We propose a new supersymmetry in field theory that generalizes standard supersymmetry and we con... more We propose a new supersymmetry in field theory that generalizes standard supersymmetry and we construct field theoretic models that provide some of its representations. This symmetry combines a finite number of standard 4D supersymmetry multiplets into a single multiplet with a new type of Kaluza-Klein embedding in higher dimensions. We suggest that this mechanism may have phenomenological applications in understanding
... Lett. B 158 (1985) 388. [4] 1. Antoniadis, J. Ellis and DV Nanopoulos, Phys. Lett. ... Lett. ... more ... Lett. B 158 (1985) 388. [4] 1. Antoniadis, J. Ellis and DV Nanopoulos, Phys. Lett. ... Lett. B 181 (1986) 263; I. Antoniadis, J. Ellis, E. Floratos, DV Nanopoulos and T. Tomaras, Phys. Lett. B 191 (1987) 96; S. Ferrara, L. Girardello, C. Kounnas and M. Porrati, Phys. Lett. ...
We obtain the non-perturbative effective potential for the dual five-dimensional N=4 strings in t... more We obtain the non-perturbative effective potential for the dual five-dimensional N=4 strings in the context of finite-temperature regarded as a breaking of supersymmetry into four space-time dimensions. Using the properties of gauged N=4 supergravity we derive the universal thermal effective potential describing all possible high-temperature instabilities of the known N=4 superstrings. These strings undergo a high-temperature transition to a new phase in which five-branes condense. This phase is described in detail, using both the effective supergravity and non-critical string theory in six dimensions. In the new phase, supersymmetry is perturbatively restored but broken at the non-perturbative level.
Decoupling limits of physical interest occur in regions of space--time where the string coupling ... more Decoupling limits of physical interest occur in regions of space--time where the string coupling diverges. This is illustrated in the celebrated example of five-branes. There are several ways to overcome this strong-coupling problem. We review those which are somehow related to two-dimensional conformal field theories. One method consists of distributing the branes over transverse space, either on a circle or over a sphere. Those distributions are connected to conformal field theories by T-dualities or lead to a new kind of sigma model where the target space is a patchwork of pieces of exact conformal-field-theory target spaces. An alternative method we discuss is the introduction of diluted F-strings, which trigger a marginal deformation of an AdS$_3\times S^3\times T^4$ background with a finite string coupling. Our discussion raises the question of finding brane configurations, their spectrum, their geometry, and their interpretation in terms of two-dimensional conformal models.
We review recent progress in string cosmology, where string dualities are applied so as to obtain... more We review recent progress in string cosmology, where string dualities are applied so as to obtain complete cosmological evolutions, free of any essential singularities. Two classes of models are analyzed. The first class consists of string gas cosmologies associated to certain thermal configurations of type II N=(4,0) models. Finite temperature is introduced along with non-trivial "gravito-magnetic" fluxes that lift the Hagedorn instabilities of the canonical ensemble and restore thermal T-duality symmetry. At a critical maximal temperature additional thermal states become massless sourcing stringy S-branes, which facilitate a bounce between the two dual, asymptotically cold phases. Unlike previous incarnations of pre-Big Bang cosmologies, the models remain perturbative throughout the cosmological evolution. The second class consists of exact solutions to classical string theory that admit a Euclidean description in terms of compact parafermionic worldsheet systems. The Eu...
In this work, we review the results of Refs [1]-[5] dedicated to the description of the early Uni... more In this work, we review the results of Refs [1]-[5] dedicated to the description of the early Universe cosmology induced by quantum and thermal effects in superstring theories. The present evolution of the Universe is described very accurately by the standard Lambda-CDM scenario, while very little is known about the early cosmological eras. String theory provides a consistent microscopic theory to account for such missing epochs. In our framework, the Universe is a torus filled with a gas of superstrings. We first show how to describe the thermodynamical properties of this system, namely energy density and pressure, by introducing temperature and supersymmetry breaking effects at a fundamental level by appropriate boundary conditions. We focus on the intermediate period of the history: After the very early "Hagedorn era" and before the late electroweak phase transition. We determine the back-reaction of the gas of strings on the initially static space-time, which then yiel...
We study heterotic ground states in which supersymmetry is broken by coupling the momentum and wi... more We study heterotic ground states in which supersymmetry is broken by coupling the momentum and winding charges of two large extra dimensions to the R-charges of the supersymmetry generators. The large dimensions give rise to towers of heavy string thresholds that contribute to the running of the gauge couplings. In the general case, these contributions are proportional to the volume of the two large dimensions and invalidate the perturbative string expansion. The problem is evaded if the susy breaking sectors arise as a spontaneously broken phase of N=4 -> N=2 -> N=0 supersymmetry, provided that N=4 supersymmetry is restored on the boundary of the moduli space. We discuss the mechanism in the case of Z_2 x Z_2 orbifolds, which requires that the twisted sector that contains the large extra dimensions has no fixed points. We analyse the full string partition function and show that the twisted sectors distribute themselves in non aligned N=2 orbits, hence preserving the solution ...
