Moduli stabilization in (string) model building: gauge fluxes and loops

We discuss the moduli stabilization arising in the presence of gauge fluxes, R-symmetry twists and non-perturbative effects in the context of 6-dimensional supergravity models. We show how the presence of D-terms, due to the gauge fluxes, is compatible with gaugino condensation, and that the two effects, combined with the R-symmetry twist, do stabilize all the Kaehler moduli present in the model, in the spirit of KKLT. We also calculate the flux-induced one-loop correction to the scalar potential coming from charged hypermultiplets, and find that it does not destabilize the minimum.Comment: Submitted for the SUSY07 proceedings, 4 pages, LaTe

Heterotic SO(32) model building in four dimensions

Four dimensional heterotic SO(32) orbifold models are classified systematically with model building applications in mind. We obtain all Z3, Z7 and Z2N models based on vectorial gauge shifts. The resulting gauge groups are reminiscent of those of type-I model building, as they always take the form SO(2n_0)xU(n_1)x...xU(n_{N-1})xSO(2n_N). The complete twisted spectrum is determined simultaneously for all orbifold models in a parametric way depending on n_0,...,n_N, rather than on a model by model basis. This reveals interesting patterns in the twisted states: They are always built out of vectors and anti--symmetric tensors of the U(n) groups, and either vectors or spinors of the SO(2n) groups. Our results may shed additional light on the S-duality between heterotic and type-I strings in four dimensions. As a spin-off we obtain an SO(10) GUT model with four generations from the Z4 orbifold.Comment: 1+37 pages LaTeX, some typos in table 4 corrected, and we have included some discussion on exceptional shift vectors which ignored in the previous version

Gauge vs. Gravity mediation in models with anomalous U(1)'s

In an attempt to implement gauge mediation in string theory, we study string effective supergravity models of supersymmetry breaking, containing anomalous gauge factors. We discuss subtleties related to gauge invariance and the stabilization of the Green-Schwarz moduli, which set non-trivial constraints on the transmission of supersymmetry breaking to MSSM via gauge interactions. Given those constraints, it is difficult to obtain the dominance of gauge mediation over gravity mediation. Furthermore, generically the gauge contributions to soft terms contain additional non-standard terms coming from D-term contributions. Motivated by this, we study the phenomenology of recently proposed hybrid models, where gravity and gauge mediations compete at the GUT scale, and show that such a scenario can respect WMAP constraints and would be easily testable at LHC.Comment: 40 pages, 5 figure

Flux Stabilization in 6 Dimensions: D-terms and Loop Corrections

We analyse D-terms induced by gauge theory fluxes in the context of 6-dimensional supergravity models. On the one hand, this is arguably the simplest concrete setting in which the controversial idea of `D-term uplifts' can be investigated. On the other hand, it is a very plausible intermediate step on the way from a 10d string theory model to 4d phenomenology. Our specific results include the flux-induced one-loop correction to the scalar potential coming from charged hypermultiplets. Furthermore, we comment on the interplay of gauge theory fluxes and gaugino condensation in the present context, demonstrate explicitly how the D-term arises from the gauging of one of the compactification moduli, and briefly discuss further ingredients that may be required for the construction of a phenomenologically viable model. In particular, we show how the 6d dilaton and volume moduli can be simultaneously stabilized, in the spirit of KKLT, by the combination of an R symmetry twist, a gaugino condensate, and a flux-induced D-term.Comment: 24 pages, 1 figure v2:minor correction

SUSY breaking mediation by throat fields

We investigate, in the general framework of KKLT, the mediation of supersymmetry breaking by fields propagating in the strongly warped region of the compactification manifold ('throat fields'). Such fields can couple both to the supersymmetry breaking sector at the IR end of the throat and to the visible sector at the UV end. We model the supersymmetry breaking sector by a chiral superfield which develops an F-term vacuum expectation value. It turns out that the mediation effect of vector multiplets propagating in the throat can compete with modulus-anomaly mediation. Moreover, such vector fields are naturally present as the gauge fields arising from isometries of the throat (most notably the SO(4) isometry of the Klebanov-Strassler solution). Their mediation effect is important in spite of their large 4d mass. The latter is due to the breaking of the throat isometry by the compact manifold at the UV end of the throat. The contribution from heavy chiral superfields is found to be subdominant.Comment: 15 pages; v2: typos corrected, references added; v3: comments adde

String vacua with flux from freely-acting obifolds

A precise correspondence between freely-acting orbifolds (Scherk-Schwarz compactifications) and string vacua with NSNS flux turned on is established using T-duality. We focus our attention to a certain non-compact Z_2 heterotic freely-acting orbifold with N=2 supersymmetry (SUSY). The geometric properties of the T-dual background are studied. As expected, the space is non-Kahler with the most generic torsion compatible with SUSY. All equations of motion are satisfied, except the Bianchi identity for the NSNS field, that is satisfied only at leading order in derivatives, i.e. without the curvature term. We point out that this is due to unknown corrections to the standard heterotic T-duality rules.Comment: 13 pages, no figures; v2: references added and rearranged, version to appear in JHE

