arXiv subjects

Hans Peter Nilles

Publications and source records attributed to Hans Peter Nilles.

At least 91 records · Page 5Linked to original sources

arXiv2003

Spontaneous localization of bulk fields: the six-dimensional case

We study N=2 supersymmetric gauge theories with $d=6$ bulk and $d=4$ brane fields charged under a U(1) gauge symmetry. Radiatively induced Fayet-Iliopoulos terms lead to an instability of the bulk fields. We compute the profile of the bulk zero modes and observe the phenomenon of spontaneous localization towards the position of the branes. While this mechanism is quite similar to the $d=5$ case, the mass spectrum of the excited Kaluza-Klein modes shows a crucial difference.

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arXiv2003

Hidden sector axions: physics and cosmology

Embedding hidden sector supergravity models in the framework of string theory leads to the appearance of axion-like degrees of freedom. Among them is the model independent axion of heterotic string theory. It has a decay constant of order of the Planck scale and could play the role of a quintessence field. In models allowing for the required $μ$ term in the TeV range, the hidden sector dynamics leads to a vacuum energy of $(0.003 {\rm eV})^4$ via a multiple see-saw effect. A solution to the strong CP-problem is provided by an additional hidden sector pseudoscalar with properties that make it an acceptable candidate for cold dark matter of the universe.

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arXiv2002

A Quintessential Axion

The model independent axion of string theory has a decay constant of order of the Planck scale. We explore the properties of this quintessence candidate (quintaxion) in the scheme of hidden sector supergravity breakdown. In models allowing for a reasonable $μ$ term, the hidden sector dynamics may lead to an almost flat potential responsible for the vacuum energy of $(0.003 {\rm eV})^4$. A solution to the strong CP-problem is provided by an additional hidden sector pseudoscalar (QCD axion) with properties that make it an acceptable candidate for cold dark matter of the universe.

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arXiv2002

Localized nonabelian anomalies at orbifold fixed points

Orbifold compactifications of 10D heterotic strings do allow different sets of chiral fermions at different fixed points. Even if the effective 4D theory is anomaly free by including the bulk fermions, there arise abelian and nonabelian fixed point anomalies from these chiral fermions. As the underlying string theory is well defined these localized fixed point anomalies of the chiral fermions are to be cancelled by a variant of the Green-Schwarz mechanism.

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arXiv2001

Nonthermal production of gravitinos and inflatinos in the Inflationary Universe

The success of primordial nucleosynthesis imposes stringent bounds on the abundance of gravitational relics. This is particularly true for gravitinos, which - for models with gravitationally mediated supersymmetry breaking - are expected to have a mass below the TeV scale and thus to decay only after nucleosynthesis has concluded. We discuss the nonthermal production of gravitinos in models with several chiral fields, to be able to distinguish the mechanisms of supersymmetry breaking and inflation. Our explicit calculations show that the superpartner of the inflaton can be significantly generated. Gravitinos can be produced both at the preheating stage and at the subsequent inflaton/inflatino decay. We verify that this production is well below the nucleosynthesis bound, provided the sector responsible for the present supersymmetry breakdown is weakly (for example, only gravitationally) coupled to the one which drove inflation.

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arXiv2001

The Inflatino Problem in Supergravity Inflationary Models

We consider the potential problems due to the production of inflatinos and gravitinos after inflation. Inflationary models with a single scale set by the microwave background anisotropies have a low enough reheat temperature to avoid problems with the thermal production of gravitinos. Moreover, the nonthermal production of gravitinos has been shown to be sufficiently small if the sector ultimately responsible for supersymmetry breaking is coupled only gravitationally to the inflationary sector. Still, in some models, inflatinos can be created during preheating with a substantial abundance. The main contribution to the gravitino abundance may thus come from their decay into the inflaton, or into its scalar partner, as well as from the inverse processes. We show that this production needs to be strongly suppressed. This suppression can be realized in the simplest scenarios which typically have a sufficiently high inflationary scale.

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arXiv2001

Hidden Sector Supergravity Breakdown

Gravity mediated supersymmetry breakdown originated from a study of gaugino condensation in a hidden sector. We review this mechanism of supergravity breakdown from the original formulation in the early eighties to its natural implementation in superstring theory and M-theory. In the latter case hidden and observable sector become separated geometrically and supersymmetry is broken dynamically on a hidden wall.

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arXiv2001

Coupled fields in external background with application to nonthermal production of gravitinos

We provide the formalism for the quantization of systems of coupled bosonic and fermionic fields in a time dependent classical background. The occupation numbers of the particle eigenstates can be clearly defined and computed, through a generalization of the standard procedure valid for a single field in which Bogolyubov coefficients are employed. We apply our formalism to the problem of nonthermal gravitino production in a two-fields model where supersymmetry is broken gravitationally in the vacuum. Our explicit calculations show that this production is strongly suppressed in the model considered, due to the weak coupling between the sector which drives inflation and the one responsible for supersymmetry breakdown.

