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 Séminaires à venir [30 suivants] [ vue agenda ]

 Mercredi 22 Novembre 2017, 11:00 à LPTHE, Bibliotheque du LPTHE, tour 13-14, 4eme etage SEM-EXCEP (Séminaire exceptionel) cond-mat Philip Phillips ( University of Illinois ) Anomalous Dimensions for Conserved Currents as a loop-hole in Noether's Theorems Abstract: For the past 30 years, the transport properties in the unusual metallic phase seen in the cuprate superconductors and many other quantum critical metals, have defied an explanation in terms of the standard building blocks of modern physics --- particles with local interactions and conservation laws. A recent proposal suggests that all of the properties of such strange metals' can be understood if the current has an anomalous dimension not determined simply by dimensional analysis. My talk will focus on trying to understand this claim. To demystify this claim, I will first show that even in the standard formulation of gauge theories, there is a loop-hole in Noether's theorems which has remained almost' (Noether was aware of it) un-noticed until now, that can allow, in principle, for the current to have any allowable dimension. The only quantum theories to date which exhibit such odd behaviour are bulk diatonic models. The corresponding boundary theory is inherently non-local as dictated by the only possible exception to Noether's theorms. The existence of currents having anomalous dimensions, a direct probe of the existence of extra hidden' dimensions, can be tested with the Aharonov-Bohm effect. I will describe this effect and its potential impact for unlocking the secret of the strange metal in the cuprates.  I will also construct the Virasoro algebra for such fractional currents and discuss the general implications for the bulk-boundary construction in holography.

 Mercredi 22 Novembre 2017, 11:00 à IHES, Amphithéâtre Léon Motchane MATH-IHES (TBA) hep-th Alen Tosenberger ( Université Libre de Bruxelles & IHES ) Mathematical modeling of cell lineage specification in mouse embryo development Abstract: Embryonic development is a self-organized process during which cells divide, interact, change fate according to a complex gene regulatory network and organize themselves in a three-dimensional (3D) space. Here, we model this complex dynamic phenomenon in the context of the acquisition of epiblast (Epi) and primitive endoderm (PrE) identities within the inner cell mass (ICM) of the preimplantation embryo in the mouse. The multiscale model describes cell division and biomechanical interactions between cells, as well as biochemical reactions inside each individual cell and in the extracellular matrix. We use the model to study the Epi and PrE lineage development and the appearance of a so-called salt-and-pepper pattern which the two lineages form.

 Mercredi 22 Novembre 2017, 13:45 à LKB, Collège de France, salle 2 SEM-PHYS-ENS (Colloquium du Département de Physique de l'ENS) quant-ph Dmitry Petrov ( LPTMS, Univ Paris-Sud ) Mesoscopic few-body problem with short-range interactions Abstract: I will describe our work with Betzalel Bazak on the N+1-body fermionic problem in three dimensions [1] and N-boson problem in two dimensions [2]. By developing a new method of solving few-body integral equations we are able to obtain precise results for ground state energies. In particular, we predict a universal pentamer state and five-body Efimov effect for the 4+1 fermionic problem and quantify the few-to-many-body crossover for two-dimensional bosons. I point to Refs. [3] and [4] as a recommended reading. [1] B. Bazak and D.S. Petrov, Five-body Efimov effect and universal pentamer in fermionic mixtures'', Phys. Rev. Lett. 118, 083002 (2017) [2] B. Bazak and D.S. Petrov, Energy of N two-dimensional bosons with zero-range interactions'', arXiv:1711.02345 [3] Y. Castin, C. Mora, and L. Pricoupenko, Four-Body Efimov Effect for Three Fermions and a Lighter Particle'', Phys. Rev. Lett. 105, 223201 (2010) [4] H.-W. Hammer and D. T. Son, Universal Properties of Two-Dimensional Boson Droplets'', Phys. Rev. Lett. 93, 250408 (2004)

