SINJE : Séminaire INformel des JEunes 2010-2011

  Vendredi 8 juillet à 14h, Salle 114

Yannis Bardoux (LPT, Orsay) : Hamiltonian derivation of the first law of black holes mechanics

The hamiltonian formulation of General Relativity is usually employed as a starting point to formulate a quantum theory by the canonical approach. However in purely classical General Relativity it already provides a powerful tool to derive the first law of black holes mechanics first discovered by Bardeen, Carter and Hawking in 1973. First of all, General Relativity will be presented in a nutshell. Then the lagrangian and hamiltonian formulations of the theory will be detailled. In their derivations we will emphazise on the presence of boundary terms which is one of the crucial points for the derivation of the first law of black hole mechanics. Finally we will deduce this law by following a simply and elegant argument presented by Wald and Sudarsky in 1990.

  Vendredi 1er juillet à 14h, Salle 114

Cesar S Lopez-Monsalvo (School of mathematical science, University of London) : Relativistic Thermal Dynamics and the Inertial Properties of Heat

Does a moving body appear cold ? This remarkably simple ques- tion,raised by P. T. Landsberg some forty years ago, highlights a profound missing link in our current understanding of classical thermodynamics and relativity. Some might interpret this as the lack of a relativistic transformation law for temperature measurements. Some others, as we do here, believe that this is a misunderstanding concerning the nature of thermal motion in relati- vistic settings. Such a view shifts the attention from a mere exercise in veloci- ty transformations to an argument about thermal equilibrium between moving observers and the inertial properties of heat. It promotes the discussion about the missing Lorentz transformation for temperature to the more elaborate - and physically meaningful - problem of heat exchange and irreversibility in a relativistic context. In this talk I will give an overview of the variational ap- proach to heat conduction in general relativity. I will discuss the conditions for stability and causality to be satisfied and present some details of our recent efforts in the development of a consistent first-order theory of relativistic dissipation.

  Vendredi 24 juin à 14h, Salle 114

Cyril Lamriben (Laboratoire FAST Orsay) : Rotating turbulence : an experimental approach of the KHM equation

Hydrodynamic turbulence is an old subject, but remains a fundamental unsolved problem of classical physics. Among the large number of facets of this subject, the Karman‐Howarth‐Monin (KHM) equation has clearly a distinct status : it is the most fundamental and only exact result for homogeneous turbulence. This equation directly derives from the Navier-Stokes equation, without any approximation, and contains the physics of the energy cascade from the large to the small scales. Importantly, this equation is still valid for anisotropic turbulence, such as rotating turbulence. Our goal is to provide an experimental assessment of the KHM equation and to study the influence of a background rotation on the energy transfers in decaying grid turbulence. The anisotropic energy flux density is determined from large data sets of Particle Image Velocimetry measurements. We show that rotation induces an anisotropy of the energy flux, which leads to an anisotropic growth of the energy distribution. Surprisingly, our results prove that this anisotropy growth is essentially driven by a nearly radial, but orientation-dependent, energy flux density.

  Vendredi 17 juin à 14h, Salle 114

Guillaume Toucas (LPT Orsay) : QCD at low energy and Effective Field Theories

« A basse énergie (inférieure au GeV), le développement perturbatif de QCD en puissance de la constante de couplage s’effondre. Il n’est plus pertinent de décrire l’interaction forte en terme des degrés de liberté fondamentaux que sont les quarks et les gluons. Une théorie effective des champs peut alors être utilisée comme description adaptée permettant de modéliser les interactions hadroniques au dessous de l’échelle de brisure de la symétrie chirale de QCD, pour laquelle interviennent les trois saveurs de quark u, d et s : c’est la Théorie Chirale des Perturbations (ChiPT), ayant pour degrés de liberté effectifs l’ensemble des particules composant l’octet des mésons légers pseudo-scalaires π, K et η. Dans un premier temps, j’aborderai la notion de théorie effective des champs dans sa généralité. Ensuite, je particulariserai le problème de QCD à basse énergie, pour aboutir à une présentation de ChiPT. Enfin, je proposerai un court exposé de mes travaux de thèse, ceux-ci ayant trait à la convergence numérique de certaines observables calculées avec ChiPT pour les trois saveurs u, d et s.