Resolutions of C^n/Z_n Orbifolds, their U(1) Bundles, and Applications to String Model Building

We describe blowups of C^n/Z_n orbifolds as complex line bundles over CP^{n-1}. We construct some gauge bundles on these resolutions. Apart from the standard embedding, we describe U(1) bundles and an SU(n-1) bundle. Both blowups and their gauge bundles are given explicitly. We investigate ten dimensional SO(32) super Yang-Mills theory coupled to supergravity on these backgrounds. The integrated Bianchi identity implies that there are only a finite number of U(1) bundle models. We describe how the orbifold gauge shift vector can be read off from the gauge background. In this way we can assert that in the blow down limit these models correspond to heterotic C^2/Z_2 and C^3/Z_3 orbifold models. (Only the Z_3 model with unbroken gauge group SO(32) cannot be reconstructed in blowup without torsion.) This is confirmed by computing the charged chiral spectra on the resolutions. The construction of these blowup models implies that the mismatch between type-I and heterotic models on T^6/Z_3 does not signal a complication of S-duality, but rather a problem of type-I model building itself: The standard type-I orbifold model building only allows for a single model on this orbifold, while the blowup models give five different models in blow down.Comment: 1+27 pages LaTeX, 2 figures, some typos correcte

Experimental tests of a seasonally changing visual preference for habitat in a long-distance migratory shorebird

Migratory shorebirds show highly organized seasonal cycles in physiological and morphological traits (body mass and composition, plumage, hormone levels, etc.), which in captivity is accompanied by restless behaviour at times when free-living birds would start migration. We introduce the idea that seasonally changing preference for habitat could motivate migrants to embark on migration and that this cognitive process could also guide them to seasonally appropriate places. We explored this by testing whether red knots (Calidris canutus), which also in captivity maintain marked circannual phenotypic rhythms, show evidence of seasonal change in preference for pictures of seasonally appropriate habitats. We first developed a method to verify whether red knots are able to memorize and discriminate contrasting pictures projected by LCD projectors. This was followed by two different experiments in which we tested for a seasonally changing preference for breeding or non-breeding habitat. When carried out during the pre-breeding season, the red knots are expected to prefer pictures of mudflats, their non-breeding habitat. At the start of the breeding season, they should prefer pictures of the tundra breeding habitat. We established that knots are able to distinguish and memorize projected images. We failed to demonstrate the predicted change in vision-based habitat preference, but for reasons of test design we do not interpret this as a strong rejection of the hypothesis. Instead, we suggest that experiments with greater numbers of individuals tested once, perhaps in combination with the provision of additional cues such as smells and sounds, will help the development of these ideas further

Neutralino Dark Matter in Mirage Mediation

We study the phenomenology of neutralino dark matter (DM) in mirage mediation scenario of supersymmetry breaking which results from the moduli stabilization in some string/brane models. Depending upon the model parameters, especially the anomaly to modulus mediation ratio determined by the moduli stabilization mechanism, the nature of the lightest supersymmetric particle (LSP) changes from Bino-like neutralino to Higgsino-like one via Bino-Higgsino mixing region. For the Bino-like LSP, the standard thermal production mechanism can give a right amount of relic DM density through the stop/stau-neutralino coannihilation or the pseudo-scalar Higgs resonance process. We also examine the prospect of direct and indirect DM detection in various parameter regions of mirage mediation. Neutralino DM in galactic halo might be detected by near future direct detection experiments in the case of Bino-Higgsino mixed LSP. The gamma ray flux from Galactic Center might be detectable also if the DM density profile takes a cuspy shape.Comment: One reference adde

Gauge Unification in Highly Anisotropic String Compactifications

It is well-known that heterotic string compactifications have, in spite of their conceptual simplicity and aesthetic appeal, a serious problem with precision gauge coupling unification in the perturbative regime of string theory. Using both a duality-based and a field-theoretic definition of the boundary of the perturbative regime, we reevaluate the situation in a quantitative manner. We conclude that the simplest and most promising situations are those where some of the compactification radii are exceptionally large, corresponding to highly anisotropic orbifold models. Thus, one is led to consider constructions which are known to the effective field-theorist as higher-dimensional or orbifold grand unified theories (orbifold GUTs). In particular, if the discrete symmetry used to break the GUT group acts freely, a non-local breaking in the larger compact dimensions can be realized, leading to a precise gauge coupling unification as expected on the basis of the MSSM particle spectrum. Furthermore, a somewhat more model dependent but nevertheless very promising scenario arises if the GUT breaking is restricted to certain singular points within the manifold spanned by the larger compactification radii.Comment: 34 pages, 4 figures, more references adde