The SEMPARIS seminar webserver hosts annoucements of all seminars taking place in Paris area, in all topics of physics, mathematics and computer science. It allows registered users to receive a selection of announcements by email on a daily or weekly basis, and offers the possibility to archive PDF or Powerpoint files, making it available to the scientific community. [ More information ]
Upcoming Seminars  [Next 30 ]  
[ scheduler view ] 
Wednesday 20 March 2019, 12:00 at LPENS, Conf. IV  FORUMENS (Forum de Physique Statistique @ ENS)  condmat.soft 


Wednesday 20 March 2019, 13:45 at DPTPHYSENS, Departement de physique  ENS  Conf IV  24, rue Lhomond  75005  SEMPHYSENS (Colloquium du Département de Physique de l'ENS)  quantph 



Abstract:  In the so called bottomup approach to controlled atomic quantum matter small quantum systems are synthesized involving single, few or many atom systems. I will discuss two examples: I will show that controlled interaction of atoms with a so called fast optical resonators leads not only to a strongly preferred emission of light into optical wave guides (Purcell effect) but also to speed higher than conventional atomic decay rates. This device will be useful for future interconnects in hybrid quantum networks: We have taken first steps towards coupling broad band (e.g. quantum dot) photons to narrow band fiber coupled atomic memories. Quantum networks will have to rely on so called quantum repeaters for large scale distribution of quantum states. Quantum repeaters remain an enormous challenge for experimenters. With quantum walks – i. e. driven discrete transport on a lattice conditioned on the spin state – we operate a tool realizing controlled coherent transport of atoms over tens of lattice sites – up to the so called quantum speed limit available. I will present the experimental realization of “ideal negative measurements” showing strong violation of the LeggettGard inequality. The experiment distinguishes quantitatively the macro realist’s world from the quantum world. A few more examples including e.g. the creation of artificial magnetic fields will be given. The ultimate aim of these experiments is the creation of quantum cellular automata. 
Wednesday 20 March 2019, 13:45 at LKB, Departement de physique  ENS  Conf IV  24, rue Lhomond  75005  SEMPHYSENS (Colloquium du Département de Physique de l'ENS)  quantph 



Abstract:  The propagation of coherent light through a thick layer of scattering material is an extremely complex physical process. However, it remains linear, and under certain conditions, if the incoming beam is spatially modulated to encode some data, the output as measured on a sensor can be modeled as a random projection of the input, i.e. its multiplication by an iid random matrix. One can leverage this principle for compressive imaging, and more generally for any data processing pipeline involving largescale random projections. This talk will discuss recent technological developments of optical coprocessors within the startup LightOn, and present a series of proof of concept experiments in machine learning, such as transfer learning, change point detection, or recommender systems. 
Wednesday 20 March 2019, 14:00 at IHES, Amphithéâtre Léon Motchane  PTIHES (Séminaire de physique théorique de l'IHES)  hepth 



Abstract:  The reduced density matrix of a subsystem induces an intrinsic internal dynamics called the ``modular flow''. The flow depends on the subsystem and the given state of the total system. It has been subject to much attention in theoretical physics in recent times because it is closely related to information theoretic aspects of quantum field theory. In mathematics, the flow has played an important role in the study of operator algebras through the work of Connes and others. It is known that the flow has a geometric nature (boosts resp. special conformal transformations) in case the subsystem is defined by a spacetime region with a simple shape. For more complicated regions, important progress was recently made by Casini et al. who were able to determine the flow for multicomponent regions for free massless fermions or bosons in 1+1 dimensions. In this introductory lecture, I describe the physical and mathematical backgrounds underlying this research area. Then I describe a new approach which is not limited to free theories, based in an essential way on two principles: The socalled ``KMScondition'' and the exchange relations between primaries (braid relations) in rational CFTs in 1+1 dimensions. A combination of these ideas and methods from operator algebras establish that finding the modular flow of a multicomponent region is equivalent to a certain matrix RiemannHilbert problem. One can therefore apply known methods for this classic problem to find or at least characterize the modular flow. 
Wednesday 20 March 2019, 14:15 at IPHT, Salle Claude Itzykson, Bât. 774  IPHTMAT (Séminaire de matrices, cordes et géométries aléatoires)  hepth 



