Standard model on a D-brane

We present a consistent string theory model which produces a simple extension of the standard model, consisting of a D3-brane at a simple orbifold singularity. We envision this as a local singularity within a warped compactification. The phenomenology of the model has some novel features. We note that, for the model to be viable, the scale of stringy physics must be in the multi-TeV range. There are natural hierarchies in the fermion spectrum and there are several possible experimental signatures of the model.     

Stacks and D-brane bundles

In this paper we describe explicitly how the twisted “bundles” on a D-brane worldvolume in the presence of a nontrivial B-field, can be understood in terms of sheaves on stacks. We also take this opportunity to provide the physics community with a readable introduction to stacks and generalized spaces.     

D-brane boundary state dynamics

We construct the open string boundary states corresponding to various time-dependent deformations of the D-brane and explore several ways in which they may be used to study stringy soliton collective coordinate quantum dynamics. Among other things, we find that D-strings have exact moduli corresponding to arbitrary chiral excitations of the basic soliton. These are presumably the duals of the BPS-saturated excitations of the fundamental type IM string. These first steps in a systematic study of the dynamics and interactions of Dirichlet-brane solitons give further evidence of the consistency of Polchinski's new approach to string soliton physics.     

D-brane solutions in a light-like linear dilaton background

The light-like linear dilaton background presents a simple time dependent solution of type II supergravity equations of motion that preserves 1/2 supersymmetry in ten dimensions. We construct supergravity D-brane solutions in a linear dilaton background starting from the known intersecting brane solutions in string theory. By applying a Penrose limit on the intersecting (NS1–NS5–NS5′)-brane solution, we find out a D5-brane in a linear dilaton background. We solve the Killing spinor equations for the brane solutions explicitly, and show that they preserve 1/4 supersymmetry. We also find a M5-brane solution in eleven-dimensional supergravity.     

Anisotropic evolution of 4-brane in a 6D generalized Randall-Sundrum model

We investigate a 6 D generalized Randall-Sundrum brane world scenario with a bulk cosmological constant.Each stress-energy tensor T_(ab)~i on the brane is shown to be similar to a constant vacuum energy.This is consistent with the Randall-Sundrum model,in which each 3-brane Lagrangian yielded a constant vacuum energy.By adopting an anisotropic metric ansatz,we obtain the 5 D Friedmann-Robertson-Walker field equations.In a slightly later period,the expansion of the universe is proportional to the square root of time,t~(1/2),which is similar to the period of the radiation-dominated regime.Moreover,we investigate the case with two a(t) and two b(t).In a large range of t,we obtain the 3 D effective cosmological constant Λ_(eff)=-2Ω/30,which is independent of the integral constant.Here,the scale factor is an exponential expansion,which is consistent with our present observation of the universe.Our results demonstrate that it is possible to construct a model that solves the dark energy problem,while guaranteeing a positive brane tension.     

p-Wave superconductors in D-brane systems

In this work we take the intersecting D-brane models to explore some properties of p-wave superconductor at strong coupling. Our studies are focused on four-dimensional spacetime, which is not completely researched as in planar case. Optimistically, the AdS/CFT approach to superconductor, or more precisely superconducting-like phase transition, can give us some intuitions about mysterious high _Tc$T_c$ superconductors. Concretely, we use defect D4/D6 and D4/D4 (noncritical) models to carry out comparative investigations. To make the system in the finite temperature bath, we assume that the superconducting phase is in the deconfined and chiral symmetry restoring phase for black D4-brane geometry. For the background fields, we use both analytical and numerical methods to solve the coupled nonlinear equations of motion. Near the phase transition, both methods give the mean filed behavior for the superconducting condensate. We then study gauge field perturbations of the systems to probe the AC conductivity. Similar to previous results, there comes out a gap in low frequency regime and the conductivity gets exponentially small as the condensation is enhanced. In contrast to previous investigations, we also compute the AC conductivity along the x direction, which needs to study a coupled sets of fluctuation modes. This shows us the anisotropic feature of p-wave superconductors.     

