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Boris Kors

Publications and source records attributed to Boris Kors.

At least 19 recordsLinked to original sources
arXiv2007

Four-dimensional String Compactifications with D-Branes, Orientifolds and Fluxes

This review article provides a pedagogical introduction into various classes of chiral string compactifications to four dimensions with D-branes and fluxes. The main concern is to provide all necessary technical tools to explicitly construct four-dimensional orientifold vacua, with the final aim to come as close as possible to the supersymmetric Standard Model. Furthermore, we outline the available methods to derive the resulting four-dimensional effective action. Finally, we summarize recent attempts to address the string vacuum problem via the statistical approach to D-brane models.

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arXiv2006

Extra-weakly Interacting Dark Matter

We investigate a new type of dark matter with couplings to ordinary matter naturally suppressed by at least one order of magnitude compared to weak interactions. Despite the extra-weak interactions massive particles of this type (XWIMPs) can satisfy the WMAP relic density constraints due to coannihilation if their masses are close to that of the lightest state of the MSSM. The region in the parameter space of a suitably extended mSUGRA model consistent with the WMAP3 constraints on XWIMPs is determined. Plots for sparticles masses are given which can be subject to test at the LHC. We also discuss the sensitivity of the analysis to the top mass. As an example for an explicit model we show that such a form of dark matter can arise in certain $Z'$ extensions of the MSSM. Specifically we consider an abelian extension with spontaneous gauge symmetry breaking via Fayet-Iliopoulos D-terms in the hidden sector. The LSP of the full model arises from the extra $U(1)_X$ sector with extra-weak couplings to Standard Model particles due to experimental constraints. With R-parity conservation the new XWIMP is a candidate for cold dark matter. In a certain limit the model reduces to the \st extension of the MSSM without a Higgs mechanism, and wider ranges of models with similar characteristics are easy to construct.

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arXiv2006

Kahler Anomalies in Supergravity and Flux Vacua

We review the subject of Kahler anomalies in gauged supergravity, emphasizing that field equations are inconsistent when the Kahler potential is non-invariant under gauge transformations or when there are elementary Fayet-Iliopoulos couplings. Flux vacua solutions of string theory with gauged U(1) shift symmetries appear to avoid this problem. The covariant Kahler anomalies involve tensors which are composite functions of the scalars as well as the gauge field strength and space-time curvature tensors. Anomaly cancellation conditions will be discussed in a sequel to this paper.

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arXiv2006

Anomaly Cancellation in Supergravity with Fayet-Iliopoulos Couplings

We review and clarify the cancellation conditions for gauge anomalies which occur when N=1, D=4 supergravity is coupled to a Kahler non-linear sigma-model with gauged isometries and Fayet-Iliopoulos couplings. For a flat sigma-model target space and vanishing Fayet-Iliopoulos couplings, consistency requires just the conventional anomaly cancellation conditions. A consistent model with non-vanishing Fayet-Iliopoulos couplings is unlikely unless the Green-Schwarz mechanism is used. In this case the U(1) gauge boson becomes massive and the D-term potential receives corrections. A Green-Schwarz mechanism can remove both the abelian and certain non-abelian anomalies in models with a gauge non-invariant Kahler potential.

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arXiv2005

On Volume Stabilization 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 Kahler potential. Concretely, we consider the known string loop corrections and tree-level alpha' 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 non-perturbative 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.

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arXiv2005

Aspects of the Stueckelberg Extension

A detailed analysis of a Stueckelberg extension of the electro-weak gauge group with an extra U(1) factor is presented for the Standard Model as well as for the MSSM. The extra gauge boson gets massive through a Stueckelberg type coupling to a pseudo-scalar, instead of a Higgs effect. This new massive neutral gauge boson Z' has vector and axial vector couplings uniquely different from those of conventional extra abelian gauge bosons, such as appear e.g. in GUT models. The extended MSSM furthermore contains two extra neutralinos and one extra neutral CP-even scalar, the latter with a mass larger than that of the Z'. One interesting scenario that emerges is an LSP that is dominantly composed out of the new neutralinos, leading to a possible new superweak candidate for dark matter. We investigate signatures of the Stueckelberg extension at a linear collider and discuss techniques for the detection of the expected sharp Z' resonance. It turns out that the substantially modified forward-backward asymmetry around the Z' pole provides an important signal. Furthermore, we also elaborate on generalizations of the minimal Stueckelberg extension to an arbitrary number of extra U(1) gauge factors.

