D=4, N=1 orientifolds with vector structure

We construct compact type IIB orientifolds with discrete groups Z_4, Z_6, Z_6', Z_8, Z_12 and Z_12'. These models are N=1 supersymmetric in D=4 and have vector structure. The possibility of having vector structure in Z_N orientifolds with even N arises due to an alternative Omega-projection in the twisted sectors. Some of the models without vector structure are known to be inconsistent because of uncancelled tadpoles. We show that vector structure leads to a sign flip in the twisted Klein bottle contribution. As a consequence, all the tadpoles can be cancelled by introducing D9-branes and D5-branes.Comment: Latex, 44 pages, 2 figures, v2: misprints and an error concerning Omega^2_{95} corrected, a comment on D5-branes with negative NSNS charge added, references and acknowledgements adde

A class of non-supersymmetric orientifolds

We study type IIB orientifolds on T^{2d}/Z_N with supersymmetry broken by the compactification. We determine tadpole cancellation conditions including anti-branes and considering different actions for the parity Omega. Using these conditions we then obtain the spectrum of tachyons and massless states. Various examples with N even correspond to type 0B orientifolds.Comment: 49 pages, Late

Aceite de almendra virgen: Métodos de extracción y composición

In this paper the extraction methods of virgin almond oil and its chemical composition are reviewed. The most common methods for obtaining oil are solvent extraction, extraction with supercritical fluids (CO2) and pressure systems (hydraulic and screw presses). The best industrial performance, but also the worst oil quality is achieved by using solvents. Oils obtained by this method cannot be considered virgin oils as they are obtained by chemical treatments. Supercritical fluid extraction results in higher quality oils but at a very high price. Extraction by pressing becomes the best option to achieve high quality oils at an affordable price. With regards chemical composition, almond oil is characterized by its low content in saturated fatty acids and the predominance of monounsaturated, especially oleic acid. Furthermore, almond oil contains antioxidants and fat-soluble bioactive compounds that make it an oil with interesting nutritional and cosmetic properties.En este trabajo se revisan los métodos de extracción del aceite de almendra virgen y su composición química. Los métodos más habituales para la obtención del aceite son la extracción con disolventes, la extracción con fluidos supercríticos (CO2) y los sistemas de presión (prensas hidráulica y de tornillo). El mayor rendimiento industrial, pero también la peor calidad de los aceites, se consigue mediante el uso de disolventes. Además, los aceites obtenidos por este método no se pueden considerar vírgenes, pues se obtienen por medio de tratamientos químicos. La extracción con fluidos supercríticos da lugar a aceites de mayor calidad pero a un precio muy elevado. La extracción mediante prensado se convierte en la mejor opción de extracción, al conseguir aceites de alta calidad a un precio asequible. En cuanto a su composición química, el aceite de almendra se caracteriza por su bajo contenido en ácidos grasos saturados y el predominio de los monoinsaturados, en especial en ácido oleico. Además, el aceite de almendra contiene compuestos bioactivos liposolubles y antioxidantes que lo convierten en un aceite con interesantes propiedades nutricionales y cosméticas

Delamination of neural crest cells requires transient and reversible Wnt inhibition mediated by DACT1/2

Delamination of neural crest (NC) cells is a bona fide physiological model of epithelial-to-mesenchymal transition (EMT), a process that is influenced by Wnt/β-catenin signalling. Using two in vivo models, we show that Wnt/β-catenin signalling is transiently inhibited at the time of NC delamination. In attempting to define the mechanism underlying this inhibition, we found that the scaffold proteins Dact1 and Dact2, which are expressed in pre-migratory NC cells, are required for NC delamination in Xenopus and chick embryos, whereas they do not affect the motile properties of migratory NC cells. Dact1/2 inhibit Wnt/β-catenin signalling upstream of the transcriptional activity of T cell factor (TCF), which is required for EMT to proceed. Dact1/2 regulate the subcellular distribution of β-catenin, preventing β-catenin from acting as a transcriptional co-activator to TCF, yet without affecting its stability. Together, these data identify a novel yet important regulatory element that inhibits β-catenin signalling, which then affects NC delamination