At low energy (under 1 GeV), the perturbative expansion of QCD in terms of the coupling constant breaks down. It is no more relevant to make a description of the strong interaction in terms of the fundamentals degrees of freedom : the quarks and the gluons. An effective field theory can then be used to formalize the dynamics of hadrons under the scale of QCD chiral symmetry breaking, which involve the three quark flavours u, d and s : Chiral Perturbation Theory (ChiPT), which contains as effective degrees of freedom the particles making up the octet of light pseudo-scalar mesons π, K and η. First, I shall address the general notion of Effective Field Theory. Then, I shall particularize this notion to the problematics of low energy QCD, in order to come off with ChiPT proper. Finally, I shall give a short presentation of my thesis work, which deals with the numerical convergence of some observables calculated by the means of ChiPT, for the three flavours u, d and s. »

  Vendredi 10 juin à 14h, Salle 114

Benoît Blossier (LPT Orsay) : A numerical approach to solve QCD

« Des efforts considérables sont accomplis expérimentalement pour tenter de mettre en évidence de la Nouvelle Physique dans le secteur des quarks du Modèle Standard de la Physique des Particules. Cela s’accompagne d’un travail théorique de grande envergure afin de quantifier leur confinement dans les hadrons, en ayant notamment recours à des simulations numériques. J’introduirai cette approche, basée sur le formalisme d’intégrale de chemin et le prolongement analytique au temps Euclidien de la théorie quantique des champs, autorisant ainsi le calcul des fonctions de Green de la théorie par méthode Monte-Carlo ; j’exposerai ce qu’on peut en extraire comme données physiques et je donnerai un avant-goût des recherches en informatique et algorithmique que cette activité suscite.

Tremendous efforts are made experimentally to detect New Physics in the quark sector of the Standard Model. It goes with a hard theoretical work to quantify their confinment within hadrons, using in particular numerical simulations. I will introduce that approach, based on the integral path formalism and the analytical continuation to Euclidean time of quantum field theory, allowing the computation of Green functions by a Monte-Carlo method ; I will describe what physical quantities one can extract and I will give a foretaste of researches in computer science and algorithmic induced by that activity. »

  Vendredi 29 avril à 14h, Salle 114

Etienne Dumonceau : Quantum groups

Quantum groups is a field of mathematical physics which is very young. Its study was impulsed by the work of Faddeev in 1979 on a possible application of the ansatz of Bethe on Quantum Scattering Inverse Problem. The method was formalized separately by Drinfel’d and Jimbo. It appears to be the study of deformation of some algebraic structures. In this talk, I will expose a possible way to familiarize with this domain. I may focus more particularly on the group of S\ell(2) and constantly refering to an example close to the spin chain problem. We will construct, one step after another, coordinate ring, algebra, coalgebra, bialgebra, and finally enveloping algebra before processing to a deformation on the last one.

  Vendredi 15 avril à 11h, Salle 114

Sylvain Le Corff (LTCI / TSI) : Approximation de lois de lissage et estimation de paramètre dans les espaces d’états généraux

L’utilisation de méthodes de Monte Carlo séquentielles pour l’approximation de lois de lissage jointes dans les espaces d’états généraux est un problème classique en statistique. Dans cette présentation, nous mettrons en avant les enjeux de telles approximations ainsi que quelques méthodes connues et largement utilisées pour les mettre en oeuvre dans les modèles de Markov cachés. Nous présenterons ensuite de nouvelles bornes d’erreur non asymptotiques générées par l’approximation de fonctionnelles additives lissées. Plus précisément, nous fournirons de nouveaux résultats concernant différentes variantes de l’algorithme Forward filtering backward smoothing. Nous montrerons que, dans ce cas, la vitesse de convergence ne dépend que du ratio T/N où T est le nombre d’observations et N le nombre de particules utilisées, contrairement aux résultats précédents qui font intervenir le ratio T/ sqrt(N). Ces résultats seront illustrés au travers de quelques simulations.

