3,821 research outputs found
On NS5-brane instantons and volume stabilization
We study general aspects of NS5-brane instantons in relation to the
stabilization of the volume modulus in Calabi-Yau compactifications of type II
strings with fluxes, and their orientifold versions. These instantons correct
the Kahler potential and generically yield significant contributions to the
scalar potential at intermediate values of string coupling constant and volume.
Under suitable conditions they yield uplifting terms that allow for
meta--stable de Sitter vacua.Comment: 29 pages, 3 figures; statements about fields G^a made more precise,
added some clarifications, typos correcte
D-instantons and twistors: some exact results
We present some results on instanton corrections to the hypermultiplet moduli
space in Calabi-Yau compactifications of Type II string theories. Previously,
using twistor methods, only a class of D-instantons (D2-instantons wrapping
A-cycles) was incorporated exactly and the rest was treated only linearly. We
go beyond the linear approximation and give a set of holomorphic functions
which, through a known procedure, capture the effect of D-instantons at all
orders. Moreover, we show that for a sector where all instanton charges have
vanishing symplectic invariant scalar product, the hypermultiplet metric can be
computed explicitly.Comment: 32 pages, 3 figures, uses JHEP3.cls; some changes in section 3.3.3;
corrected formula for the contact potentia
S-duality in Twistor Space
In type IIB string compactifications on a Calabi-Yau threefold, the
hypermultiplet moduli space must carry an isometric action of the modular
group SL(2,Z), inherited from the S-duality symmetry of type IIB string theory
in ten dimensions. We investigate how this modular symmetry is realized at the
level of the twistor space of , and construct a general class of
SL(2,Z)-invariant quaternion-Kahler metrics with two commuting isometries,
parametrized by a suitably covariant family of holomorphic transition
functions. This family should include corrected by D3-D1-D(-1)-instantons
(with fivebrane corrections ignored) and, after taking a suitable rigid limit,
the Coulomb branch of five-dimensional N=2 gauge theories compactified on a
torus, including monopole string instantons. These results allow us to
considerably simplify the derivation of the mirror map between type IIA and IIB
fields in the sector where only D1-D(-1)-instantons are retained.Comment: 29 pages, 1 figur
Supergravity description of spacetime instantons
We present and discuss BPS instanton solutions that appear in type II string
theory compactifications on Calabi-Yau threefolds. From an effective action
point of view these arise as finite action solutions of the Euclidean equations
of motion in four-dimensional N=2 supergravity coupled to tensor multiplets. As
a solution generating technique we make use of the c-map, which produces
instanton solutions from either Euclidean black holes or from Taub-NUT like
geometries.Comment: 35 pages, some clarifications adde
Heterotic-Type II duality in the hypermultiplet sector
We revisit the duality between heterotic string theory compactified on K3 x
T^2 and type IIA compactified on a Calabi-Yau threefold X in the hypermultiplet
sector. We derive an explicit map between the field variables of the respective
moduli spaces at the level of the classical effective actions. We determine the
parametrization of the K3 moduli space consistent with the Ferrara-Sabharwal
form. From the expression of the holomorphic prepotential we are led to
conjecture that both X and its mirror must be K3 fibrations in order for the
type IIA theory to have an heterotic dual. We then focus on the region of the
moduli space where the metric is expressed in terms of a prepotential on both
sides of the duality. Applying the duality we derive the heterotic
hypermultiplet metric for a gauge bundle which is reduced to 24 point-like
instantons. This result is confirmed by using the duality between the heterotic
theory on T^3 and M-theory on K3. We finally study the hyper-Kaehler metric on
the moduli space of an SU(2) bundle on K3.Comment: 27 pages; references added, typos correcte
Completeness in supergravity constructions
We prove that the supergravity r- and c-maps preserve completeness. As a
consequence, any component H of a hypersurface {h=1} defined by a homogeneous
cubic polynomial such that -d^2 h is a complete Riemannian metric on H defines
a complete projective special Kahler manifold and any complete projective
special Kahler manifold defines a complete quaternionic Kahler manifold of
negative scalar curvature. We classify all complete quaternionic Kahler
manifolds of dimension less or equal to 12 which are obtained in this way and
describe some complete examples in 16 dimensions.Comment: 29 page
D3-instantons, Mock Theta Series and Twistors
The D-instanton corrected hypermultiplet moduli space of type II string
theory compactified on a Calabi-Yau threefold is known in the type IIA picture
to be determined in terms of the generalized Donaldson-Thomas invariants,
through a twistorial construction. At the same time, in the mirror type IIB
picture, and in the limit where only D3-D1-D(-1)-instanton corrections are
retained, it should carry an isometric action of the S-duality group SL(2,Z).