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arXiv2004

How Stueckelberg Extends the Standard Model and the MSSM

Abelian vector bosons can get massive through the Stueckelberg mechanism without spontaneous symmetry breaking via condensation of Higgs scalar fields. This appears very naturally in models derived from string theory and supergravity. The simplest scenarios of this type consist of extensions of the Standard Model (SM) or the minimal supersymmetric standard model (MSSM) by an extra U(1)_X gauge group with Stueckelberg type couplings. For the SM, the physical spectrum is extended by a massive neutral gauge boson Z' only, while the extension of the MSSM contains a CP-even neutral scalar and two extra neutralinos. The new gauge boson Z' can be very light compared to other models with U(1)' extensions. Among the new features of the Stueckelberg extension of the MSSM, the most striking is the possibility of a new lightest supersymmetric particle (LSP) chi_{St}^0 which is mostly composed of Stueckelberg fermions. In this scenario the LSP of the MSSM chi_1^0 is unstable and decays into chi_{St}^0. Such decays alter the signatures of supersymmetry and have impact on searches for supersymmetry in accelerator experiments. Further, with R-parity invariance, chi_{St}^0 is the new candidate for dark matter.

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arXiv2004

Hierarchically Split Supersymmetry with Fayet-Iliopoulos D-terms in String Theory

We show that in string theory or supergravity with supersymmetry breaking through combined F-terms and Fayet-Iliopoulos D-terms, the masses for charged scalars and fermions can be hierarchically split. The mass scale for the gauginos and higgsinos of the MSSM is controlled by the gravitino mass m_{3/2}, as usual, while the scalars get extra contributions from the D-terms of extra abelian U(1) factors, which can make them much heavier. The vanishing of the vacuum energy requires that their masses lie below {m_{3/2} M_{Pl}}^{1/2}, which for m_{3/2}=O(TeV) sets a bound of 10^{10-13} GeV. Thus, scalars with non-vanishing U(1) charges typically become heavy, while others remain light, producing a spectrum of scalars with masses proportional to their charges, and therefore non-universal. This is a modification of the split supersymmetry scenario, but with a light gravitino. We discuss how Fayet-Iliopoulos terms of this size can arise in orientifold string compactifications with D-branes. Furthermore, within the frame work of D-term inflation, the same vacuum energy that generates the heavy scalar masses can be responsible for driving cosmological inflation.

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arXiv2004

On the Moduli Dependence of Nonperturbative Superpotentials in Brane Inflation

We discuss string corrections to the effective potential in various models of brane inflation. These corrections contribute to the mass of the inflaton candidate and may improve its slow-roll properties. In particular, in orientifold string compactifications with dynamical D3- and D7-branes, the corrections induce inflaton dependence in the part of the superpotential that arises from gaugino condensation or other nonperturbative effects. The additional terms are in part required by supersymmetry. We explicitly discuss D3/D7-inflation, where flat directions of the potential can be lifted, and the KKLMMT model of warped brane inflation, in which the corrections open up the possibility of flattening the potential and canceling unwanted contributions to the inflaton mass.

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arXiv2004

A Supersymmetric Stueckelberg U(1) Extension of the MSSM

A Stueckelberg extension of the MSSM with only one abelian vector and one chiral superfield as an alternative to an abelian extension with Higgs scalars is presented. The bosonic sector contains a new gauge boson Z' which is a sharp resonance, and a new CP-even scalar, which combines with the MSSM Higgs bosons to produce three neutral CP-even massive states. The neutral fermionic sector has two additional fermions which mix with the four MSSM neutralinos to produce an extended 6x6 neutralino mass matrix. For the case when the LSP is composed mostly of the Stueckelberg fermions, the LSP of the MSSM will be unstable, which leads to exotic decays of sparticles with many leptons in final states. Prospects for supersymmetry searches and for dark matter are discussed.

hep-ph↗
arXiv2004

Loop Corrections to Volume Moduli and Inflation in String Theory

The recent progress in embedding inflation in string theory has made it clear that the problem of moduli stabilization cannot be ignored in this context. In many models a special role is played by the volume modulus, which is modified in the presence of mobile branes. The challenge is to stabilize this modified volume while keeping the inflaton mass small compared to the Hubble parameter. It is then crucial to know not only how the volume modulus is modified, but also to have control over the dependence of the potential on the inflaton field. We address these questions within a simple setting: toroidal N=1 type IIB orientifolds. We calculate corrections to the superpotential and show how the holomorphic dependence on the properly modified volume modulus arises. The potential then explicitly involves the inflaton, leaving room for lowering the inflaton mass through moderate fine-tuning of flux quantum numbers.

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arXiv2004

Effective Action and Soft Supersymmetry Breaking for Intersecting D-brane Models

We consider a generic scenario of spontaneous breaking of supersymmetry in the hidden sector within N=1 supersymmetric orientifold compactifications of type II string theories with D-branes that support semi-realistic chiral gauge theories. The soft breaking terms in the visible sector of the models are computed in a standard way without specifying the breaking mechanism, which leads to expressions that generalize those formerly known for heterotic or type I string models. The elements of the effective tree level supergravity action relevant for this, such as the Kahler metric for the matter fields, the superpotential of the visible sector and the gauge kinetic functions, are specified by dimensional reduction and duality arguments. As phenomenological applications we argue that gauge coupling unification can only occur in special regions of the moduli space; we show that flavor changing neutral currents can be suppressed sufficiently for a wide range of parameters, and we briefly address the issues of CP violation, electric dipole moments and dark matter, as well.