  Vendredi 18 mars à 14h, Salle 114

Antonin Coutant (LPT Orsay) : From Unruh effect to Hawking radiation

In this talk, I will present the basics of the phenomenon of Unruh effect. The notion of particle is a central concept in modern elementary physics. However, it appears that this concept becomes fuzzy when it comes to non- inertial systems. The most famous example of this is the Unruh effect. Indeed, it has been shown that a uniformly accelerated detector in the vacuum will feel a thermal bath surrounding it. I will present this effect by appealing to simple quantum mechanical argument like the Fermi golden rule. I will also make the link with the presence of a Rindler horizon seen by an accelerated observer.

Furthermore the physics of Unruh effect can also describe what hap- pens in the vicinity of the horizon of a black hole. In this context the Unruh thermal bath becomes the well-known Hawking radiation, leading to the quan- tum evaporation of black holes.

  Vendredi 4 mars à 11h, Salle 114

Matthieu Barbier (LPTMS Orsay) : Kinetic theories and granular gases

At first, statistical mechanics had a foundational aim : explaining the laws of macroscopic physics by deriving them from the hamiltonian mechanics for large ensembles of particles. One of its main realiza- tions is the kinetic theory of gases, which starts with a N-body Liou- ville equation and recovers Navier-Stokes hydrodynamics, then goes even further than that. I will present the main points of this approach, and illustrate it in the case of granular gases : here, particles are macroscopic and inelastic grains (powders, sand or asteroids), in systems ranging from avalanches to planetary rings. These systems are dissipative and highly out-of-equilibrium, which causes their hydrodynamics to be even more amusing than usual.

  Vendredi 25 février à 14h, Salle 114

Benjamin Leveque (CPHT - Ecole Polytechnique) : Renormalisation of \Phi^4 theories through the method of flow equations

In this talk, I will present an approach to the renormalization problem in QFT. This one is called the flow equation method and has already been used to show the renormalizability of \Phi^4 theories (both massive and massless), QED and abelian gauge theories with spontaneous symmetry breaking in Euclidian signature. In the Euclidian picture, this method makes use of functional integral, which is in this context well defined. I will apply it in the simplest case, which is a massive scalar field with \Phi^4 interaction.

I will then attempt to show how one can generalize the method to gauge theories where renormalizability imposes new constrains ; Slavnov-Taylor identities should also be satisfied.

  Vendredi 18 février à 14h, Salle 114

Thomas Heattel (Laboratoire de Topologie et Dynamique, Orsay) : Geometry of symmetric spaces

Cet exposé se veut élémentaire, une introduction à plusieurs objets géométriques.

Un espace symétrique est une variété riemannienne où la symétrie géodésique est définie en chaque point. C’est également un espace homogène sous l’action d’un groupe de Lie. Je donnerai les principaux exemples d’espaces symétriques, puis m’intéresserai à leurs quotients compacts ou de volume fini. Dans le cas des surfaces hyperboliques, je présenterai l’espace de Teichmüller, qui paramètre les structures hyperboliques sur une surface donnée.

Dans les autres cas, je présenterai les phénomènes de rigidité qui apparaissent.

Je terminerai en exposant des liens entre la dynamique sur ces espaces et des questions arithmétiques.