Abstract:  I will discuss recent results on the classification of supersymmetric solutions of sixdimensional (2,0) supergravity. All such solutions with a lightlike isometry can be obtained from solutions of the less supersymmetric (1,0) theory, implying that no new solutions can be found in this framework. Supersymmetric solutions of sixdimensional supergravity are an important ingredient for the construction of black hole microstate geometries, I will comment on possible implications of our results on this program. In particular, we find that for generic values of the moduli the 3form fluxes on all 3cycles of the geometry must be proportional to each other and thus rational multiples of the total flux associated to the black hole. 
Thursday 21 March 2019, 11:00 at CPHT, Salle de Conference Louis Michel (Bât.6 CPHT)  SEMCPHT (Séminaire du CPHT)  hepth 



Abstract:  Providing a microscopic derivation of the entropy of supersymmetric asymptotically AdS_5 black holes has been an open problem for 15 years. In the talk I will present progress in this direction. On the gravity side of the AdS/CFT correspondence, I will describe a BPS limit of black hole thermodynamics by first focussing on a supersymmetric family of complexified solutions and then reaching extremality. In this limit the black hole entropy is the Legendre transform of the onshell gravitational action with respect to a set of chemical potentials subject to a specific constraint. The latter follows from supersymmetry and regularity in the Euclidean bulk geometry. The gravitational analysis instructs us that the dual N=1 superconformal field theory is defined on a twisted S^1 x S^3 with complexified chemical potentials obeying the constraint, and localization allows to compute the corresponding partition function exactly. This computation defines a generalization of the supersymmetric Casimir energy, whose Legendre transform at large N precisely reproduces the Bekenstein Hawking entropy of the black hole. 
Thursday 21 March 2019, 11:00 at LPTHE, bibliothèque du LPTHE, tour 1314, 4eme étage  SEMDARBOUX (Séminaire Darboux  physique théorique et mathématiques)  hepth 



Abstract:  We study the eigenfunctions of discrete Schrödinger operators on finite graphs of large size, and their (de)localization properties. To measure delocalization, we use a criterion of "quantum ergodicity" borrowed from the subject of quantum chaos. We can prove delocalization in this sense for some families of graphs, typically : regular graphs and their small perturbations. We show that quantum ergodicity is related to ``spectral delocalization'', namely the fact that the spectrum of the ideal infinite system is absolutely continuous. 
Thursday 21 March 2019, 11:30 at
IPN,
Bâtiment 100, Salle A201 ( $ $ )  IPNTHEO (Séminaire du groupe de physique théorique de l'IPN Orsay)  nuclth 



Abstract:  Finitesize instabilities are unphysical phase transitions that plague several parameterizations of effective interactions used in selfconstistent meanfield calculations. They take the form of largeamplitude oscillations of the isovector density, or the vector (spin) density if time reversal symmetry in not enforced. I will briefly review a study we made concerning the isovector instabilities and discuss a quantitative tool that we have developed in order to detect and avoid them during the fit of the parameters of an interaction. Since the so called bubble (or semibubble) nuclei show significant oscillations in proton and neutron densities often quantified by a depletion factor, it is interesting to see how the proximity of an isovector instability may be correlated with this depletion factor. Finally, I will revisit some results for several nuclei considered as having a possible bubble structure and question the predictive power of the zero or finiterange interactions used for these predictions. 
Thursday 21 March 2019, 14:00 at CPHT, Salle de Conference Louis Michel (Bât.6 CPHT)  SEMCPHT (Séminaire du CPHT)  hepth 



