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Theorique d'Orsay

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Agenda > Séminaires / Seminars > Jeunes Dernier ajout : vendredi 3 octobre 2014.

SINJE : Séminaire INformel des JEunes 2009-2010

Le séminaire SINJE (séminaire Jeunes) a généralement lieu le vendredi matin de 11h à 12h, salle 114 du LPT.

-  Vendredi 9 juillet à 11h, Bâtiment 210, salle 114

— Blaise Goutéraux (LPT, Orsay) : Genesis of the concept of black hole

— Antonin Coutant (LPT, Orsay) : Analog black hole in fluid mechanics

— Guillaume Toucas (LPT, Orsay) : Spontaneous breaking of chiral symmetry and the spectrum of light pseudo-scalar mesons

— Joachim Rambeau (LPT, Orsay) : Shuffling a deck of cards

-  Vendredi 18 juin 2010 à 11h, Bâtiment 210, salle 114

Bartjan Van Tent (LPT, Orsay) : The Cosmic Microwave Background

After a reminder of basic cosmology to set the scene, I’ll focus on the cosmic microwave background (CMB) and what its observations can tell us. In particular I’ll introduce cosmic perturbation theory and derive some simple analytical expressions that explain the main features of the CMB power spectrum and their dependence on cosmological parameters. Time permitting I’ll also discuss some more advanced topics like the polarization of the CMB and the CMB bispectrum (non-Gaussianity).

-  Vendredi 11 juin 2010 à 11h, Bâtiment 210, salle 114

Sébastien Descotes-Genon (LPT, Orsay) : A hitchhiker’s guide to the Standard Model - PART 2/2

Le Modèle Standard constitue une référence incontournable en physique des particules, au point que toute activité théorique ou expérimentale dans ce domaine se définit par rapport à lui. Après avoir rappelé certaines notions qui sous-tendent cette construction (théorie quantique des champs, théorie des groupes), je passerai en revue les trois interactions (électromagnétique, forte et faible). Je m’efforcerai de mettre en évidence quelques-uns de ses succès les plus notables, certaine de ses zones d’ombre, et ses limitations.

-  Vendredi 28 mai 2010 à 11h, Bâtiment 210, salle 114

Sofiane Faci (APC, Universite Paris 7 : Conformal Quantization of the massless spin1 on de Sitter space

We quantize the Maxwell (``massless spin one") de Sitter field in a conformally invariant gauge. This quantization is invariant under the SO$_0(2,4)$ group and consequently under the de Sitter group. We obtain a new de Sitter-invariant two-point function which is very simple. Our method relies on the one hand, on a geometrical point of view which uses the realization of Minkowski, de Sitter and anti-de Sitter spaces as intersections of the null cone in $\setR^6$ and a moving plane, and on the other hand, on a canonical quantization scheme of the Gupta-Bleuler type.

-  Vendredi 21 mai 2010 à 11h, Bâtiment 210, salle 114

Sébastien Descotes-Genon (LPT, Orsay) : A hitchhiker’s guide to the Standard Model

Le Modèle Standard constitue une référence incontournable en physique des particules, au point que toute activité théorique ou expérimentale dans ce domaine se définit par rapport à lui. Après avoir rappelé certaines notions qui sous-tendent cette construction (théorie quantique des champs, théorie des groupes), je passerai en revue les trois interactions (électromagnétique, forte et faible). Je m’efforcerai de mettre en évidence quelques-uns de ses succès les plus notables, certaine de ses zones d’ombre, et ses limitations.

-  Mercredi 21 avril 2010 à 14h, Bâtiment 210, salle 114

Christos Charmousis (LPT, Orsay) : Black holes in anti de Sitter spacetime

We will start by discussing anti de Sitter space and its basic properties. We will then study the static adS black holes and its thermodynamics.

-  Vendredi 16 avril 2010 à 11h, Bâtiment 210, salle 114

Elia Zarinelli (LPTMS, Orsay) : Surface tension in Kac glass models

In this talk I will present an introduction to the physics of supercooled liquids. The surface tension is a key ingredient of the mosaic theory for the glass transition. I will show how this quantity can be calculated with a spin-glass model with Kac interactions.

