Boris Körs

Noncommutative compactifications of type I strings on tori with magnetic background flux

Weconstructsix-andfour-dimensionaltoroidalcompacticationsofthe type-IstringwithmagneticfluxontheD-branes. Theopenstringsinthisbackground probe a noncommutative internal geometry. Phenomenologically appealing features suchaschiralfermionsandsupersymmetrybreakinginthegaugesectorarenaturally realized by these vacua. We investigate the spectra of such noncommutative string compactications and in a bottom-up approach discuss the possibility to obtain the standard or some GUT like model.

The Standard Model from stable intersecting brane world orbifolds

Abstract We analyze the perturbative stability of non-supersymmetric intersecting brane world models on tori. Besides the dilaton tadpole, a dynamical instability in the complex structure moduli space occurs at string disc level, which drives the background geometry to a degenerate limit. We show that in certain orbifold models this latter instability is absent as the relevant moduli are frozen. We construct explicit examples of such orbifold intersecting brane world models and discuss the phenomenological implications of a three generation Standard Model which descends naturally from an SU (5) GUT theory. It turns out that various phenomenological issues require the string scale to be at least of the order of the GUT scale. As a major difference compared to the Standard Model, some of the Yukawa couplings are excluded so that the standard electroweak Higgs mechanism with a fundamental Higgs scalar is not realized in this set-up.

Type I strings with F- and B-flux

We present non-supersymmetric toroidal compactifications of type-I string theory with both constant background NSNS two-form flux and non-trivial magnetic flux on the various D9-branes. The non-vanishing B-flux admits four-dimensional models with three generations of chiral fermions in standard model like gauge groups. Additionally, we consider the orbifold 4/2, again with both kinds of background flux present, leading to non-supersymmetric as well as supersymmetric models in six dimensions. All models have T-dual descriptions as intersecting brane worlds.

Supersymmetric 4D orientifolds of Type IIA with D6-branes at angles

We study a certain class of four-dimensional = 1 supersymmetric orientifolds for which the world-sheet parity transformation is combined with a complex conjugation in the compact directions. We investigate in detail the orientifolds of the 3, 4, 6 and 6 toroidal orbifolds finding solutions to the tadpole cancellation conditions for all models. Generically, all the massless spectra turn out to be non-chiral.

Stabilization of the compactification volume by quantum corrections.

We discuss prospects for stabilizing the volume modulus of N = 1 supersymmetric type IIB orientifold compactifications using only perturbative corrections to the Kähler potential. Concretely, we consider the known string loop corrections and tree-level α corrections. They break the no-scale structure of the potential, which otherwise prohibits stabilizing the volume modulus. We argue that when combined, these corrections provide enough flexibility to stabilize the volume of the internal space without nonperturbative effects, although we are not able to present a completely explicit example within the limited set of currently available models. Furthermore, a certain amount of fine-tuning is needed to obtain a minimum at large volume.

Supersymmetric orientifolds in 6D with D-branes at angles

Abstract We study a new class of N=1 supersymmetric orientifolds in six space-time dimensions. The world-sheet parity transformation is combined with a permutation of the internal complex coordinates. In contrast to ordinary orientifolds the twisted sectors contribute to the Klein bottle amplitude leading to new tadpoles to be cancelled by twisted open string sectors. They arise from open strings stretched between D7-branes intersecting at non-trivial angles. We study in detail the Z 3 , Z 4 and Z 6 permutational orientifolds obtaining in all cases anomaly free massless spectra.

Fluxes in heterotic and type II string compactifications

Abstract In this paper we consider heterotic compactifications on K3 × T 2 as well as type II compactifications on K3-fibred Calabi–Yau spaces with certain fluxes for the gauge and R–R field strengths F and H turned on. By providing an identification of corresponding fluxes we show that the well-known N =2 heterotic/typexa0II string–string duality still holds for a subset of all possible fluxes, namely those which arise from six-dimensional gauge fields with internal magnetic flux on the common two-sphere P b 1 , which is the base space of the typexa0II K3-fibration. On the other hand, F - and H -fluxes without P b 1 -support, such as heterotic F -fluxes on the torus T 2 or typexa0II H -fluxes on cycles of the K3-fibre cannot be matched in any simple way, which is a challenge for heterotic/typexa0II string–string duality. Our analysis is based on the comparison of terms in the effective low-energy heterotic and typexa0II actions which are induced by the fluxes, such as the Green–Schwarz couplings related to flux-induced U (1) anomalies, the effective superpotential and the Fayet–Iliopoulos scalar potential.

Asymmetric orbifolds, noncommutative geometry and type I string vacua

Abstract We investigate the D-brane contents of asymmetric orbifolds. Using T-duality we find that the consistent description of open strings in asymmetric orbifolds requires to turn on background gauge fields on the D-branes. We derive the corresponding noncommutative geometry arising on such D-branes with mixed Neumann–Dirichlet boundary conditions directly by applying an asymmetric rotation to open strings with pure Dirichlet or Neumann boundary conditions. As a concrete application of our results we construct asymmetric type I vacua requiring open strings with mixed boundary conditions for tadpole cancellation.

The effective two-loop Euler-Heisenberg action for scalar and spinor QED in a general constant background field

Abstract. Using the Wordline formalism of QED we compute the two-loop effective action induced by a charged scalar, respectively spinor particle in a general constant electromagnetic field.

A New Class of Supersymmetric Orientifolds with D-branes at Angles

We describe a new class of supersymmetric orientifolds which combine the world-sheet parity transformation with a complex conjugation in the compact directions. As an example, we investigate in detail the orientifold of the Z3 toroidal orbifold in six and four dimensions. We demonstrate how the solution to the tadpole cancellation conditions, the resulting gauge groups and the massless spectra depend on the choice of the complex structures on the tori, giving rise to a variety of inequivalent models. We also summarize the results for the orientifolds of the Z4, Z6,and Z 0 orbifolds in four and six dimensions.