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arXiv2001

Quadratic Divergences in Kaluza-Klein Theories

We investigate the so-called ``Kaluza-Klein regularisation'' procedure in supersymmetric extensions of the standard model with additional compact dimensions and Scherk-Schwarz mechanism for supersymmetry breaking. This procedure uses a specific mathematical manipulation to obtain a finite result for the scalar potential. By performing the full calculation, we show that the finiteness of this result is not only a consequence of the underlying supersymmetry, but also the result of an implicit fine-tuning of the coefficients of the terms that control the ultraviolet behaviour. The finiteness of the Higgs mass at one-loop level seems therefore to be an artefact of the regularisation scheme, and quadratic divergences are expected to reappear in higher orders of perturbation theory.

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arXiv2001

The Cosmological Constant and the Brane World Scenario

Although the cosmological constant has primarily cosmological consequences, its smallness poses one of the basic problems in particle physics. Various attempts have been made to explain this mystery, but no satisfactory solution has been found yet. The appearance of extra dimensions in the framework of brane world systems seems to provide some new ideas to address this problem form a different point of view. We shall discuss some of these new approaches and see whether or not they lead to an improvement of the situation. We shall conclude that we are still far from a solution of the problem.

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arXiv2000

Dark Energy in Extra Dimensions and String Theory: Consistency Conditions

The smallness of the cosmological constant is one of the basic problems in particle physics and cosmology. Various attempts have been made to explain this mystery, but no satisfactory solution has been found yet. The appearance of extra dimensions in the framework of brane world systems seems to provide some new ideas to address this problem from a different point of view. We shall discuss some of these new approaches and see whether or not they lead to an improvement of the situation. We shall conclude that we are still far from a solution of the problem.

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arXiv2000

The Cosmological Constant Problem from a Brane-World Perspective

We point out several subtleties arising in brane-world scenarios of cosmological constant cancellation. We show that solutions with curvature singularities are inconsistent, unless the contribution to the effective four-dimentional cosmological constant of the physics that resolves the singularities is fine-tuned. This holds for both flat and curved branes. Irrespective of this problem, we then study an isolated class of flat solutions in models where a bulk scalar field with a vanishing potential couples to a 3-brane. We give an example where the introduction of a bulk scalar potential results in a nonzero cosmological constant. Finally we comment on the stability of classical solutions of the brane system with respect to quantum corrections.

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arXiv2000

On the low-energy limit of string and M-theory

We discuss the possible applications of string theory for the construction of generalizations of the $SU(3)\times SU(2)\times U(1)$ standard model of strong and electroweak interactions. This includes an investigation of effective $d=4$ dimensional supergravity theories that could be derived from higher dimensional string theories ($d=10$) and M-theory ($d=11$).It is shown how the question of unification of gauge and gravitational coupling constants could find a solution within this framework. The mechanism of hidden sector supersymmetry breakdown and its consequences for the pattern for soft supersymmetry breaking terms are discussed in detail.

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arXiv2000

A Comment on Self-Tuning and Vanishing Cosmological Constant in the Brane World

In this note we elaborate on various five dimensional contributions to the effective 4D cosmological constant in brane systems. In solutions with vanishing 5D cosmological constant we describe a non-local mechanism of cancellation of vacuum energy between the brane and the singularities. We comment on a hidden fine tuning which is implied by this observation.

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arXiv2000

Anomalous U(1) gauge symmetries and heterotic-typeI/II string duality

Anomalous U(1) gauge symmetries can appear both in heterotic and type I string theories. In the heterotic case we find a single anomalous U(1), while in open string theories several such symmetries can appear. Nonetheless, there is a conjectured duality symmetry that might connect these two theories. We review the properties of anomalous gauge symmetries in various string theories as well as the status of this heterotic-type I/II duality. We also comment on the possible phenomenological applications of anomalous gauge symmetries in string theory.

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arXiv2000

Remarks on anomalous U(1) symmetries in string theory

Anomalous U(1) gauge symmetries appear frequently both in heterotic and type I/type II string theory. In the heterotic case we find at most a single anomalous U(1), while in open string theories several such symmetries can appear. We review the properties of the anomalous gauge bosons, the appearance of Fayet-Iliopoulos terms and the concept of heterotic-type I duality. Phenomenological applications of these symmetries might be different in the type I/type II case compared to the heterotic one.

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arXiv1999

Target-Space Duality in Heterotic and Type I Effective Lagrangians

We study the implications of target-space duality symmetries for low-energy effective actions of various four-dimensional string theories. In the heterotic case such symmetries can be incorporated in simple orbifold examples. At present a similar statement cannot be made about the simplest type IIB orientifolds due to an obstruction at the level of gravitational anomalies. This fact confirms previous doubts concerning a conjectured heterotic-type IIB orientifold duality and shows that target-space symmetries can be a powerful tool in studying relations between various string theories at the level of the effective low-energy action. Contraints on effective Lagrangians from these symmetries are discussed in detail. In particular, we consider ways of extending T-duality to include additional corrections to the Kaehler potential in heterotic string models with N=2 subsectors.

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arXiv1999

Supersymmetry Breakdown at Distant Branes: The Super-Higgs Mechanism

A compactification of 11-dimensional supergravity with two (or more) walls is considered. The whole tower of massive Kaluza-Klein modes along the fifth dimension is taken into account. With the sources on the walls, an explicit composition in terms of Kaluza-Klein modes of massless gravitino (in the supersymmetry preserving case) and massive gravitino (in the supersymmetry breaking case) is obtained. The super-Higgs effect is discussed in detail.

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