Boris Kors

String loop corrections to Kähler potentials in orientifolds

We determine one-loop string corrections to Kahler potentials in type-IIB orientifold compactifications with either = 1 or = 2 supersymmetry, including D-brane moduli, by evaluating string scattering amplitudes.

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

A Stueckelberg extension of the standard model

mathcal{N}=1

Aspects of the Stueckelberg extension

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.

Effective action and soft supersymmetry breaking for intersecting D-brane models

Abstract 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 a low mass and appear in e+e− collisions as a sharp resonance. The branching ratios into f 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×U(1)Y.

A supersymmetric Stueckelberg U(1) extension of the MSSM

A detailed analysis of a Stueckelberg extension of the electro-weak gauge group with an extra abelian U(1)X 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.

An orientifold with fluxes and branes via T-duality

Abstract 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.

Hierarchically split supersymmetry with Fayet–Iliopoulos D-terms in string theory

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 6 × 6 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.

Kaehler anomalies in Supergravity and flux vacua

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.

How Stueckelberg extends the standard model and the MSSM

Abstract 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 , 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.