 Mercredi 22 Novembre 2017, 14:00 à IHES, Amphithéâtre Léon Motchane ( Cours de l'IHES ) MATH-IHES (TBA) hep-th Vincent Vargas ( ENS, Paris ) Liouville conformal field theory and the DOZZ formula (1/4) Abstract: Liouville conformal field theory (LCFT hereafter), introduced by Polyakov in his 1981 seminal work "Quantum geometry of bosonic strings", can be seen as a random version of the theory of Riemann surfaces. LCFT appears in Polyakov's work as a 2d version of the Feynman path integral with an exponential interaction term. Since then, LCFT has emerged in a wide variety of contexts in the physics literature and in particular recently in relation with 4d supersymmetric gauge theories (via the AGT conjecture). A major issue in theoretical physics was to solve the theory, namely compute the correlation functions. In this direction, an intriguing formula for the three point correlations of LCFT was proposed in the middle of the 90's by Dorn-Otto and Zamolodchikov-Zamolodchikov, the celebrated DOZZ formula. The purpose of the course is twofold (based on joint works with F. David, A. Kupiainen and R. Rhodes). First, I will present a rigorous probabilistic construction of Polyakov's path integral formulation of LCFT. The construction is based on the Gaussian Free Field. Second, I will show that the three point correlation functions of the probabilistic construction indeed satisfy the DOZZ formula. This establishes an explicit link between probability theory (or statistical physics) and the so-called conformal bootstrap approach of LCFT.

 Mercredi 22 Novembre 2017, 14:00 à LPTM, 4.13 St Martin II ( Séminaire déplacé au mercredi 22 novembre ) SEM-LPTM-UCP (Seminaires du LPTM , Universite de Cergy Pontoise) math-ph Hongjie Bi ( LPTM UCP Cergy Pontoise ) Collective behavior in frequency weighted coupled oscillators and in adaptively coupled oscillators. Abstract: We report on a novel collective state, occurring in globally coupled nonidentical oscillators in the proximity of the point where the transition from the system’s incoherent to coherent phase converts from explosive to continuous. In such a state, the oscillators form quantized clusters, where neither their phases nor their instantaneous frequencies are locked. The oscillators’ instantaneous speeds are different within the clusters, but they form a characteristic cusped pattern and, more importantly, they behave periodically in time so that their average values are the same. Given its intrinsic specular nature with respect to the recently introduced Chimera states, the phase is termed the Bellerophon state. We provide an analytical and numerical description of Bellerophon states, and furnish practical hints on how to seek them in a variety of experimental and natural systems.

 Mercredi 22 Novembre 2017, 14:15 à IPHT, Salle Claude Itzykson, Bât. 774 IPHT-MAT (Séminaire de matrices, cordes et géométries aléatoires) hep-th Dan Israel ( Jussieu ) K3 automorphisms and non-geometric compactifications Abstract: I will present an interesting class of non-geometric string compactifications from three different perspectives: mirrored automorphisms of K3 surfaces, asymmetric Gepner models and gauged supergravity.

 Mercredi 22 Novembre 2017, 16:30 à IHES, Amphithéâtre Léon Motchane ( Séminaire de probabilités et physique statistique de l’IHES ) MATH-IHES (TBA) hep-th Ben Wallace ( IST ) Two-point function of O(n) models below the critical dimension Abstract: We will discuss the asymptotic behaviour of the critical two-point function for a long-range version of the n-component $|varphi|^4$ model and the weakly self-avoiding walk (WSAW) on the d-dimensional Euclidean lattice with d=1,2,3. The WSAW corresponds to the case n=0 via a supersymmetric integral representation. We choose the range of the interaction so that the upper-critical dimension of both models is $d+epsilon$. Our main result is that, for small $epsilon$ and small coupling strength, the critical two-point function exhibits mean-field decay, confirming a prediction of Fisher, Ma, and Nickel. The proof makes use of a renormalisation group method of Bauerschmidt, Brydges, and Slade, as well as a cluster expansion. This is joint work with Martin Lohmann and Gordon Slade.