Abstract:  Partial compositeness can be used to explain the fermion mass hierarchy and predict the sfermion mass spectrum in a supersymmetric model. It assumes that the Higgs and thirdgeneration matter superfields are elementary fields, while the first two matter generations are composite, with a linear mixing between elementary superfields and supersymmetric operators with large anomalous dimensions. This then gives rise to a splitlike supersymmetric model that explains the fermion mass hierarchy while simultaneously predicting an inverted sfermion mass spectrum with a gravitino LSP that is consistent with LHC and flavor constraints. The model therefore intricately connects the sector responsible for the generation of flavor with supersymmetry breaking to produce a unique sparticle spectrum. 
Thursday 21 March 2019, 14:00 at LPTM, 4.13 St Martin II  SEMLPTMUCP (Seminaires du LPTM , Universite de Cergy Pontoise)  condmat 



Abstract:  One of the most puzzling recent experimental discoveries in condensed matter physics has been the observation of quantum oscillations in insulating materials SmB6 and YbB12 [1,2]. Both materials are strongly correlated f electron systems for which a gap develops due to a hybridization between conduction electrons and strongly correlated f electrons, and thus a large resistivity at low temperatures can be measured. Our understanding of quantum oscillations is rooted in the existence of a Fermi surface; electron bands, which form the Fermi surface, form Landau levels in a magnetic field. When the magnetic field strength is changed, the energy of these Landau levels changes which lead to an oscillatory behavior in almost all of the observable properties. However, insulating materials like SmB6 and YbB12 do not possess a Fermi surface, thus there are no electrons, which can form Landau levels, close to the Fermi energy. On the other hand, SmB6 and YbB12 are both good candidates for topological Kondo insulator. Naturally, the question arises, if these quantum oscillations can be due to the interplay between topology and strong correlations. We here answer this question by showing results of dynamical mean field theory in a magnetic field for a two dimensional topological Kondo insulator. We demonstrate that the gap closing, described for a noninteracting continuum model with momentum dependent hybridization [3], persists for a topological Kondo insulator on a two dimensional (2D) lattice. Furthermore, we demonstrate that the amplitude of quantum oscillations is strongly enhanced due to correlations, which makes them easily observable in quantities like magnetization and resistivity over a wide range of magnetic fields before the magnetic breakdown occurs. 
Thursday 21 March 2019, 14:30 at
IHES,
Amphithéâtre Léon Motchane ( Cours de l'IHES )  MATHIHES (TBA)  hepth 



Abstract:  Bridgeland stability structure/condition on a triangulated category is a vast generalization of the notion of an ample line bunlde (or polarization) in algebraic geometry. The origin of the notion lies in string theory, and is applicable to derived categories of coherent sheaves, quiver representations and Fukaya categories. In a category with Bridgeland stability every objects carries a canonical filtration with semistable pieces, an analog of HarderNarasimhan filtration. It is expected that for categories over complex numbers Bridgeland stability structures often admit analytic enhancements, similar to the relation between ample bundles and usual Kaehler metrics. In a sense, this should be a generalization DonaldsonUhlenbeckYau theorem which syas that a holomorphic vector bundle over compact Kaehler manifold is polystable if and only if it admits a metrization satisfying hermitean YangMills equation. In my course I will talk about a nonarchimedean analog of analytic Bridgeland stability. I will show several examples, results and conjectures. In particular, I'll introduce nonarchimedean moment map equations, generalized honeycomb diagrams, and hypothetical stability on derived categories of coherent sheaves on maximally degenerating varieties over nonarchimedean fields. 
Thursday 21 March 2019, 15:30 at LPTENS, Bibliotheque Joel Scherk  SEMEXCEP (Séminaire exceptionel)  hepph 