  Vendredi 4 février à 14h, Amphi 1 du bâtiment 210

Nawal Quennous (ESPCI Paris) : Transport and deformation of an elastic filament in a cellular flow

The interaction of a deformable body with a viscous flow is found in a wide range of situa- tions, ranking from biology to polymer science. Numerous studies both experimental and theoretical have been devoted to this field, but a large number of questions remains to be answered. The experimental study of simple model systems, such as homogeneous elas- tic fibers, evolving in a controlled flow geometry, could help understanding the complex interactions that govern the dynamics. Here we address the fundamental question of the modification of the transport of an object induced by its deformation in a viscous flow as studied by Young et al. In this context, we experimentally study the deformation and transport of an isolated elastic fiber in a viscous cellular flow at low Reynolds number, namely a lattice of counter-rotative vortices. We show that the fiber can buckle when approaching a stagnation point. By tuning either the flow or the fiber properties, we measure the onset of this buckling instability. The buckling threshold is determined by the relative intensity of viscous and elastic forces, the elasto-viscous number Sp. We directly compare our experimental results to theoretical predictions by Young et al. Moreover we show that flexible fibers escape faster from a vortex (formed by closed streamlines) compared to rigid ones. As a consequence, the deformation of the fiber changes its transport properties in the cellular flow.

  Vendredi 14 Janvier à 11h, Salle 114

Ben Still (PPRC, Quenn Mary, University of London) : Neutrino oscillations in the T2K experiment

The Tokai to Kamioka neutrino oscillation experiment is the first in a new generation of super beam neutrino experiments. The main aim of the experiment is determination of the third and final lepton mixing angle θ13, through observation of the sub-dominant oscillation νμ → νe. T2K also aims to improve the accuarcy to which the θ23 mixing angle is measured, using νμ → νμ disappearence. The oscillation is a maximum as νμ travel 295 km from the near detec- tors to the far Super-Kamiokande detector. This allows for a sensitive measurement of the maximal nature of the θ23 mixing and maximises the chance of observing the subdominant θ13 mixing. To achieve these goals T2K must understand the systematic uncertain- ties behind the composition of the beam, interaction of the neutrinos and extrapo- lation of this information from near to far detectors.

  Vendredi 17 Décembre à 16h, Salle 114

Aurélien Decelle (LPTMS Orsay) : Inference and learning in Community Structure

Detecting community structure from network topology is a well known problem with many possible applications. A large number of studies was conducted over the last decade, but a principal approach that would for instance output that a random graph does not have any community structure is still missing. In the usual approach finding community is to group nodes of graph together according to a given rule, which is in general that : inside a community there will be more edges than between different communities. Based on a random graph model for a community structure I will first show the existence of a phase transition between possible and impossible community inference. This phase transition is related to some known results from statistical physics of spin glasses, for optimal inference the partition function of a corresponding spin glass model needs to be computed. Then I will turn to real­world networks and inspired by the theoretical results I introduce a new message passing algorithm which is able to learn parameters of the community structure (number of communities, ...), and to infer the most likely community assignment. As an application I will present some results on real­world networks.

  Vendredi 3 Décembre à 11h, Salle 110 du bâtiment 210

Debottam Das (LPT Orsay) : Light pseudoscalars in the NMSSM : Some interesting possibilities

One of the interesting properties of the NMSSM is that it can provide a light ( 10 GeV) pseudoscalar which may lead to exciting phenomenology related to both Higgs hunting and dark matter annihilations. However, such scenarios are challenged by several experimental constraints, especially those arising from the fermionic decays of the pseudoscalar. In this talk, with a prolong introduction, I shall discuss the phenomenology related to light pseudoscalar in some detail, particularly focusing on its contribution to produce the correct relic abundance of the very light neutralino dark matter

  Vendredi 19 Novembre à 11h, salle 22 (rez-de-chaussée du bâtiment 210)

Julien Baglio (LPT Orsay) : One glimpse in the life of the Higgs particle(s)

The Higgs boson is a jester : the key component of the electroweak symmety breaking of the Standard Model and at the origin of the masses, but still hiding to mankind and laughing after us. In this Monkey seminar we will discuss why the Higgs boson is necessary in the Standard Model and present its production and decays at hadron colliders. We will on the road give a theoretical analysis of the uncertainties around these productions and decay rates and finish with a presentation of its supersymmetric brothers at the LHC.