 Jeudi 23 Novembre 2017, 11:00 à CPHT, Salle de conférences, bât. 6 IPN-X (Séminaire commun de physique théorique des particules IPN-CPHT-X) hep-ph Marcela Pelaez ( Republica U., Montevideo ) Small parameters in infrared QCD Abstract: Lattice simulations show that the running coupling constants obtained through QCD vertices differ in the infrared even though their bare values coincide. For instance, at low momenta the strength of the quark-gluon coupling is about twice the ghost-gluon coupling. None of them diverge in the infrared as it is predicted by standard perturbation theory. Moreover, the ghost-gluon coupling remains moderate even in the infrared. This observation motivates the use of perturbation theory in the ghost-gluon sector in the frame of a massive deformation of QCD Lagrangian in Landau gauge. However, perturbation theory in the quark sector within this massive Lagrangian doesn´t bring as good results as in the pure Yang-Mills case, as should be expected from the larger value of the quark-gluon coupling constant. We propose a controlled systematic expansion in full QCD based in two small parameters: first the Yang.Mills sector couplings and second the inverse of the number of colors (large-Nc limit). The second approximation relies on the fact that the general features of QCD can be observed in the large-Nc limit. This systematic expansion allows us to properly introduced the use of the renormalization group for the rainbow resummation. At leading order, this double expansion leads to the well-known rainbow approximation for the quark propagator whose solution shows spontaneous chiral symmetry breaking for sufficiently large quark-gluon coupling constant.

 Jeudi 23 Novembre 2017, 11:00 à LPTHE, bibliothèque SEM-DARBOUX (Séminaire Darboux - physique théorique et mathématiques) hep-th|math|math.MP Ilia Itenberg ( IMJ-PRG ) Tropical geometry and enumeration of curves Abstract: The main goal of the talk is to give an introduction to tropical geometry making an emphasis on the tropical approach to enumeration of algebraic curves in the plane. This approach, based on Grigory Mikhalkin's correspondence theorem, reduces many enumerative problems of Gromov-Witten type (complex or real) to a count of certain rectilinear graphs that are called tropical curves. We discuss enumeration of tropical curves with different multiplicities, in particular, with the polynomial multiplicities proposed recently by Florian Block and Lothar Göttsche; the enumeration with the help of these multiplicities can be interpreted as quantization of the number of complex solutions in the enumerative problems under consideration.

 Jeudi 23 Novembre 2017, 14:00 à LPT, salle 110, salle cosmologie, 1er etage, bat 210, lpt orsay SEM-LPT (Séminaire de Physique des Particules du LPT) hep-ph Gabriel Jung ( LPT Orsay ) Non-Gaussianity in two-field inflation beyond the slow-roll approximation Abstract: In this talk, I will discuss the level of bispectral non-Gaussianity produced in two-field inflation models with standard kinetic terms. Even though the Planck satellite has so far not detected any primordial non-Gaussianity, it has tightened the constraints significantly, and it is important to better understand what regions of inflation model space have been ruled out, as well as prepare for the next generation of experiments that might reach the important milestone of Delta f_NL(local) = 1. I will present our results on the non-Gaussianity parameter f_NL in the case of sum potentials and show that it is very difficult to satisfy simultaneously the conditions for a large f_NL and the observational constraints on the spectral index n_s in the slow-roll approximation. I will then discuss the case of monomial potentials and show explicitly the small region of parameter space in which this is possible. Finally, I will extend these results beyond the slow-roll approximation and illustrate them with two explicit inflation models.

 Jeudi 23 Novembre 2017, 14:30 à LPTENS, LPTENS Library STRINT (Strings, integrability and beyond) hep-th|math-ph Yunfeng Jiang ( ETH Zurich ) Algebraic geometry and Bethe ansatz Abstract: In this talk, I will discuss how to apply methods of modern computational algebraic geometry to Bethe ansatz. I will show that algebraic geometry provides natural mathematical languages and powerful tools to understand the structure of solutions of Bethe ansatz equations (BAE). In particular, I will present new methods to count the number of physical solutions with fixed quantum numbers based on Gr\"obner basis and quotient ring. This method can be applied to study the completeness of Bethe ansatz. I will also discuss an analytical method to compute the sum of on-shell physical quantities over all physical solutions without explicitly solving BAE. This method has important applications in calculating the sum rules of OPE coefficients in $\mathcal{N}=4$ super-Yang-Mills theory.

 Jeudi 23 Novembre 2017, 14:30 à IHES, Amphithéâtre Léon Motchane MATH-IHES (TBA) hep-th Petr Grinevich ( L.D.Landau Institute for Theoretical Physics & IHES ) Differential operators with meromorphic eigenfunctions and indefinite scalar products (Based on joint works with S.P. Novikov and R.G. Novikov) Abstract: Meromorphic solutions of soliton equations usually do not fit in the standard spectral transform scheme. We show, that the spectral theory for the corresponding linear problems should be formulated in terms of Pontrjagin spaces - pseudo-Hilbert spaces with a finite number of negative squares. This observation uses the following property: all eigenfucntions of these linear operators with special singularities are meromorphic for all values of spectral parameter. We also discuss a two-dimensional analog of this property.