-  Jeudi 18 mars 2010 à 11h, Bâtiment 210, salle 114

Juien Baglio (LPT, Orsay) : Predictions for Higgs production at the Tevatron and the associated uncertainties

The Standard Model crucially relies on the Higgs mechanism, which generates masses to the W/Z bosons and restores unitarity in WW scattering. In the past decades its predictions have successfully passed experimental tests, apart from one key ingredient : the Higgs boson itself. The purpose of this SINJE is to present the update of the theoretical predictions for the production cross sections of the Standard Model Higgs boson at the Fermilab Tevatron collider, focusing on the two main search channels, the gluon-gluon fusion mechanism gg—>H and the Higgs-strahlung processes qqbar —> VH with V = W/Z, including all relevant higher order QCD and electroweak corrections in perturbation theory. The various uncertainties affecting these predictions are then estimated : the scale uncertainties which are viewed as a measure of the unknown higher order effects, the uncertainties from the parton distribution functions and the related errors on the strong coupling constant, as well as the uncertainties due to the use of an effective theory approach in the determination of the radiative corrections in the gg—>H process at next-to- next-to-leading order. We find that while the cross sections are well under control in the Higgs-strahlung processes, the theoretical uncertainties are rather large in the case of the gluon-gluon fusion channel, possibly shifting the central values of the next-to-next-to- leading order cross sections by more than 40%. These uncertainties are thus significantly larger than the 10% error assumed by the CDF and D0 experiments in their recent analysis that have excluded the Higgs mass range MH = 162-166 GeV at the 95% confidence level. These exclusion limits should be, therefore, reconsidered in the light of these large theoretical uncertainties.

-  Vendredi 12 mars 2010 à 11h, Bâtiment 210, salle 114

Emeline Cluzel (IPhT, CEA Saclay) : DBI Inflation

In the last ten years, D-brane inflation has become a time honoured subject which offers an alternative setup for the model of inflation. In a first section, I will recall why the paradigm of inflation was introduced. Then I will introduce brane inflation and explain the basic differences between Dirac-Born-Infeld (DBI) inflation and standard one-field slow-roll inflation. Finally, I will focus on the creation of particules and their backreaction in DBI inflation and put this in perspective with cosmological observations.

-  Vendredi 26 février 2010 à 11h, Bâtiment 210, salle 114

Philippe Doublet (LAL, Orsay) : Designing a detector for a future e-e+ linear collider - Precision measurements based on particle flow & high granularity calorimeters

The upcoming observations at the LHC will trigger the start of a future experiment (an electron-position collider), meant to do precision measurements. Precision measurements will help disentangling several models probed by the LHC. We will show how the expected physics (Higgs, SUSY, extra dimensions,...) gives requirements on a particle detector. Then we will explain why these various final states require the concept of Particle Flow and how it is already applied with new prototypes.

-  Vendredi 19 février 2010 à 14h, Bâtiment 210, salle 114

Lorenzo Battarra (APC, Université Paris 7) : Cosmological singularities in AdS/CFT

Since its discovery in 1997 by Maldacena, the AdS/CFT correspondence has opened an increasingly large number of new research frontiers in fundamental physics. The original conjecture states the exact duality between a string theory (which contains gravity) in AdS (anti De Sitter) curved background and a conformally invariant, supersymmetric (non-gravitational) gauge theory defined on the conformal boundary of this space. The duality is of the strong/weak coupling type, namely it links the ``difficult" regime of the gravitational theory to the perturbative regime of the CFT, and vice-versa. Providing a dual definition of a quantum gravitational theory, AdS/CFT can in principle be used to address the problem of cosmological singularities, for which we expect the classical gravity to fail. The formation of a cosmological singularity is mapped to an instability of the CFT : I will describe an explicit realization of this principle, and the issues related to the possible regularizations of the field theory instability.

-  Jeudi 11 février 2010 à 11h, Bâtiment 210, salle 114

Grégory Schehr (LPT Orsay) : An introduction to first-passage problems in non-equilibrium statistical physics and related questions in the theory of random polynomials

I will present a pedagogical overview of first passage problems in a variety of physical situations ranging from simple random walk to the ordering dynamics of Ising systems or the heat equation with random initial conditions. In particular, I will focus on the so-called persistence $P(t)$, i.e. the probability, for a stochastic process, that it has not changed sign between time $0$ and time $t$. I will shortly describe various analytical tools, including rigorous results as well as mean-field type approximation and field-theoretic techniques, to tackle these problems. I will finally show a connection between the persistence probability for the heat equation and the statistical properties of the real roots of random polynomials.

-  Mercredi 3 février 2010 à 14h, Bâtiment 200, salle 166 - "LSD session"

Freddy Poirier (SERA LAL, Orsay) : Beam dynamics considerations for future e+e- accelerators

The future e+e- colliders (ILC/CLIC, SuperB) with high quality and high intensity beams are projects that require a thorough Research & Development on beam dynamics* for their design. After a quick snapshot of the future colliders and some basic knowledge of accelerator physics, we’ll tackle some of the R&D on positron productions and capture for the various accelerators. The beam dynamics and beam based alignment (BBA) in linear accelerators as well as the use of emittance measurement tools such as the laser-wire will also be discussed.

