten-dimensional point of view.
In particular, we demonstrate that there exists an attractive class of de
Sitter solutions, whose geometry, fluxes and source terms can be entirely
written in terms of the universal forms that are defined on all SU(3)-structure
manifolds. These are the forms J and Omega, defining the SU(3)-structure
itself, and the torsion classes. The existence of such universal de Sitter
solutions is governed by easy-to-verify conditions on the SU(3)-structure,
rendering the problem of finding dS solutions purely geometrical. We point out
that the known (unstable) solution coming from the compactification on SU(2)x
SU(2) is of this kind.Comment: 20 pages, 3 figures, v2: added reference
On moduli and effective theory of N=1 warped flux compactifications
The moduli space of N=1 type II warped compactions to flat space with generic
internal fluxes is studied. Using the underlying integrable generalized complex
structure that characterizes these vacua, the different deformations are
classified by H-twisted generalized cohomologies and identified with chiral and
linear multiplets of the effective four-dimensional theory. The Kaehler
potential for chiral fields corresponding to classically flat moduli is
discussed. As an application of the general results, type IIB warped Calabi-Yau
compactifications and other SU(3)-structure subcases are considered in more
detail.Comment: 54 pages; v3: comments and references added, version published in
JHE
Dynamic SU(2) Structure from Seven-branes
We obtain a family of supersymmetric solutions of type IIB supergravity with
dynamic SU(2) structure, which describe the local geometry near a stack of four
D7-branes and one O7-plane wrapping a rigid four-cycle. The deformation to a
generalized complex geometry is interpreted as a consequence of nonperturbative
effects in the seven-brane gauge theory. We formulate the problem for
seven-branes wrapping the base of an appropriate del Pezzo cone, and in the
near-stack limit in which the four-cycle is flat, we obtain an exact solution
in closed form. Our solutions serve to characterize the local geometry of
nonperturbatively-stabilized flux compactifications.Comment: 49 pages, 2 figures; v2: minor corrections, references adde
The problematic backreaction of SUSY-breaking branes
In this paper we investigate the localisation of SUSY-breaking branes which,
in the smeared approximation, support specific non-BPS vacua. We show, for a
wide class of boundary conditions, that there is no flux vacuum when the branes
are described by a genuine delta-function. Even more, we find that the smeared
solution is the unique solution with a regular brane profile. Our setup
consists of a non-BPS AdS_7 solution in massive IIA supergravity with smeared
anti-D6-branes and fluxes T-dual to ISD fluxes in IIB supergravity.Comment: 27 pages, Latex2e, 5 figure
AdS Vacua, Attractor Mechanism and Generalized Geometries
We consider flux vacua attractor equations in type IIA string theory
compactified on generalized geometries with orientifold projections. The
four-dimensional N=1 superpotential in this compactification can be written as
the sum of the Ramond-Ramond superpotential and a term described by
(non)geometric flux charges. We exhibit a simple model in which supersymmetric
AdS and Minkowski solutions are classified by means of discriminants of the two
superpotentials. We further study various configurations without Ramond-Ramond
flux charges. In this case we find supersymmetric AdS vacua both in the case of
compactifications on generalized geometries with SU(3) x SU(3) structures and
on manifolds with an SU(3)-structure without nongeometric flux charges. In the
latter case, we have to introduce correction terms into the prepotential in
order to realize consistent vacua.Comment: 35 pages, accepted version in JHE
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