Anisotropic evolution of energy gap in Bi2212 superconductor

We present a systematic analysis of the energy gap in underdoped Bi2212 superconductor as a function of temperature and hole doping level. Within the framework of the theoretical model containing the electron-phonon and electron-electron-phonon pairing mechanism, we reproduced the measurement results of modern ARPES experiments with very high accuracy. We showed that the energy-gap amplitude is very weakly dependent on the temperature but clearly dependent on the level of doping. The evidence for a non-zero energy gap above the critical temperature, referred to as a pseudogap, was also obtained.     

D-brane models and flux supersymmetry breaking

We describe recent work on constructing four-dimensional string models with moduli stabilized by field strength fluxes and chiral gauge sectors close to the Standard Model from D-brane configurations. We discuss how the interplay of both ingredients relates to phenomenological issues, in particular the appearance of soft terms on the D-brane gauge sector induce from non-supersymmetric flux backgrounds. To cite this article: A.M. Uranga, C. R. Physique 5 (2004).

Résumé

Nous décrivons des travaux récents de constructions de modèles de cordes à 4 dimensions ayant des modules stabilisés par des flux, ainsi que des secteurs chiraux de jauge proches du modèle standard, réalisés à partir de configurations de D-branes. Nous expliquons comment les liens entre les différents ingrédients relient des aspects phénoménologiques, en particulier l'apparition de termes doux dans le secteur de jauge des D-branes à partir de champs de fonds non-supersymétriques avec flux. Pour citer cet article : A.M. Uranga, C. R. Physique 5 (2004).     

Yukawa coupling structure in intersecting D-brane models

The structure of Yukawa coupling matrices is investigated in type IIA T ⁶/(Z₂ x Z₂) orientifold models with intersecting D-branes. Yukawa coupling matrices are difficult to be made realistic in conventional models in which the generation structure emerges by the multiple intersection of D-branes in the factorized T ⁶ = T ² x T ² x T ². We study the new type of flavor structure, where Yukawa couplings are dynamically generated, and show this type of models lead to non-trivial structures of Yukawa coupling matrices, which can be realistic.Received: 8 December 2004, Revised: 22 December 2004, Published online: 9 March 2005     

Properties of fermion mixings in intersecting D-brane models

We consider the Yukawa couplings for quarks and leptons in the context of Pati–Salam model using intersecting D-brane models where the Yukawa coupling matrices are rank one in a simple choice of family replication. The CKM mixings can be explained by perturbing the rank 1 matrix using higher order terms involving new Higgs fields available in the model. We show that the near bi-large neutrino mixing angles can be naturally explained, choosing the light neutrino mass matrix to be type II seesaw dominant. The predicted value of U_e3$U_{e3}$ is in the range ?0.05–0.15$?0.05\text{–}0.15$. In the quark sector, V_cb$V_{cb}$ is naturally close to the strange/bottom quark mass ratio and we obtain an approximate relation V_ubV_cb?(ms/mb)²V_us$V_{ub}{V}_{cb}?{(m_s/m_b)}^2{V}_{us}$. The geometrical interpretations of the neutrino mixings are also discussed.     

Statistical anisotropy from vector curvaton in D-brane inflation

We investigate the possibility of embedding the vector curvaton paradigm in D-brane models of inflation in type IIB string theory in a simple toy model. The vector curvaton is identified with the U(1)$U(1)$ gauge field that lives on the world volume of a D3-brane, which may be stationary or undergoing general motion in the internal space. The dilaton is considered as a spectator field which modulates the evolution of the vector field. In this set-up, the vector curvaton is able to generate measurable statistical anisotropy in the spectrum and bispectrum of the curvature perturbation assuming that the dilaton evolves as e^−?∝a²$e^{-?}\propto a^2$ where a(t)$a(t)$ is the scale factor. Our work constitutes a first step towards exploring how such distinctive features may arise from the presence of several light fields that naturally appear in string theory models of cosmology.     

Hierarchies from D-brane instantons in globally defined Calabi-Yau orientifolds

We construct the first globally consistent semirealistic type I string vacua on an elliptically fibered manifold where the zero modes of the Euclidean D1-instanton sector allow for the generation of nonperturbative Majorana masses of an intermediate scale. In another class of global models, a D1-brane instanton can generate a Polonyi-type superpotential breaking supersymmetry at an exponentially suppressed scale.