 Jeudi 23 Novembre 2017, 16:00 à LPT, 114 LPT-PTH (Particle Theory Seminar of LPT Orsay) hep-ph Roman Zwicky ( Edinburgh University ) On RG-flow Theorems in d=4 Abstract: I review, pedagogically, renormalisation group flow theorems in 4D making the connection to Zamalodchikov’s famous c-theorem in 2D. I’ll discuss the well-established a-theorem (Euler term), from the viewpoint of the Komargodski-Schwimmer dilaton effective action approach, and then report on some work in progress on the flow of the $\Box R$-term.

 Vendredi 24 Novembre 2017, 10:00 à IPHT, Salle Claude Itzykson, Bât. 774 ( https://courses.ipht.cnrs.fr/?q=fr/node/194 ) COURS (Cours) cond-mat Pierfrancesco Urbani ( IPhT ) Statistical physics of glassy systems: tools and applications (3/6) Abstract: The complex behavior of a large variety of systems can often be ascribed to the competition of many quasi-optimal equilibria. In these cases metastability deeply affects both the structural and dynamical properties. Glasses are the prototype of such systems, with glassy behavior arising not only in condensed matter but also in a wide variety of fields ranging from optimization to computer science. \par In these lectures I will introduce different kinds of glassy systems and I will describe their fundamental similarities, which emerge from their statistical description. Then I will focus on a representative model, the random perceptron, and I will discuss the main techniques that can be used to solve it. \par These techniques provide a versatile theoretical toolbox that can be applied to several problems such as for example the physics of the jamming transition, its interpretation in the context of constraint satisfaction, as well as the design of new efficient algorithms to solve statistical problems. \\ 1.Introduction to glassy systems: structural glasses and constraint satisfaction problems. \\ 2.The random perceptron model. The replica approach. The replica symmetric solution and phase diagram. \\ 3.Replica symmetry breaking and marginal stability. The full RSB solution of the random perceptron. \\ 4.The jamming transition as a satisfiability threshold; critical exponents. \\ 5.Belief propagation approach and the algorithmic version of the Thouless-Anderson-Palmer equations. \\ Supported by Investissements d'Avenir'' LabEx PALM (ANR-10-LABX-0039-PALM) Fichiers attachés: 2017-2018.pdf (4503344 bytes) 2017_Urbani.pdf (5052625 bytes)

 Vendredi 24 Novembre 2017, 10:30 à IHES, Amphithéâtre Léon Motchane ( Cours de l'IHES ) MATH-IHES (TBA) hep-th Vincent Vargas ( ENS, Paris ) Liouville conformal field theory and the DOZZ formula (2/4) Abstract: Liouville conformal field theory (LCFT hereafter), introduced by Polyakov in his 1981 seminal work "Quantum geometry of bosonic strings", can be seen as a random version of the theory of Riemann surfaces. LCFT appears in Polyakov's work as a 2d version of the Feynman path integral with an exponential interaction term. Since then, LCFT has emerged in a wide variety of contexts in the physics literature and in particular recently in relation with 4d supersymmetric gauge theories (via the AGT conjecture). A major issue in theoretical physics was to solve the theory, namely compute the correlation functions. In this direction, an intriguing formula for the three point correlations of LCFT was proposed in the middle of the 90's by Dorn-Otto and Zamolodchikov-Zamolodchikov, the celebrated DOZZ formula. The purpose of the course is twofold (based on joint works with F. David, A. Kupiainen and R. Rhodes). First, I will present a rigorous probabilistic construction of Polyakov's path integral formulation of LCFT. The construction is based on the Gaussian Free Field. Second, I will show that the three point correlation functions of the probabilistic construction indeed satisfy the DOZZ formula. This establishes an explicit link between probability theory (or statistical physics) and the so-called conformal bootstrap approach of LCFT.