-  Mercredi 27 janvier 2010, 17h, salle 114

Joachim Rambeau (LPT, Orsay) : Statistics of the maximal value for non-intersecting Brownian paths

The system of non-intersecting Brownian paths has been introduced by Pierre-Gilles de Gennes to study chains of polymers under steric constraints. Later Michael Fisher reviewed in details this problem (that he named ’vicious walkers’) with an emphasis on domain walls produced at a certain kind of phase transition. The maximal value statistics of vicious Brownian paths is in line with the problem of extreme value statistics, and is closely related to random matrix theory and its applications. After a recall of some basic aspects of Brownian motion, I will introduce the path integral methode used to compute the probability density function of the maximal value M and/or the time tau_M to reach it for various Brownian motion processes. First we will look at one Brownian path with periodic boundary conditions. Then we will add a second Brownian path, with the non- intersecting condition. The general result for an arbitrary number of paths will be shown. At this time we will explore the strong connection with random matrix theory, and its applications. (A ’back to basics’ approach will permits non-statisticians to understand a large part -if not the totality- of the talk)

-  Vendredi 22 janvier 2010, 14h, salle 114

Gaetan Borot (IPhT, CEA) : Algebraic geometry applied to enumeration of self avoiding loops

We study a statistical model of self avoiding loops drawn on a random lattice. This is called the O(n) model on the random lattice, and was introduced in 1989 by I. Kostov. It is strongly related to subjects of active research currently. 1. The continuum limit of this model should describe 2d quantum gravity coupled to matter (i.e. 2d gravity + conformal field theory), and the parameter n of the model allows to reach any central charge c =< 1 for the CFT. A lot of work is in progress on (2d gravity + CFT), by CFT techniques, aiming at computing all observables. 2. From the point of view of combinatorics, it is one example of enumerating discrete surfaces carrying structure. 3. This model has a representation as a matrix model, which generalizes the 1-hermitian matrix model. In arxiv :math-ph/0910.5896 with B. Eynard, we showed that the technology (based on algebraic geometry) available to solve matrix models extends completely to the O(n) model. So, the purpose of this talk is to give a big picture of these methods, which also arise in many other topics in theoretical physics, and to show them at work for the O(n) model. Matrix models will be left aside in this presentation.

-  Vendredi 8 janvier 2010 à 11h, salle 114

Antonin Coutant (LPT, Orsay) : Black holes and causality

La relativité générale d’Einstein permet de décrire avec précision la gravitation. Pour cela, l’espace-temps lui-même devient un objet dynamique. Sa géométrie encode alors les interactions gravitationnelles. Mais il faut alors renoncer à l’interprétation usuel du concept de coordonnées. Celles-ci perdent en effet leur sens physique. Il est alors important de comprendre quelle sont les observables physique, et comment décrire des phénomènes/systèmes de façon indépendante d’un système de coordonnées. Nous verrons que la géométrie d’un espace temps possède une structure causale intrinsèque, c’est-à-dire un ensemble de mouvement possibles ou non tenant compte de la contrainte que la vitesse de la lumière est une limite absolue. La compréhension de cette structure nous amène à définir les transformations conformes et diagrammes de Penrose. Nous pourrons alors définir de façon naturelle les notions d’horizons et trou noirs. Cette définition indépendante du jeu de coordonnée permet d’obtenir des résultats très généraux sur la physique des trous noirs.

-  Vendredi 11 décembre à11h, salle 110

Jean-Marie Stephan (IPhT, CEA) : Entanglement entropy, quantum dimers, and exotic phases of matter

There has been growing interest in studying phases of matter that cannot be described by the two cornerstones of condensed matter theory : Fermi liquid and Landau symmetry-breaking theories. One example is provided by the fractional quantum Hall effect, which exhibits a new kind of order, called topological order. In this talk we will focus on a simple toy model ("quantum dimers") to gain insight on this topic. We will also introduce entanglement entropy, and show that this tool can help to characterize these exotic phases. Finally, we will discuss possible applications to quantum computing.

-  Vendredi 4 décembre à14h, salle 110

Flavien Kiefer (IAP) : Colliding D-brane and anti-D-brane system

A popular class of inflationary models inspired by string theory, such as the KKLMMT setup, makes use of the separation between a D-brane and an anti D-brane, which attracts each other, as an inflation field. Graceful exit from inflation and preheating are then obtained by brane/anti-brane annihilation into closed string modes. It is driven by the process known as open string tachyon condensation, which is not fully understood. We first describe the system in three different regimes, for which the brane/anti-brane attraction has different,yet complementary, origins. Then we study the classical dynamics of the system below the critical distance where the would-be tachyon becomes massless. The combined dynamics of the brane separation and the tachyon is obtained from a non-linear Born-Infeld type Lagrangian. We show that, for arbitrary initial conditions on the initial separation and on the relative velocity, tachyon condensation does not generically happen. Fortunately, coupling the system to four-dimensional gravity in a FRW universe, Hubble friction seems to force tachyon condensation to happen, whatever the initial conditions are.

