arXiv subjects

Oscar Loaiza-Brito

Publications and source records attributed to Oscar Loaiza-Brito.

33 records · Page 2Linked to original sources

arXiv2013

More stable dS vacua from S-dual non-geometric fluxes

Stable vacua obtained from isotropic tori compactification might not be fully stable provided the existence of runaway directions in the Kaehler directions of anisotropy. By implementing a genetic algorithm we report the existence of explicit flux configurations leading to stable de Sitter and Anti- de Sitter vacua, consisting on Type IIB compactifications on a 6-dimensional anisotropic torus threaded with standard and S-dual invariant non-geometric fluxes in the presence of orientifold 3-planes. In all dS vacua the masses of the complex structure moduli are heavier than the Hubble scale suggesting that the axio-dilaton and Kaeahler moduli are natural candidates for small-field inflation. In the way, we also report new solutions on isotropic and semi-isotropic tori compactifications. Finally, we observe that, since all our solutions are obtained in the absence of solitonic objects, they are good candidates to be lifted to stable solutions in extended supersymmetric theories.

hep-th↗

arXiv2011

Closed String Tachyon: Inflation and Cosmological Collapse

By compactifying a critical bosonic string theory on an internal non-flat space with a constant volume, we study the role played by the closed string tachyon in the cosmology of the effective four-dimensional space-time. The effective tachyon potential consists on a negative constant related to the internal curvature space and a polynomial with only quadratic and quartic terms of the tachyon field. Based on it, we present a solution for the tachyon field and the scale factor, which describes an accelerated universe which expands to a maximum value before collapsing. At early times, the closed string tachyon potential behaves as a cosmological constant driving the Universe to an expansion. The value of the cosmological constant is determined by the curvature of the internal space which also fixes the value of the vacuum energy. As time evolves, inflation is present in our models, and it finishes long before the collapsing. At late times, we show that the collapse of the Universe starts as soon as the tachyon field condensates. We also comment on the consistency of our solution at early times at which quantum aspects become important under the perspective of quantum cosmology. Finally we briefly mention the relation among the curvature of the internal space and the value of the energy vacuum in a non-constant internal volume scenario.

gr-qc↗

arXiv2011

Spherically symmetric solution in a space-time with torsion

By using the method of group analysis, we obtain a new exact evolving and spherically symmetric solution of the Einstein-Cartan equations of motion, corresponding to a space-time threaded with a three-form Kalb-Ramond field strength. The solution describes in its more generic form, a space-time which scalar curvature vanishes for large distances and for large time. In static conditions, it reduces to a classical wormhole solution and to a exact solution with a localized scalar field and a torsion kink, already reported in literature. In the process we have found evidence towards the construction of more new solutions.

gr-qc↗

arXiv2011

Near-Horizon geometry from flux compactification

We study the conditions an arbitrary flux configuration must fulfill in order to construct a 4d space-time of the type $AdS_2\times S^2$ from a type IIB supergravity flux compactification in which NS-NS fluxes are included. We present a solution consisting on a compactification in the presence of 3-form NS-NS and RR fluxes. The internal manifold is a SU(3) structure six-dimensional manifold, with null curvature and with torsion. By preserving two supersymmetries in the four-dimensional low energy theory, we find a way to obtain the $AdS_2\times S^2$ geometry as a near-horizon solution by compactification in non-Calabi-Yau manifolds.

hep-th↗

arXiv2011

Topological effects on string vacua

We review some topological effects on the construction of string flux-vacua. Specifically we study the effects of brane-flux transitions on the stability of D-branes on a generalized tori compactificaction, the transition that a black hole suffers in a background threaded with fluxes and the connections among some Minkowsky vacua solutions.