Chiral N=1 4d Orientifolds with D-branes at Angles

D6-branes intersecting at angles allow for phenomenologically appealing constructions of four dimensional string theory vacua. While it is straightforward to obtain non-supersymmetric realizations of the standard model, supersymmetric and stable models with three generations and no exotic chiral matter require more involved orbifold constructions. The T^6/(Z_4 x Z_2 x OmegaR) case is discussed in detail. Other orbifolds including fractional D6-branes are treated briefly.Comment: Invited brief review, 16 pages, 2 figures; references adde

Complex structure moduli stability in toroidal compactifications

In this paper we present a classification of possible dynamics of closed string moduli within specific toroidal compactifications of Type II string theories due to the NS-NS tadpole terms in the reduced action. They appear as potential terms for the moduli when supersymmetry is broken due to the presence of D-branes. We particularise to specific constructions with two, four and six-dimensional tori, and study the stabilisation of the complex structure moduli at the disk level. We find that, depending on the cycle on the compact space where the brane is wrapped, there are three possible cases: i) there is a solution inside the complex structure moduli space, and the configuration is stable at the critical point, ii) the moduli fields are driven towards the boundary of the moduli space, iii) there is no stable solution at the minimum of the potential and the system decays into a set of branes.Comment: 24 pages, JHEP3.cls, 19 figures. A few references adde

Inflationary Scenarios from Branes at Angles

We describe a simple mechanism that can lead to inflation within string-based brane-world scenarios. The idea is to start from a supersymmetric configuration with two parallel static Dp-branes, and slightly break the supersymmetry conditions to produce a very flat potential for the field that parametrises the distance between the branes, i.e. the inflaton field. This breaking can be achieved in various ways: by slight relative rotations of the branes with small angles, by considering small relative velocities between the branes, etc. If the breaking parameter is sufficiently small, a large number of e-folds can be produced within the D-brane, for small changes of the configuration in the compactified directions. Such a process is local, i.e. it does not depend very strongly on the compactification space nor on the initial conditions. Moreover, the breaking induces a very small velocity and acceleration, which ensures very small slow-roll parameters and thus an almost scale invariant spectrum of metric fluctuations, responsible for the observed temperature anisotropies in the microwave background. Inflation ends as in hybrid inflation, triggered by the negative curvature of the string tachyon potential. In this paper we elaborate on one of the simplest examples: two almost parallel D4-branes in a flat compactified space.Comment: 29 pages, 9 eps figures, using JHEP3.cls, published in JHE

Charge exchange in collisions of 1-100-keV Sn3+ ions with H2 and D2

Absolute cross sections for single electron capture by Sn3+ colliding with H2 and D2 have been measured and calculated in the energy range of 1-100 keV. The cross sections are determined by measuring the change in ion beam current with varying target density and by measuring the yields of charged target fragments by means of a time-of-flight spectrometer. The results for D2 show good agreement with our seven-state semiclassical calculations, while for H2 the experimental results increase more strongly than the calculations toward lower energies. This discrepancy is attributed to vibrational effects, not included in the calculations, that lead to the breakdown of the Franck-Condon approximation

D-Terms from Generalized NS-NS Fluxes in Type II

Orientifolds of type II string theory admit a certain set of generalized NS-NS fluxes, including not only the three-form field strength H, but also metric and non-geometric fluxes, which are related to H by T-duality. We describe in general how these fluxes appear as parameters of an effective N=1 supergravity theory in four dimensions, and in particular how certain generalized NS-NS fluxes can act as charges for R-R axions, leading to D-term contributions to the effective scalar potential. We illustrate these phenomena in type IIB with the example of a certain orientifold of T^6/Z_4.Comment: 31+1 pages, uses utarticle.cls; v2: references adde