We prove that this is the case in the one-instanton approximation, by
constructing a holomorphic action of SL(2,Z) on the linearized twistor space.
Using the modular invariance of the D4-D2-D0 black hole partition function, we
show that the standard Darboux coordinates in twistor space have modular
anomalies controlled by period integrals of a Siegel-Narain theta series, which
can be canceled by a contact transformation generated by a holomorphic mock
theta series.Comment: 42 pages; discussion of isometries is amended; misprints correcte
An extreme proto-cluster of luminous dusty starbursts in the early Universe
We report the identification of an extreme proto-cluster of galaxies in the
early Universe whose core (nicknamed Distant Red Core, DRC) is formed by at
least ten dusty star-forming galaxies (DSFGs), confirmed to lie at via detection of [CI](1-0), CO(6-5), CO(4-3),
CO(2-1) and emission lines, detected
using ALMA and ATCA. The spectroscopically-confirmed components of the
proto-cluster are distributed over a region
and have a collective obscured star-formation rate (SFR) of , considerably higher than has been seen before in any
proto-cluster of galaxies or over-densities of DSFGs at . Most of
the star formation is taking place in luminous DSFGs since no Ly
emitters are detected in the proto-cluster core, apart from a Ly blob
located next to one of the DRC dusty components and extending over . The total obscured SFR of the proto-cluster could rise to if all the members of an over-density
of bright DSFGs discovered around DRC in a wide-field LABOCA 870-m image
are part of the same structure. The total halo mass of DRC could be as high as
and could be the progenitor of a Coma-like
cluster at . The relatively short gas-depletion times of the DRC
components suggest either the presence of a mechanism able to trigger extreme
star formation simultaneously in galaxies spread over a few hundred kpc or the
presence of gas flows from the cosmic web able to sustain star formation over
several hundred million years.Comment: Submitted to ApJ. Minor updates added, including a change of the
source name. Comments welcom
Magnetism of small V clusters embedded in a Cu fcc matrix: an ab initio study
We present extensive first principles density functional theory (DFT)
calculations dedicated to analyze the magnetic and electronic properties of
small V clusters (n=1,2,3,4,5,6) embedded in a Cu fcc matrix. We consider
different cluster structures such as: i) a single V impurity, ii) several
V dimers having different interatomic distance and varying local atomic
environment, iii) V and iv) V clusters for which we assume compact
as well as 2- and 1-dimensional atomic configurations and finally, in the case
of the v) V and vi) V structures we consider a square pyramid and a
square bipyramid together with linear arrays, respectively. In all cases, the V
atoms are embedded as substitutional impurities in the Cu network. In general,
and as in the free standing case, we have found that the V clusters tend to
form compact atomic arrays within the cooper matrix. Our calculated non
spin-polarized density of states at the V sites shows a complex peaked
structure around the Fermi level that strongly changes as a function of both
the interatomic distance and local atomic environment, a result that
anticipates a non trivial magnetic behavior. In fact, our DFT calculations
reveal, in each one of our clusters systems, the existence of different
magnetic solutions (ferromagnetic, ferrimagnetic, and antiferromagnetic) with
very small energy differences among them, a result that could lead to the
existence of complex finite-temperature magnetic properties. Finally, we
compare our results with recent experimental measurements.Comment: 7 pages and 4 figure
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