Théorie des cordes

Résumé

En physique fondamentale, la théorie des cordes est un cadre théorique dans lequel les particules ponctuelles de la physique des particules sont représentées par des objets unidimensionnels appelés cordes. La théorie décrit comment ces cordes se propagent dans l'espace et interagissent les unes avec les autres. Sur des échelles de distance supérieures à l'échelle de la corde, cette dernière ressemble à une particule ordinaire, avec ses propriétés de masse, de charge et autres, déterminées par l'état vibratoire de la corde. En théorie des cordes, l'un de ces états vibratoires correspond au graviton, une particule décrite par la mécanique quantique qui véhicule l'interaction gravitationnelle. Ainsi, la théorie des cordes est une théorie de la gravité quantique. Puisque la théorie des cordes fournit potentiellement une description unifiée de la gravité et de la physique des particules, elle est candidate à une théorie du tout, un modèle mathématique autonome qui décrit toutes les forces

Source officielle

https://fr.wikipedia.org/wiki/Th%C3%A9orie_des_cordes

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Chargement

Tackling the Supersymmetric Flavour Problem in String Models

Christopher John Andrey

In this work we address one of the phenomenological issues of beyond the Standard Model scenarios which embed Supersymmetry, namely the Supersymmetric Flavour Problem, in the context of String Theory. Indeed, the addition of new interactions to the Standard Model generically spoils its flavour structure which is one of its major achievements since it for example leads to a very elegant understanding of the absence of flavour changing neutral currents in the leptonic sector and of the stability of the proton, thanks to accidental symmetries. We focus on a subset of the phenomenologically dangerous operators, namely the soft scalar masses. One way out of the Supersymmetric Flavour Problem is to geographically separate the observable and hidden sectors along a fifth dimension, gravity being the only interaction propagating in the bulk. In such scenarios, the soft scalar masses are vanishing at the classical level since there is no direct contact term between the observable and hidden multiplets and tend to be universal at the loop-level. However such setups hardly ever come about in String Theory, which is one of the most promising candidates of quantum gravity. In order to make contact with the five-dimensional picture, we focus on the prototypical case of the E8 × E8 Heterotic M-Theory which, in a certain regime, effectively looks five-dimensional and embeds matter fields on two end-of-the-world branes. In these scenarios, not only gravity but also vector multiplets propagate in the five-dimensional bulk, effectively spoiling the sequestered picture. However, since the contact terms responsible for the appearance of soft scalar masses arise due to the exchange of heavy vectors, they do enjoy a current-current structure which can be exploited to inhibit the emergence of soft scalar masses by postulating a global symmetry in the hidden sector. In order to assess the possibility of realising such a mechanism, we first study the full dependence of the Kähler potential on both the moduli and the matter fields in the case of orbifold and Calabi-Yau compactifications. We then determine whether an effective sequestering may be achieved thanks to a global symmetry and argue that whereas for orbifold models our strategy can naturally be put at work, it can only be implemented in a subset of Calabi-Yau models.

EPFL2011

Chargement

We elaborate on the idea that five-dimensional models where sequestering is spoiled due to contact interactions induced by vector multiplets may still be mildly sequestered if a global version of the gauge symmetry associated to the latter survives in the hidden sector. Interestingly, it has been argued that although in such a situation non-trivial current current contact interactions are induced by the heavy vector modes, these do not induce soft scalar masses, as a consequence of the global symmetry. We perform a detailed study of how this hybrid mechanism can be implemented in supergravity and string models, focusing on the prototypical case of heterotic M-theory orbifolds. We emphasize that in general the mechanism works only up to subleading effects suppressed by the ratio between the global symmetry breaking scale in the hidden sector and the vector mass scale or the Planck scale. We also argue that this mild sequestering mechanism allows to rehabilitate the scenario of dilaton domination of supersymmetry breaking, which is incompatible with dilaton stabilization in its original version, by exploiting the fact that hidden brane fields do contribute to the cosmological constant but not to soft terms, thanks to the global symmetry. (C) 2010 Elsevier B.V. All rights reserved.

2010

Chargement

Constraints on modular inflation in supergravity and string theory

Claudio Scrucca

We perform a general algebraic analysis on the possibility of realising slow-roll inflation in the moduli sector of string models. This problem turns out to be very closely related to the characterisation of models admitting metastable vacua with non-negative cosmological constant. In fact, we show that the condition for the existence of viable inflationary trajectories is a deformation of the condition for the existence of metastable de Sitter vacua. This condition depends on the ratio between the scale of inflation and the gravitino mass and becomes stronger as this parameter grows. After performing a general study within arbitrary supergravity models, we analyse the implications of our results in several examples. More concretely, in the case of heterotic and orientifold string compactifications on a Calabi-Yau in the large volume limit we show that there may exist fully viable models, allowing both for inflation and stabilisation. Additionally, we show that subleading corrections breaking the no-scale property shared by these models always allow for slow-roll inflation but with an inflationary scale suppressed with respect to the gravitino scale. A scale of inflation larger than the gravitino scale can also be achieved under more restrictive circumstances and only for certain types of compactifications.

2008

Chargement

Tackling the Supersymmetric Flavour Problem in String Models

Christopher John Andrey

EPFL2011