hep-th↗

arXiv2009

Minkowski vacuum transitions in (non-geometric) flux compactifications

In this work we study the generalization of twisted homology to geometric and non-geometric backgrounds. In the process we describe the necessary conditions to wrap a network of D-branes on twisted cycles. If the cycle is localized in time, we show how by an instantonic brane mediation, some D-branes transform into fluxes on different backgrounds including non-geometric fluxes. As a consequence, we show that in the case of a IIB six-dimensional torus compactification on a simple orientifold, the flux superpotential is not invariant by this brane-flux transition, allowing the connection among different Minkowski vacuum solutions. For the case in which non-geometric fluxes are turned on, we also discuss some topological restrictions for the transition to occur. In this context, we show that there are some vacuum solutions protected to change by a brane-flux transition.

hep-th↗

arXiv2007

Effects of brane-flux transition on black holes in string theory

Extremal N=2 black holes in four dimensions can be described by an ensemble of D3-branes wrapped on internal supersymmetric three-cycles of Calabi-Yau threefolds on which type IIB superstring theory is compactified. We construct a similar configuration, with extra RR and NS-NS three-form fluxes being turned on. We can avoid the Freed-Witten anomaly on the D3-branes by enforcing the pullback of these extra fluxes to the D3-branes to vanish at the classical level. In the setup the D3-brane charge is not conserved since it is classified as a trivial class in twisted K-theory. Consequently, the D3-branes may disappear by encountering an instantonic D5-brane localized in time. We discuss what happens on the black hole described by such disappearing D3-branes, relying mainly on topological arguments. Especially, we argue that another RR three-form flux will be left as a lump of remnant which is localized in the uncompactified four-dimensional space-time and that it may carry the same amount of degrees of freedom to describe a black hole, in cooperation with the original NS-NS flux, after this transition of the D3-branes.

hep-th↗

arXiv2007

Freed-Witten anomaly in general flux compactification

Turning on a NS-NS three-form flux in a compact space drives some D-branes to be either Freed-Witten anomalous or unstable to decay into fluxes by the appearance of instantonic branes. By applying T-duality on a toroidal compactification, the NS-flux is transformed into metric fluxes. We propose a T-dual version of the Atiyah-Hirzebruch Spectral Sequence upon which we describe the Freed-Witten anomaly and the brane-flux transition driven by NS and metric fluxes in a twisted torus. The required conditions to cancel the anomaly and the appearance of new instantonic branes are also described. In addition, we give an example in which all D6-branes wrapping Freed-Witten anomaly-free three-cycles in the twisted torus T^6/Z(2)XZ(2) are nevertheless unstable to be transformed into fluxes. Evenmore we find a topological transformation between RR, NS-NS and metric fluxes driven by a chain of instantonic branes.

hep-th↗

arXiv2005

log(M_Pl/m_3/2)

Flux compactifications of string theory seem to require the presence of a fine-tuned constant in the superpotential. We discuss a scheme where this constant is replaced by a dynamical quantity which we argue to be a `continuous Chern--Simons term'. In such a scheme, the gaugino condensate generates the hierarchically small scale of supersymmetry breakdown rather than adjusting its size to a constant. A crucial ingredient is the appearance of the hierarchically small quantity exp(- ) which corresponds to the scale of gaugino condensation. Under rather general circumstances, this leads to a scenario of moduli stabilization, which is endowed with a hierarchy between the mass of the lightest modulus, the gravitino mass and the scale of the soft terms, m_modulus ~ m_3/2 ~ ^2 m_soft. The `little hierarchy' is given by the logarithm of the ratio of the Planck scale and the gravitino mass, ~ log(M_Pl/m_3/2) ~ 4pi^2. This exhibits a new mediation scheme of supersymmetry breakdown, called mirage mediation. We highlight the special properties of the scheme, and their consequences for phenomenology and cosmology.

hep-th↗

arXiv2004

Branes and Fluxes in Orientifolds and K-theory

RR fields in string backgrounds including orientifold planes and branes on top of them are classified by K-theory. Following the idea introduced in hep-th/0103183, we also classify such fluxes by cohomology. Both of them are compared through the Atiyah-Hirzebruch Spectral Sequence. Some new correlations between branes on orientifold planes $Op^\pm$ and obstructions to the existence of some branes are found. Finally, we find a topological condition that avoid the presence of global gauge anomalies in lower dimensional systems.