 Vendredi 24 Novembre 2017, 14:00 à LPTHE, Library LPTHE-PPH (Particle Physics at LPTHE) hep-ph Giacomo Cacciapaglia ( IPNL ) Light (pseudo)scalars in composite Higgs models Abstract: The possibility that the Higgs boson is a composite state of an underlying confining dynamics is still open. After a brief recap of the main features and properties of this class of models, I will discuss the reasons why we expect the presence of light (pseudo)scalars as a generic prediction of composite Higgs models. Their phenomenology at the LHC will also be touched upon.

 Lundi 27 Novembre 2017, 11:00 à IPHT, Salle Claude Itzykson, Bât. 774 IPHT-PHM (Séminaire de physique mathématique) math-ph Aurélien Grabsch Truncated linear statistics of eigenvalues Abstract: Since the pioneer work of Wigner, random matrix theory have been applied to many fields in physics. Invariant ensembles have played a prominent role in physical applications, when eigenvalues and eigenvectors are uncorrelated. Many important physical observables take the form of linear statistics of eigenvalues $\{\lambda_i\}_{i=1,\ldots,N}$, i.e. $L = \sum_{i=1}^N f(\lambda_i)$, where $N$ is the total number of eigenvalues and $f$ is any given function depending on the physical situation under consideration. We have recently introduced a new type of problem: motivated by the analysis of the statistical physics of fluctuating one-dimensional interfaces, we have studied the distribution of \textit{truncated} linear statistics $\tilde{L}=\sum_{i=1}^K f(\lambda_i)$, where the summation is restricted to number $K < N$ of the eigenvalues. In this talk, I will analyse two cases: first the case where the linear statistics is restricted to the largest (or smallest) $K$ eigenvalues. Then, I will discuss the case where any eigenvalue can contribute to the truncated linear statistics, without restriction on the ordering. I will show that, in a certain regime, this last problem can be mapped onto a system of non-interacting fermions which can have either positive or negative temperature. \\ \\ Refs: \\ - Truncated linear statistics associated with the top eigenvalues of random matrices. Aurélien Grabsch, Satya N. Majumdar and Christophe Texier. J. Stat. Phys. 167(2), 234-259 (2017) \\ - Truncated linear statistics associated with the eigenvalues of random matrices II. Partial sums over proper time delays for chaotic quantum dots. Aurélien Grabsch, Satya N. Majumdar and Christophe Texier. J. Stat. Phys. 167(2) 1452-1488 (2017)

 Lundi 27 Novembre 2017, 11:00 à LPTMC, tower 12-13 room 5-23 SEM-LPTMC (Séminaire du Laboratoire de Physique Théorique de la Matière Condensée) cond-mat.mes-hall Andrej Mesaros ( LPS Orsay ) Fractionalized particles on defects in topological insulators and superconductors Abstract: Recent experiments on one-dimensional and two-dimensional materials have been very successful in realizing topological states of electrons, with one of ultimate goals being the creation of emergent Majorana particles. Such emergent particles have fractional charge, spin, and/or quantum statistics, so they are interesting both fundamentally and for applications, but remain hard to realize. We will discuss the use of intrinsic and extrinsic topological defects as a way to realize fractional particles. By combining analytic and numerical approaches we predict Majorana particles, having favorable energetic properties, in certain vortices of two-dimensional superconductors consistent with recent experiments at INSP. We will also focus on lattice dislocations in topological insulators as realizations of more complex fractional particles. General properties of fractionalized particles will also be discussed.

 Lundi 27 Novembre 2017, 14:00 à IPHT, Salle Claude Itzykson, Bât. 774 IPHT-STA (Séminaire de Physique Statistique, CEA/Saclay) cond-mat Igor B. Mekhov ( Univ. Paris-Saclay (CEA SPEC), St. Petersburg State Univ., Univ. of Oxford ) Weak measurements and quantum optical lattices for strongly correlated bosons and fermions Abstract: While optical lattices are well-established systems, the quantum nature of light is neglected in all setups so far. We show theoretically that the light quantization significantly broadens the physical picture enabling to go beyond effects expected in many-body dissipative systems. We prove that the quantum backaction of weak global measurement constitutes a novel source of competitions in many-body systems (in addition to atom tunnelling and interactions) [1,2], leading to novel effects: multimode oscillations of macroscopic superposition states, protection and break-up of fermion pairs [2], as well as generation of antiferromagnetic states [3]. Novel processes beyond the standard Hubbard models can be designed by the measurement, entering the field of non-Hermitian many-body physics: long-range correlated pair tunnelling and Raman-like second-order transitions beyond the typical quantum Zeno dynamics [2]. We demonstrate the generation of multipartite mode entanglement and feedback control of many-body states [4]. The quantization of optical lattice potentials enables quantum simulations of various long-range interacting systems unobtainable using classical optical lattices [5]. This leads to new quantum phases (dimers, trimers, etc. of matter waves similar to valence bond solids) beyond density orders (e.g. supersolids and density waves) directly benefiting from the collective light-matter interaction. [1] W. Kozlowski, S. F. Caballero-Benitez, and I. B. Mekhov, Scientific Rep. 7, 42597 (2017)