  Vendredi 12 Novembre à 11h, salle 114

MartheTeinturier (LAL Orsay) : L’expérience ATLAS

Depuis le 30 mars dernier, le LHC délivre des collisions pp à 7TeV dans le centre de masse. Avec près de 45 pb-1 de donnés enregistrées, des premiers résultats sortent, passant de la validation (ou non) du Monte Carlo aux premières exclusions dans SUSY. Lors de cette *courte* présentation, après avoir présentée le LHC et ATLAS, je reviendrais sur l’ensemble des résultats obtenus avec les données à 7 TeV.

  Vendredi 22 Octobre à 11h, salle 114

Max Ebbinghaus (LPT Orsay) : Stochastic transport on dynamic network

Intracellular transport along microtubules is bidirectional because molecular motors of different types may move in opposite directions on the filament. Models taking into account the mutual exclusion of particles turn out to be unrealistically inefficient on static networks due to jam formation. Lane changes and particle-particle interactions will be shown not to solve the problem. Instead, we propose another possible mechanism to recover efficient transport : We consider a stochastic lattice gas model with two particle species which includes dynamics of the underlying lattice. This model allows for a transition from a jammed to a homogenous phase in which bidirectional transport is much more efficient than in the static lattice case. In particular, the flux of particles along the lattice becomes density-dependent if the lattice is dynamic enough. This is a necessary condition for a non- vanishing transport capacity in extremely elongated geometries such as axons. Our results are generic for several types of lattice dynamics but rely on a limited lifetime of a lattice site which inhibits the formation of clusters.

  Vendredi 15 Octobre à 10h30, salle 114

Marc Geiller (APC Paris 7) : Quantum gravity in a coconutshell

The search of a satisfactory quantum theory of gravity, that would represent a synthesis of general relativity and quantum mechanics, has been at the center of most research in theoretical physics for a while. The aim of this introductory talk is to first review some conceptual issues related to the definition of a quantum theory of gravity, before explaining the basic foundations of a candidate theory known as loop quantum gravity. Loop quantum gravity is a proposal for a non-perturbative theory of quantum gravity in which no classical background metric is used. We will try to show how this theory leads to a certain notion of quantum space-time geometry, and maybe discuss its application to cosmology, where it leads to an avoidance of the big-bang singularity.

  Vendredi 1er Octobre à 11h, salle 114

Andreas Goudelis (LPT Orsay) : Detection of WIMP-like dark matter in some extensions of the Standard Model

There exists strong evidence witnessing the existence of some large quantity of yet unidentified matter form, dominating the matter content of the universe. Whereas the gravitational evidence for its existence are really strong, we know very little on its true nature. Interestingly, it has been known for a long time that dark matter cannot consist of the usual Standard Model particles : it constitutes one of the few concrete experimental evidence that physics beyond the Standard Model should exist. One of the most popular classes of candidates for dark matter are Weakly Interacting Massive Particles (WIMPs), i.e. particles possessing masses and couplings falling roughly within the electroweak scale. Apart from offering a natural mechanism for explaining dark matter, WIMPs also provide us with a hope that they could be detected through their non-gravitational interactions. In such an eventuality, we shall certainly get closer to the goal of identifying the true nature of dark matter. In this presentation, we shall introduce the basic ways through which we hope to detect dark matter. Then, we shall examine the capacity of some experiments to reconstruct WIMP properties in a model-independent manner. As a second step, we shall analyze the perspectives for dark matter detection in some extensions of the Standard Model. At first, we shall present results on a minimal extension of the SM by a real, singlet, scalar field. Then, we shall investigate two supersymmetric scenarios trying to adress the "little hierarchy problem" of the MSSM : the so-called BMSSM, where the MSSM superpotential is extended with the inclusion of some non-renormalizable operators, and a variant of the MSSM with non-universal higgs masses at the GUT scale leading to a light Higgs boson.