We propose a new supersymmetry in field theory that generalizes standard supersymmetry and we con... more We propose a new supersymmetry in field theory that generalizes standard supersymmetry and we construct field theoretic models that provide some of its representations. This symmetry combines a finite number of standard 4D supersymmetry multiplets into a single multiplet with a new type of Kaluza-Klein embedding in higher dimensions. We suggest that this mechanism may have phenomenological applications in understanding
... Lett. B 158 (1985) 388. [4] 1. Antoniadis, J. Ellis and DV Nanopoulos, Phys. Lett. ... Lett. ... more ... Lett. B 158 (1985) 388. [4] 1. Antoniadis, J. Ellis and DV Nanopoulos, Phys. Lett. ... Lett. B 181 (1986) 263; I. Antoniadis, J. Ellis, E. Floratos, DV Nanopoulos and T. Tomaras, Phys. Lett. B 191 (1987) 96; S. Ferrara, L. Girardello, C. Kounnas and M. Porrati, Phys. Lett. ...
We obtain the non-perturbative effective potential for the dual five-dimensional N=4 strings in t... more We obtain the non-perturbative effective potential for the dual five-dimensional N=4 strings in the context of finite-temperature regarded as a breaking of supersymmetry into four space-time dimensions. Using the properties of gauged N=4 supergravity we derive the universal thermal effective potential describing all possible high-temperature instabilities of the known N=4 superstrings. These strings undergo a high-temperature transition to a new phase in which five-branes condense. This phase is described in detail, using both the effective supergravity and non-critical string theory in six dimensions. In the new phase, supersymmetry is perturbatively restored but broken at the non-perturbative level.
Decoupling limits of physical interest occur in regions of space--time where the string coupling ... more Decoupling limits of physical interest occur in regions of space--time where the string coupling diverges. This is illustrated in the celebrated example of five-branes. There are several ways to overcome this strong-coupling problem. We review those which are somehow related to two-dimensional conformal field theories. One method consists of distributing the branes over transverse space, either on a circle or over a sphere. Those distributions are connected to conformal field theories by T-dualities or lead to a new kind of sigma model where the target space is a patchwork of pieces of exact conformal-field-theory target spaces. An alternative method we discuss is the introduction of diluted F-strings, which trigger a marginal deformation of an AdS$_3\times S^3\times T^4$ background with a finite string coupling. Our discussion raises the question of finding brane configurations, their spectrum, their geometry, and their interpretation in terms of two-dimensional conformal models.
We review recent progress in string cosmology, where string dualities are applied so as to obtain... more We review recent progress in string cosmology, where string dualities are applied so as to obtain complete cosmological evolutions, free of any essential singularities. Two classes of models are analyzed. The first class consists of string gas cosmologies associated to certain thermal configurations of type II N=(4,0) models. Finite temperature is introduced along with non-trivial "gravito-magnetic" fluxes that lift the Hagedorn instabilities of the canonical ensemble and restore thermal T-duality symmetry. At a critical maximal temperature additional thermal states become massless sourcing stringy S-branes, which facilitate a bounce between the two dual, asymptotically cold phases. Unlike previous incarnations of pre-Big Bang cosmologies, the models remain perturbative throughout the cosmological evolution. The second class consists of exact solutions to classical string theory that admit a Euclidean description in terms of compact parafermionic worldsheet systems. The Eu...
In this work, we review the results of Refs [1]-[5] dedicated to the description of the early Uni... more In this work, we review the results of Refs [1]-[5] dedicated to the description of the early Universe cosmology induced by quantum and thermal effects in superstring theories. The present evolution of the Universe is described very accurately by the standard Lambda-CDM scenario, while very little is known about the early cosmological eras. String theory provides a consistent microscopic theory to account for such missing epochs. In our framework, the Universe is a torus filled with a gas of superstrings. We first show how to describe the thermodynamical properties of this system, namely energy density and pressure, by introducing temperature and supersymmetry breaking effects at a fundamental level by appropriate boundary conditions. We focus on the intermediate period of the history: After the very early "Hagedorn era" and before the late electroweak phase transition. We determine the back-reaction of the gas of strings on the initially static space-time, which then yiel...
We study heterotic ground states in which supersymmetry is broken by coupling the momentum and wi... more We study heterotic ground states in which supersymmetry is broken by coupling the momentum and winding charges of two large extra dimensions to the R-charges of the supersymmetry generators. The large dimensions give rise to towers of heavy string thresholds that contribute to the running of the gauge couplings. In the general case, these contributions are proportional to the volume of the two large dimensions and invalidate the perturbative string expansion. The problem is evaded if the susy breaking sectors arise as a spontaneously broken phase of N=4 -> N=2 -> N=0 supersymmetry, provided that N=4 supersymmetry is restored on the boundary of the moduli space. We discuss the mechanism in the case of Z_2 x Z_2 orbifolds, which requires that the twisted sector that contains the large extra dimensions has no fixed points. We analyse the full string partition function and show that the twisted sectors distribute themselves in non aligned N=2 orbits, hence preserving the solution ...
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Papers by Costas Kounnas