 Mardi 28 Novembre 2017, 14:00 à LPTMC, tower 22-23 room 3-17 ( 2ème partie d'un mini cours en 2 séances de 2h (de 10h à 12h) ) COURS (Cours) cond-mat.mes-hall Benoît Douçot ( LPTHE ) Introduction à la classification des isolants et supraconducteurs topologiques Abstract: Peu de temps après la découverte fondamentale par Kane et Mele en 2005 de modèles réalistes d’isolants topologiques invariants par symétrie de renversement du temps, et la confirmation expérimentale de l’existence de la phase d’effet Hall quantique de spin par Molenkamp et collaborateurs, est apparue une impressionnante classification des isolants et supraconducteurs topologiques pour des Hamiltoniens quadratiques de fermions. Cette classification résulte de la confluence remarquable entre deux domaines de recherche a priori très éloignés. Le premier est l’étude de la localisation d’ Anderson due au désordre en présence de symétries discrètes, comme le renversement du temps, éventuellement étendue au cas supraconducteur. En 1996, M. Zirnbauer a montré qu’il existe 10 classes de symétries discrètes possibles pour de tels systèmes. En suivant ce fil conducteur,Schnyder, Ryu, Furasaki et Ludwig ont identifié en 2008 lesquelles de ces classes, en fonction de la dimension de l’espace, permettent de stabiliser des états de bord échappant à la localisation d’Anderson. Le deuxième domaine de recherche impliqué est l’étude de la topologie des états de Bloch pour des systèmes invariants par translation. En 2009, A. Kitaev a compris comment incorporer les contraintes provenant des symétries discrètes sur cette topologie, et il a abouti à une classification identique à la précédente ! Le but de ces deux cours est de donner un aperçu de cette classification, d’expliquer certaines des idées mathématiques sous-jacentes, et de montrer comment elle peut être utilisée. Un prérequis utile (mais non nécessaire) est d’avoir suivi les cours récents de Jean-Noël Fuchs et Tristan Cren sur les isolants et supraconducteurs topologiques.

 Mardi 28 Novembre 2017, 14:00 à APC, 483 A - Malevitch APC-TH (Seminar of the theory group of APC) hep-th Elias Kiritsis ( APC, Université Paris Diderot ) Scale invariant Lifshitz hydrodynamics and Torsional Newton-Cartan geometry Abstract: Hydrodynamics is the generic low energy theory that describes fluids and generalizes thermodynamics. There are however quantum field theories, whose  hydrodynamics is not known. A large class are non-relativistic theories with Lifshitz scaling (appearing in condensed matter systems). The gauge/gravity correspondence has provided a novel and precise correspondence between fluid dynamics and the gravitational physics of horizons. We use this to find the hydrodynamic equations that describe non- relativistic quantum field theories with Lifshitz scaling and hyperscaling violation. We also find the (torsional) Newton-Cartan gravity plays an important role.

 Mardi 28 Novembre 2017, 16:00 à IPHT, Salle Claude Itzykson, Bât. 774 IPHT-HEP (Séminaire de physique des particules et de cosmologie) hep-ph Nicolas Wschebor ( Universidad de la Republica de Uruguay, Montevideo ) Proving conformal invariance in critical scalar theories in any dimension Abstract: Conformal invariance in three dimension has a tremendous renewed interest due to the surprisingly good results obtained by using the "conformal bootstrap" in last five years. In this talk, the interest of this symmetry is reviewed and its existence in critical (scale invariant) theories in any dimension is discussed. In particular, using Wilson renormalization group, we show that if no integrated vector operator of scaling dimension -1 exists in a given model, then scale invariance implies conformal invariance. By using the Lebowitz inequalities, we prove that this necessary condition (or another similar necessary condition proposed by Polchinski many years ago) is fulfilled in all dimensions less than four for the Ising universality class. This shows, in particular, that scale invariance implies conformal invariance for the three-dimensional Ising model. Finally, the extension of this result to other critical systems is discussed.