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arXiv2004

A Stueckelberg Extension of the Standard Model

An extension of the standard model of electro-weak interactions by an extra abelian gauge boson is given, in which the extra gauge boson and the hypercharge gauge boson both couple to an axionic scalar in a form that leads to a Stueckelberg mass term. The theory leads to a massive Z$'$ whose couplings to fermions are uniquely determined and suppressed by small mixing angles. Such a Z$'$ could have low mass and appear in $e^+e^-$ collisions as a sharp resonance. The branching ratios into $f\bar f$ species, and the forward-backward asymmetry are found to have distinctive features. The model also predicts a new unit of electric charge $e'=Q'e$, where $Q'$ is in general irrational, in the coupling of the photon with hidden matter that is neutral under $SU(2)_L\times U(1)_Y$.

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arXiv2003

Brane/Flux Interactions in Orientifolds

Combining the benefits of D-branes and background fluxes in string compactifications opens up the possibility to explore phenomenologically interesting brane world models with stabilized moduli. However, it is difficult to determine interaction effects among open strings and fluxes in the effective action. We derive the full bosonic Lagrangian of a (spontaneously broken) N=4 supersymmetric model with D3-branes and NSNS and RR 3-form fluxes in an orientifold of type IIB, that, without fluxes, would be T-dual to type I theory. In the limit where backreaction in form of a warp factor is neglected, the effective action can be obtained through a procedure that combines dimensional reduction and T-duality, and it is found to be in agreement with results from gauged supergravity. This provides evidence for the consistency of this commonly used approximation scheme.

hep-th↗
arXiv2003

An Orientifold with Fluxes and Branes via T-duality

String compactifications with non-abelian gauge fields localized on D-branes, with background NSNS and RR 3-form fluxes, and with non-trivial warp factors, can naturally exist within T-dual versions of type I string theory. We develop a systematic procedure to construct the effective bosonic Lagrangian of type I T-dualized along a six-torus, including the coupling to gauge multiplets on D3-branes and the modifications due to 3-form fluxes. Looking for solutions to the ten-dimensional equations of motion, we find warped products of Minkowski space and Ricci-flat internal manifolds. Once the warp factor is neglected, the resulting no-scale scalar potential of the effective four-dimensional theory combines those known for 3-form fluxes and for internal Yang-Mills fields and stabilizes many of the moduli. We perform an explicit comparison of our expressions to those obtained from N=4 gauged supergravity and find agreement. We also comment on the possibility to include D9-branes with world-volume gauge fluxes in the background with 3-form fluxes.

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arXiv2002

The Standard Model on the Quintic

We describe the general geometrical framework of brane world constructions in orientifolds of type IIA string theory with D6-branes wrapping 3-cycles in a Calabi-Yau 3-fold, and point out their immediate phenomenological relevance. These branes generically intersect in points, and the patterns of intersections govern the chiral fermion spectra and issues of gauge and supersymmetry breaking in the low energy effective gauge theory on their world volume. In particular, we provide an example of an intersecting brane world scenario on the quintic Calabi-Yau with the gauge group and the chiral spectrum of the Standard Model and discuss its properties in some detail. Additionally we explain related technical advancements in the construction of supersymmetric orientifold vacua with intersecting D-branes. Six-dimensional orientifolds of this type generalize the rather limited set of formerly known orbifolds of type I, and the presented techniques provide a short-cut to obtain their spectra. Finally, we comment on lifting configurations of intersecting D6-branes to M-theory on non-compact G_2 manifolds.

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arXiv2002

Orientifolds of K3 and Calabi-Yau Manifolds with Intersecting D-branes

We investigate orientifolds of type II string theory on K3 and Calabi-Yau 3-folds with intersecting D-branes wrapping special Lagrangian cycles. We determine quite generically the chiral massless spectrum in terms of topological invariants and discuss both orbifold examples and algebraic realizations in detail. Intriguingly, the developed techniques provide an elegant way to figure out the chiral sector of orientifold models without computing any explicit string partition function. As a new example we derive a non-supersymmetric Standard-like Model from an orientifold of type IIA on the quintic Calabi-Yau 3-fold with wrapped D6-branes. In the case of supersymmetric intersecting brane models on Calabi-Yau manifolds we discuss the D-term and F-term potentials, the effective gauge couplings and the Green-Schwarz mechanism. The mirror symmetric formulation of this construction is provided within type IIB theory. We finally include a short discussion about the lift of these models from type IIB on K3 to F-theory and from type IIA on Calabi-Yau 3-folds to M-theory on G_2 manifolds.

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