-  Séminaire reporté à une date ultérieure

Hélène Granclaude (IPhT, CEA Saclay) : Fluctuations and extreme-value statistics in growing networks

Models of networks are relevant to describe many real systems, such as the Internet, social phenomena, energy landscapes or biological systems. Many of these systems exhibit a topological scale invariance, where few highly connected nodes (hubs) dominate the topology of the networks. Growing networks with a preferential attachment have received considerable interest, as they provide a natural explanation for the observed scalefreeness. We are interested in understanding how such networks evolve in time, and how their most connected nodes behave.

-  Vendredi 23 octobre à 11h, salle 114

Mical Was (LAL) : Searching for Gravitational Waves associated with Gamma Ray Bursts

Gravitational waves have been experimentally sought for the past 40 years. Current interferometric detectors are on the verge of detection and are able to establish relevant limits on astrophysical events. After a short introduction on Gravitational Wave physics and observational challenges. I will present present the search for GWs associated with Gamma Ray Bursts, the most energetic explosions known in the universe after the Big Bang.

-  Vendredi 16 octobre à 11h, salle 114

Cédric Tournier-Colletta (Institut Jean Lamour, Nancy) : Electron-electron and electron-phonon interactions at surface

Many crystals exhibit electronic states localized at the surface. These surface states result from the breaking of translation symmetry in the perpendicular direction. In this talk, I will present two experimental techniques, angle-resolved photoemission (ARPES) and scanning tunneling spectroscopy (STS), which are perfect to study these surface states. From a theoretical point of view, in order to understand precisely the measurements, one has to go beyond the “one-electron” approach and use “many-body” theory. Electrons are indeed interacting with each other, but also with other degrees of freedom, e.g. phonons. As a firt example, I will present some results about the Ag(111) surface state. In this case, interaction effects are quite small allowing the use of Fermi liquid theory. Then we will look at the K/Si(111) sqrt3 x sqrt3 case, where electron-phonon interaction is very strong, leading to the bipolaronic quantum phase transition.

-  Vendredi 9 octobre à 11h, salle 110

Céline Nadal (LPTMS) : Non-intersecting Brownian interfaces and Wishart random matrices

The system of N nonintersecting elastic lines was first studied by De Gennes. These lines can be viewed as the trajectories in time of N nonintersecting Brownian motions or random walkers, called "vicious walkers" by Fisher. We have studied a system of N nonintersecting fluctuating elastic interfaces at thermal equilibrium, in presence of substrate inducing an additional potential for each interface. For a large system and with an appropriate choice of the external confining potential, the joint distribution of the heights of the N nonintersecting interfaces at a fixed point on the substrate can be mapped to the joint distribution of the eigenvalues of a Wishart matrix of size N with complex entries, thus providing a physical realization of the Wishart matrix. Exploiting this analogy to random matrix, we could calculate analytically some statistical properties of the model (as the distribution of the height of the upper/lower interface and of the center of mass).

-  Vendredi 2 octobre à 11h, salle 114

Andrey Tayduganov (LPT Orsay/LAL) : An introduction to effective Hamiltonians and rare B-decays

The basic starting point for any phenomenology of weak decays of hadrons is the effective weak Hamiltonian which is simply a series of effective vertices (local operators) multiplied by effective coupling constants.This series is known as the operator product expansion (OPE). The essential point of OPE is that it allows to separate the problem of calculating the amplitude into two distinct parts : the short distance (perturbative) calculation of the couplings and the long-distance (non-perturbative) calculation of the matrix elements of the local operators. An interesting example of an effective weak Hamiltonian application are flavor-changing neutral current (FCNC) transitions in radiative and semileptonic B-decays. Such interactions are absent at tree level in the Standard Model, while they can be mediated through loop processes, what provides a window for New Physics beyond the SM searches and makes them interesting to study.

-  Mercredi 23 septembre à 11h, salle 114

Max Ebbinghaus (Universität Des Saarlandes/LPT Orsay) : Geometrical aspects and aging in microtubule dynamics

Microtubules, a semi-flexible polymer, are a major component of the cytoskeleton in an eucaryotic cell. They exhibit a characteristic behavior termed "dynamic instability" which refers to the rapid switching between a polymerizing and a depolymerizing state. In this talk, a model for microtubule dynamics will be presented, aimed at explaining some of the experimentally observed properties of the microtubule network in living cells. The focus of the model lies on the influence of the cell geometry on the growth of microtubules as well as on the interaction with proteins that bind to microtubules and have an important effect on the microtubule dynamics. Within this model, we observe aging behavior which would allow for further experimental verification of the association mechanism of proteins tracking the plus ends of growing microtubules.