Standard Grand Unification from Superstrings

Recent developments about the construction of standard $SO(10)$ and $SU(5)$ grand unified theories from 4-dimensional superstrings are presented. Explicit techniques involving higher level affine Lie algebras, for obtaining such stringGUTs from symmetric orbifolds are discussed. Special emphasis is put on the different constraints and selection rules for model building in this string framework, trying to disentangle those which are generic from those depending on the orbifold construction proposed. Some phenomenological implications from such constraints are briefly discussed.Comment: 7 pages, Latex, no figures. Talk at Susy 95. Paris, May 199

In quest of "just" the Standard Model on D-branes at a singularity

In this note we explore the possibility of obtaining gauge bosons and fermionic spectrum as close as possible to the Standard Model content, by placing D3-branes at a ZN orbifold-like singularity in the presence of D7-branes. Indeed, we find that this is plausible provided a sufficiently high N is allowed for and the singular point is also fixed by an orientifold action. If extra charged matter is not permitted then the singularity should necessarily be non-supersymmetric. Correct hypercharge assignments require a dependence on some Abelian gauge D7-groups. In achieving such a construction we follow a recent observation made in Ref. [hep-th/0105155] about the possibility that, the three left handed quarks, would present different U(2) transformation properties.Comment: 18 pages, 1 table, no figures, v2: typos correcte

Standard-like models from intersecting D4-branes

We construct a one-parameter set of intersecting D4-brane models, with six stacks, that yield the (non-supersymmetric) standard model plus extra vector-like matter. Twisted tadpoles and gauge anomalies are cancelled, and the model contains all of the Yukawa couplings to the tachyonic Higgs doublets that are needed to generate mass terms for the fermions. A string scale in the range 1-10 TeV and a Higgs mass not much greater than the current bound is obtained for certain values of the parameters, consistently with the observed values of the gauge coupling constants.Comment: LaTeX file, 12 pages, v3 to appear in Phys. Lett.

Tachyon-free Non-supersymmetric Type IIB Orientifolds via Brane-Antibrane Systems

We derive the rules to construct type IIB compact orientifolds in six and four dimensions including D-branes and anti-D-branes. Even though the models are non-supersymmetric due to the presence of the anti-D-branes, we show that it is easy to construct large classes of models free of tachyons. Brane-antibrane annihilation can be prevented for instance by considering models with branes and antibranes stuck at different fixed points in the compact space. We construct several anomaly-free and tachyon-free six-dimensional orientifolds containing D9-branes and anti-D5-branes. This setup allows to construct four-dimensional chiral models with supersymmetry unbroken in the bulk and in some D-brane sectors, whereas supersymmetry is broken (at the string scale) in some `hidden' anti-D-brane sector. We present several explicit models of this kind. We also comment on the role of the non-cancelled attractive brane-antibrane forces and the non-vanishing cosmological constant, as providing interesting dynamics for the geometric moduli and the dilaton, which may contribute to their stabilization.Comment: 30 pages, Latex, no figures. Minor correction

On the quantization of the N=2 supersymmetric non linear sigma model

A method for quantizing the bidimensional N=2 supersymmetric non-linear sigma model is developed. This method is both covariant under coordinate transformations (concerning the order relevant for calculations) and explicitly N=2 supersymmetric. The OPE of the supercurrent is computed accordingly, including also the dilaton. By imposing the N=2 superconformal algebra the equations for the metric and dilaton are obtained. In particular, they imply that the dilaton is a constant.Comment: 16 page

The Dilaton Potential from N= 1*

Recent understanding of {\cal N}=1* supersymmetric theory (mass deformed {\cal N}=4) has made it possible to find an exact superpotential which encodes the properties of the different phases of the theory. We consider this superpotential as an illustrative example for the source of a nontrivial scalar potential for the string theory dilaton and study its properties. The superpotential is characterized by the rank of the corresponding gauge group (N) and integers p,q,k labelling the different massive phases of the theory. For generic values of these parameters, we find the expected runaway behaviour of the potential to vanishing string coupling. But there are also supersymmetric minima at weak coupling stabilizing the dilaton field. An interesting property of this potential is that there is a proliferation of supersymmetric vacua in the confining phases, with the number of vacua increasing with N and leading to a kind of staircase potential. For a range of parameters, it is possible to obtain realistic values for the gauge coupling.Comment: Latex, 21 pages, 4 figures, v2: typos and eq.(2.1) corrected, v3: comments on possible embeddings of the model in string theory adde

Double Field Theory: A Pedagogical Review

Double Field Theory (DFT) is a proposal to incorporate T-duality, a distinctive symmetry of string theory, as a symmetry of a field theory defined on a double configuration space. The aim of this review is to provide a pedagogical presentation of DFT and its applications. We first introduce some basic ideas on T-duality and supergravity in order to proceed to the construction of generalized diffeomorphisms and an invariant action on the double space. Steps towards the construction of a geometry on the double space are discussed. We then address generalized Scherk-Schwarz compactifications of DFT and their connection to gauged supergravity and flux compactifications. We also discuss U-duality extensions, and present a brief parcours on world-sheet approaches to DFT. Finally, we provide a summary of other developments and applications that are not discussed in detail in the review.Comment: 121 pages, invited review for Class. Quantum Grav; v2: Updated reference

Standard-like models from Intersecting D5-branes

We construct intersecting D5-brane orbifold models that yield the (non-supersymmetric) standard model up to vector-like matter and charged-singlet scalars. The models are constrained by the requirement that twisted tadpoles cancel, and that the gauge boson coupled to the weak hypercharge $U(1)_Y$ does not get a string-scale mass via a generalised Green-Schwarz mechanism. Gauge coupling constant ratios close to those measured are easily obtained for reasonable values of the parameters, consistently with having the string scale close to the electroweak scale, as required to avoid the hierarchy problem.Comment: 13 pages, LaTeX file, version to appear in Physics Letters