SINJE : Séminaire INformel des JEunes 2012-2013

  Mardi 16 avril 2013 à 11h, salle 114

Stéphane Perrard (Laboratoire Matière et Systèmes Complexes) : Quantization of a particle guided by its own pilot-wave

The association of a particle and a wave can be obtained even at a macroscopic scale, using a simple experimental set up. A liquid bath is set into vertical oscillation so that any drop deposit on it has never the time to break the air layer under it. The drop is always ejected from the bath by the vibration and can then live for hours. The impact generates waves at the surface of the bath which can propel the drop. It becomes a walker, the self-propelled entity formed by a bouncing droplet and its associated surface wave. This system has already shown surprising wave-particle duality as single particle diffraction or Bohr-Sommerfeld quantization of level when the drop is submitted to a transverse force. We study now its motion when it is submitted to a central force, which can be tuned at will. In particular, I will present our results in the case of a 2D harmonic potential well. In the case of a strong coupling between the waves and the bouncing drop, the walker exhibits a discrete set of state where the angular momentum and the spatial extend of each level are quantized. Disordered trajectories also appear, as an intermittency between the pure eigenmodes through transitions between them. Finally I will compare these results to the quantum eigenstates in a 2D potential well.

  Mardi 4 décembre 2012 et Mardi 11 décembre 2012 à 11h, salle 114

Michel Dubois-Violette (LPT Orsay) : An introduction to noncommutative geometry

We shall make a general elementary introduction to noncommutative geometry. We shall describe the generalizations of the classical structures such as the topological structures, the smooth structures and the group-like structures in the noncommutative setting. Concepts with no classical counterpart such as Morita equivalence will be pointed out (and explained). Many illustrative examples, in particular examples of physical applications, will be described.

  Mardi 13 Novembre à 11h, salle 114

Marco Crisostomi (LPT Orsay) : Recent developments in massive gravity

Recently, a nonlinear completion of the Fierz-Pauli model has been proved to be ghost free. I will review the basic phenomenology of this model looking at the spherically symmetric solutions and at the cosmological ones. I will fo- cus on the differences and the analogies respect to General Relativity, in order to discuss the effective viability of this theory. Moreover I will present the cosmological perturbations, showing if really we can trust about them.

  Mercredi 24 octobre à 11h, salle 114

Guillaume Toucas (LPT, Orsay) : Mécanisme de brisure de symétrie chirale pour les trois saveurs de quarks légers et extrapolation de résultats de Chromodynamique Quantique sur réseau

In this thesis, we focus on some aspects concerning hadronic phenomena at low energy, below 1 GeV, where the spontaneous breaking of chiral symmetry takes place. Below this scale, the spectrum of Quantum ChromoDynamics reduces to an octet of light pseudo-scalar mesons (π, K and η). But because of confinement, QCD under 1 GeV is highly non-perturbative, it is thus not possible to describe at low energy the dynamics of these mesons in terms of gluons and quarks, in that case the three light quarks u,d, and s. Two main alternatives exist to circumvent this major obstacle : Lattice QCD and Effective Field Theories. Lattice QCD is concerned with the numerical computations of various hadronic observables, while Effective Field Theories correspond to analytical frameworks adapted to a particular energy scale. In the case of QCD at low energy, this role is devoted to Chiral Perturbation Theory (ChiPT). This theory can be built either from two quark flavours (u and d), or three (u,d, and s). Using the numerical results from Lattice QCD, it is possible to obtain numerical values for the unknown parameters that ChPT contains. It was however observed that the series expansions of hadronic observables stemming from ChiPT calculations do not “behave well” in the three-flavour case. Indeed, previous works has shown that there could exists, at the numerical level, a competition between the Leading and the Next-to-Leading order (LO and NLO) ; i.e., instead of the usually expected hierarchy LO>>NLO, one would have LO NLO. The main part of the thesis work consists in the description and the use of a modified version of ChiPT allowing this numerical competition in the chira l series that was called “Resummed ChiPT”. Within this “Resummed” framework, we proceed to fitting data from lattice calculations to hadronic observables computed in ChiPT : decay cons tants and masses of π, K and η, the Kl3 form factors, and check the consistency of our claim about the numerical competition in ChiPT expansions. In the last part, we discuss topological quantities that are intrinsically tied to the very complex structure of the QCD vacuum, in the light of (resummed) ChiPT, in their analytical and numerical aspects.

  Mardi 21 septembre à 11h, salle 114

Antonin Coutant (LPT, Orsay) : Stability properties of Hawking radiation in the presence of ultraviolet violation of local Lorentz invariance

In this presentation, we study several features of Hawking radiation in the presence of ultraviolet Lorentz violations. These violations are implemented by a modified dispersion relation that becomes nonlinear at short wavelengths. The motivations of this work arise on the one hand from the developing field of analog gravity, where we aim at measuring the Hawking effect in fluid flows that mimic black hole space-times, and on the other hand from the possibility that quantum gravity effects might be approximately modeled by a modified dispersion relation. We develop several studies on various aspects of the problem. First we obtain precise characterizations about the deviations to the Hawking result of black hole radiation, which are induced by dispersion. Second, we describe in detail an instability named black hole laser, which arises in the presences of two horizons, where Hawking radiation is self-amplified and induces an exponentially growing in time emitted flux. Third, we study the emergence, both in white hole flows or for massive fields, of a macroscopic standing wave, spontaneously produced from the Hawking effect, and known as “undulation”.

  Mardi 18 septembre à 11h, salle 114

Yannis Bardoux (LPT, Orsay) : Black hole solutions of modified gravity theories

The main interest of the work exposed in this PhD defence is to explore new black hole solutions in a more general framework than General Relativity by taking into account the presence of a cosmological constant, of higher dimensions, of exotic matter fields or of higher curvature terms. These extensions to General Relativity can be derived in the context of String Theory. It is also by studying natural extensions to General Relativity that we can more deeply understand the theory of Einstein. A first extension will be detailed with the introduction of higher dimensions and p-form fields which constitute the natural generalization of the electromagnetic interaction. We will build in this framework new static black hole solutions where p-form fields allow to shape the geometry of the horizon. Secondly, we will present the general extension of Einstein theory in any dimension which produces second order field equations : Lovelock theory. We will determine in this context a large class of solutions in dimension 6 for which the theory is reduced to Einstein-Gauss-Bonnet theory with the presence of p-form fields. Thirdly, we will study a generalization of General Relativity in dimension 4 whose modification is induced by a conformally coupled scalar field. We will namely exhibit a new black hole solution with a flat horizon in the presence of axionic fields. To conclude this PhD defence, thermodynamical aspects of these gravitational theories will be studied. In this way, we will be able to determine the mass and the charges of these new solutions and we will examine phase transition phenomena in the presence of a conformally scalar field.