 Mardi 28 Novembre 2017, 17:15 à DPT-PHYS-ENS, Salle Jean Jaurès (29 rue d'Ulm) SEM-PHYS-ENS (Colloquium du Département de Physique de l'ENS) physics.optics John Dudley ( Université de Franche Comté ) Extreme events in nature, rogue wave in optics Abstract: Understanding extreme events in nature is intrinsically challenging because the events themselves are rare, and often appear in environments where measurements are difficult. A particular case of interest concerns the infamous oceanic rogue or freak waves that have been associated with many catastrophic maritime disasters. Studying such rogue waves is problematic, and the phenomena remain very poorly understood. On the other hand, significant experiments have been reported in optics in recent years, where advanced measurement techniques have been used to quantify the appearance of extreme localised optical fields that have been termed "optical rogue waves". In fact, there is a rigorous analogy between the physics of wave propagation on the ocean and light pulse propagation in optical fibre, and this has opened up possibilities to explore general properties of extreme value dynamics using a convenient benchtop optical environment. The purpose of this talk will be to review these results in optics and discuss the impact they have had on oceanography. The talk will provide suitable introduction to aspects of ocean physics and optics, and will be accessible to non-specialists.

 Mercredi 29 Novembre 2017, 11:30 à LPTENS, LPTENS library STR-LPT-ENS-HE (Séminaire commun LPTENS/LPTHE) hep-th Sofian Teber ( LPTHE ) Critical behavior of (2+1)-dimensional QED Abstract: The talk will focus on dynamical chiral symmetry breaking (DCSB) in three-dimensional QED with N four-component fermions. The interest in QED3 comes from its similarities to (3+1)-dimensional QCD and the fact that DCSB may be studied systematically in this simpler model. Additionally, QED3 is often used as an effective field theory model describing planar condensed matter physics systems. In this talk, we shall present recent results related to the computation of the critical fermion flavor number, Nc, which is such that DCSB takes place for N < Nc. The method envolves solving the Schwinger-Dyson gap equation in an arbitrary nonlocal gauge up to next-to-leading order in the 1/N-expansion. Our results suggest that DCSB should take place for integer values N smaller or equal to 3. If times allows, a new mapping between QED3 and the so-called reduced QED, describing planar Dirac liquids at the infra-red Lorentz invariant fixed point, will be presented together with new results on dynamical gap generation in these liquids.

 Mercredi 29 Novembre 2017, 14:00 à IHES, Amphithéâtre Léon Motchane MATH-IHES (TBA) hep-th Vincent Vargas ( ENS, Paris ) Liouville conformal field theory and the DOZZ formula (3/4) Abstract: Liouville conformal field theory (LCFT hereafter), introduced by Polyakov in his 1981 seminal work "Quantum geometry of bosonic strings", can be seen as a random version of the theory of Riemann surfaces. LCFT appears in Polyakov's work as a 2d version of the Feynman path integral with an exponential interaction term. Since then, LCFT has emerged in a wide variety of contexts in the physics literature and in particular recently in relation with 4d supersymmetric gauge theories (via the AGT conjecture). A major issue in theoretical physics was to solve the theory, namely compute the correlation functions. In this direction, an intriguing formula for the three point correlations of LCFT was proposed in the middle of the 90's by Dorn-Otto and Zamolodchikov-Zamolodchikov, the celebrated DOZZ formula. The purpose of the course is twofold (based on joint works with F. David, A. Kupiainen and R. Rhodes). First, I will present a rigorous probabilistic construction of Polyakov's path integral formulation of LCFT. The construction is based on the Gaussian Free Field. Second, I will show that the three point correlation functions of the probabilistic construction indeed satisfy the DOZZ formula. This establishes an explicit link between probability theory (or statistical physics) and the so-called conformal bootstrap approach of LCFT.