hep-th↗

arXiv2003

Instantonic Branes, Atiyah-Hirzebruch Spectral Sequence, and SL(2,Z) duality of N=4 SYM

We study the equivalence among orientifold three-planes in the context of the Atiyah-Hirzebruch Spectral Sequence. This equivalence refers to the fact that two different cohomology classes in the same cohomology group, which classify the orientifolds, are lifted to the same trivial class in K-theory. The physical interpretation of this mathematical algorithm is given by the role of D-brane instantons. By following some recent ideas, we extend the sequence to include a classification of NS-NS fluxes. We find that such equivalences, in the low energy limit of the dynamics on the worldvolume of type IIB D3-branes on top of the orientifolds, are interpreted as the SL(2,Z) duality in four dimensional N=4 SYM theories.

hep-th↗

arXiv2003

The Exchange of Orientifold Two-Planes in M-theory

We propose an M-theory lift picture of the exchange among type IIA orientifold two-planes. This consists in wrapping a M5-brane on a three-cycle in the transverse space of the M-theory orientifold plane OM2. A flux quantization condition for the three-form self-dual field strength, on the worldvolume of the M5-brane is computed. This condition establishes the value which explains the relative charge between two different OM2-planes. Also, we find that the exchange of the four types of orientifold two-planes in string theory, has a common picture in M-theory. Moreover, we find that the assignment of the extra charge is fixed by cohomology and by the flux quantization of the field strength G in M-theory. We conclude that cohomology is sufficient to describe some orientifold properties in M-theory, that at string theory level, only K-theory is able to explain.

hep-th↗

arXiv2001

The fate of the type I non-BPS D7-brane

We describe the fate of the Type I non-BPS D7-brane, which is tachyonic but carries a non-trivial K-theory $\IZ_2$ charge. It decays to topologically non-trivial gauge field configurations on the background D9-branes. In the uncompactified theory the decay proceeds to infinity, while with a transverse torus the decay reaches a final state, a toron gauge configuration with vanishing Chern classes but non-trivial $\IZ_2$ charge. A similar behaviour is obtained for the type I non-BPS D8-brane, and other related systems. We construct explicit examples of type IIB orientifolds with non-BPS D7-branes, which are hence non-supersymmetric, but for which supersymmetry is restored upon condensation of the tachyon. We also report on the interesting structure of non-BPS states of type IIA theory in the presence of an O6-plane, their M-theory lifts, the relation between string theory K-theory and M-theory cohomology, and its interplay with NS-NS charged objects. We discuss several new effects, including: i) transmutation between NS-NS and RR torsion charges, ii) non-BPS states classified by K-theory but not by cohomology in string theory, but whose lift to M-theory is cohomological.

hep-th↗

arXiv2000

Topics on Strings, Branes and Calabi-Yau Compactifications

Basics of some topics on perturbative and non-perturbative string theory are reviewed. After a mathematical survey of the Standard Model of particle physics and GUTs, the bosonic string kinematics for the free case and with interaction is described. The effective action of the bosonic string and the spectrum is also discussed. Five perturbative superstring theories and their spectra is briefly outlined. Calabi-Yau three-fold compactifications of heterotic strings and their relation to some four-dimensional physics are given. T-duality in closed and open strings are surveyed. D-brane definition is provided and some of their properties and applications to brane boxes configurations, in particular to the cube model are discussed. Finally, non-perturbative issues like S-duality, M-theory, F-theory and basics of their non-perturbative Calabi-Yau compactifications are considered.

hep-th↗

arXiv2000

Lectures on Strings, D-branes and Gauge Theories

In these lectures we review the basic ideas of perturbative and non-perturbative string theory. On the non-perturbative side we give an introduction to D-branes and string duality. The elementary concepts of non-BPS branes and noncommutative gauge theories are also discussed.

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