Consistent uplifting of AdS vacua in string theory often requires extra light degrees of freedom in addition to those of a (Kähler) modulus. Here we consider the possibility that de Sitter and Minkowski vacua arise due to hidden sector matter interactions. We find that, in this scheme, the hierarchically small supersymmetry breaking scale can be explained by the scale of gaugino condensation and that interesting patterns of the soft terms arise. In particular, a matter-dominated supersymmetry breaking scenario and a version of the mirage mediation scheme appear in the framework of spontaneously broken supergravity. © 2006 Elsevier B.V. All rights reserved.

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(-〈X〉) 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 ∼ 〈X〉 m3/2 ∼ 〈X〉2 msoft. The 'little hierarchy' 〈X〉 is given by the logarithm of the ratio of the Planck scale and the gravitino mass, 〈X〉 ∼ log(MPl/m3/2) ∼ 4π2. 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. © 2005 American Institute of Physics.

The most promising MSSM candidates of the heterotic string reveal some distinctive properties. These include gauge-top unification, a specific solution to the μ-problem and mirage pattern for the gaugino masses. The location of the top- and the Higgs-multiplets in extra dimensions differs significantly from that of the other quarks and leptons leading to a characteristic signature of suppressed soft breaking terms, reminiscent of a scheme known as natural supersymmetry. © 2012 Elsevier B.V.

We revisit the implications of naturalness and gauge unification in the minimal supersymmetric standard model. We find that precision unification of the couplings in connection with a small μ parameter requires a highly compressed gaugino pattern as it is realized in mirage mediation. Due to the small mass difference between the gluino and lightest supersymmetric particle (LSP), collider limits on the gluino mass are drastically relaxed. Without further assumptions, the relic density of the LSP is very close to the observed dark matter density due to coannihilation effects. © 2013 American Physical Society.

We show that hierarchically small vacuum expectation values of the superpotential in supersymmetric theories can be a consequence of an approximate R symmetry. We briefly discuss the role of such small constants in moduli stabilization and understanding the huge hierarchy between the Planck and electroweak scales.

We complete our search for MSSM vacua in the Z6 - II heterotic orbifold by including models with 3 Wilson lines. We estimate the total number of inequivalent models in this orbifold to be 107. Out of these, we find almost 300 models with the exact MSSM spectrum, gauge coupling unification and a heavy top quark. Models with these features originate predominantly from local GUTs. The scale of gaugino condensation in the hidden sector is correlated with properties of the observable sector such that soft masses in the TeV range are preferred. © 2008 Elsevier B.V. All rights reserved.

We review recent progress in embedding the supersymmetric standard model into string theory. We discuss how, with the incorporation of certain aspects of grand unification, a search strategy can be developed that allows one to efficiently find rather large numbers of promising string vacua. Global string-derived models with the following features are discussed: (i) exact MSSM spectrum below the unification scale; (ii) R-parity; (iii) hierarchical Yukawa couplings with non-trivial mixing; (iv) solution to the μ problem; (v) see-saw suppressed neutrino masses. © 2008 Springer-Verlag / Società Italiana di Fisica.

We re-derive the R symmetries for the Z6-II orbifold with non-trivial Wilson lines and find expressions for the R charges which differ from those in the literature. © 2013 Elsevier B.V.

In the context of flux compactifications, metastable vacua with a small positive cosmological constant are obtained by combining a sector where supersymmetry is broken dynamically with the sector responsible for moduli stabilization, which is known as the F-uplifting. We analyze this procedure in a model-independent way and study phenomenological properties of the resulting vacua. © SISSA 2007.

We study the origin of non-Abelian discrete flavor symmetries in superstring theory. We classify all possible non-Abelian discrete flavor symmetries which can appear in heterotic orbifold models. These symmetries include D4 and Δ (54). We find that the symmetries of the couplings are always larger than the symmetries of the compact space. This is because they are a consequence of the geometry of the orbifold combined with the space group selection rules of the string. We also study possible breaking patterns. Our analysis yields a simple geometric understanding of the realization of non-Abelian flavor symmetries. © 2007 Elsevier B.V. All rights reserved.