 Mercredi 29 Novembre 2017, 14:15 à IPHT, Salle Claude Itzykson, Bât. 774 IPHT-MAT (Séminaire de matrices, cordes et géométries aléatoires) hep-th Swapnamay Mondal ( Jussieu ) SYK like tensor models for black hole probe Abstract: Infrared dynamics of SYK model (and tensor models with similar large N physics), exhibit some of the non trivial features of near extremal black holes, such as maximal chaos and nearly AdS$_2$ symmetry. In this note we present a class of tensor models that can be thought of as describing probes to such a black hole. Our models are obtained by adding probe fields to SYK like tensor models. Dynamics of the original model in unaffected by the probe fields in large N limit. Further all four point functions exhibit maximal chaos in large N limit, a non trivial feature expected of a black hole probe.

 Jeudi 30 Novembre 2017, 10:00 à IHP, 314 RENC-THEO (Rencontres Théoriciennes) hep-th Harvey Reall ( DAMTP, Cambridge, UK ) On the well-posedness of Lovelock and Horndeski theories of gravity Abstract: Lovelock theories are the most general theories of a metric tensor with second order equations of motion. Horndeski theories are the most general four-dimensional theories of a metric tensor and a scalar field with second order equations of motion. Many hundreds of papers have been written about these theories. But it is unknown whether they satisfy a basic consistency requirement, namely well-posedness of the initial value problem. I will discuss this problem and explain why the method used to establish well-posedness of the Einstein equation fails for Lovelock theories and the most general Horndeski theories.

 Jeudi 30 Novembre 2017, 11:45 à IHP, 314 RENC-THEO (Rencontres Théoriciennes) hep-th Manuela Kulaxizi ( Trinity College Dublin ) Einstein gravity from conformal field theory Abstract: We will analyse the Regge limit of certain four point functions in CFTs and will argue that holographic CFTs must be described by Einstein gravity.

 Jeudi 30 Novembre 2017, 16:00 à LPT, 114 LPT-PTH (Particle Theory Seminar of LPT Orsay) hep-ph Ilja Doršner ( University of Split ) Radiative neutrino mass generation through leptoquarks in models of unification Abstract: I will discuss implementation of a one-loop neutrino mass mechanism within a framework of grand unification when the loop particles comprise scalar leptoquarks and quarks of the matching electric charge. I will, in particular, demonstrate viability of the two phenomenologically distinct regimes for the leptoquark mass scales and subsequently outline potential implications of these findings for a construction of realistic grand unified models.

 Vendredi 1 Décembre 2017, 10:00 à IPHT, Salle Claude Itzykson, Bât. 774 ( https://courses.ipht.cnrs.fr/?q=fr/node/194 ) COURS (Cours) cond-mat Pierfrancesco Urbani ( IPhT ) Statistical physics of glassy systems: tools and applications (4/6) Abstract: The complex behavior of a large variety of systems can often be ascribed to the competition of many quasi-optimal equilibria. In these cases metastability deeply affects both the structural and dynamical properties. Glasses are the prototype of such systems, with glassy behavior arising not only in condensed matter but also in a wide variety of fields ranging from optimization to computer science. \par In these lectures I will introduce different kinds of glassy systems and I will describe their fundamental similarities, which emerge from their statistical description. Then I will focus on a representative model, the random perceptron, and I will discuss the main techniques that can be used to solve it. \par These techniques provide a versatile theoretical toolbox that can be applied to several problems such as for example the physics of the jamming transition, its interpretation in the context of constraint satisfaction, as well as the design of new efficient algorithms to solve statistical problems. \\ 1.Introduction to glassy systems: structural glasses and constraint satisfaction problems. \\ 2.The random perceptron model. The replica approach. The replica symmetric solution and phase diagram. \\ 3.Replica symmetry breaking and marginal stability. The full RSB solution of the random perceptron. \\ 4.The jamming transition as a satisfiability threshold; critical exponents. \\ 5.Belief propagation approach and the algorithmic version of the Thouless-Anderson-Palmer equations. \\ Supported by `Investissements d'Avenir'' LabEx PALM (ANR-10-LABX-0039-PALM) Fichiers attachés: 2017-2018.pdf (4503344 bytes) 2017_Urbani.pdf (5052625 bytes)

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