Abstract:  I will discuss two novel proposals to probe Dark Matter (DM) with existing and upcoming data. 1. Cosmic rays constitute our arguably unique direct access to energy domains of 10 TeV and above, and a wealth of data is delivered/expected from current/nearfuture telescopes (ANTARES, IceCube, KM3NeT, HESSII, CTA, LHAASO, CALET,...). Heavy DM constitutes therefore an ideal BSM target for these experiments: I will discuss the theory and phenomenology of DM models that evade challenges like the socalled unitarity bound, and propose related searches at such telescopes. 2. DM lighter than a GeV is notoriously a challenge for direct detection experiments. I will propose to rely on the DM component that is unavoidably accelerated by scatterings of cosmic rays, that make it possible to detect DM at experiments with large energy thresholds and volumes, like SuperKamiokande and DUNE. I will derive a new strong limit from public data and discuss search strategies at current and future neutrino experiments. 
Thursday 21 March 2019, 16:00 at LPT, 114  LPTPTH (Particle Theory Seminar of LPT Orsay)  hepph 



Abstract:  In this talk a new production mechanism for vector dark matter (VDM) is presented in which the VDM is produced at the end of inflation. This mechanism relies on a pseudo scalar coupling between the inflaton and the vector field strength which leads to a tachyonic instability and exponential production of one transverse polarization of the vector field, reaching its maximum near the end of inflation. These polarized transverse vectors can account for the observed dark matter relic density in the mass range microeV to tens of TeV. Furthermore, since they are produced coherently with very high occupation number and with a single polarization, the nature of the VDM today is in the form of helical dark matter fields who's typical size is determined by the Hubble scale at the end of inflation. Some of the potential phenomenology of these objects is also discussed. 
Friday 22 March 2019, 11:00 at LPTHE, Bibliothèque  SEMLPTHE (Séminaire du LPTHE)  condmat.statmechhepth 



Abstract:  We review the effect of introducing anisotropic couplings for lattice gauge theories and show that these can lead to new, ``layered'', phases, beyond the bulk confining and Coulomb phases, known in abelian gauge theories. The new phases can be interpreted as describing the effects of flux compactifications and generalize the KaluzaKlein paradigm of extra dimensions. Extra dimensions have been, in fact, used to describe defects in condensed matter systems and anisotropic gauge theories can be also understood as allowing the description of more elaborate defects. Coupling matter to the gauge fields leads to a consistent, nonperturbative, description of chiral fermions, provided the anomalies are properly cancelled, and can describe edge states and their currents while scalar fields lead to a quite elaborate phase diagram. The challenges of assembling these ingredients to study supersymmetric theories will be touched on, as a way of describing how such systems can be consistently closed, a la ParisiSourlas. 
Friday 22 March 2019, 11:00 at IHES, Amphithéâtre Léon Motchane  MATHIHES (TBA)  math 



Abstract:  Consider a simplicial complex that allows for an embedding into R^d. How many faces of dimension d/2or higher can it have? How dense can they be? This basic question goes back to Descartes' "Lost Theorem" and Euler's work on polyhedra. Using it and other fundamental combinatorial problems, we introduce a version of the Kähler package beyond positivity, allowing us to prove the hard Lefschetz theorem for toric varieties (and beyond) even when the ample cone is empty. A particular focus lies on replacing the HodgeRiemann relations by a nondegeneracy relation at torusinvariant subspaces, allowing us to state and prove a generalization of theorems of Hall and Laman in the setting of toric varieties and, more generally, the face rings of Hochster, Reisner and Stanley. This has several applications including full characterization of the possible face numbers of simplicial rational homology spheres, a generalization of the crossing lemma.) 
Friday 22 March 2019, 14:00 at OBSPARIS, Salle de l'atelier, 77 Av. Denfert Rochereau, Paris 14e  SEMEXCEP (Séminaire exceptionel)  grqc 



Abstract:  The physical history of the Universe is completed by including the quantum planckian and superplanckian phase before Inflation in the Standard Model of the Universe in agreement with observations. In the absence of a complete quantum theory of gravity, we start from quantum physics and its foundational milestone: the universal classicalquantum (or waveparticle) duality, which we extend to gravity and the Planck domain. A new quantum precursor phase of the Universe appears beyond the Planck scale. Relevant cosmological examples as the Cosmic Microwave Background, Inflation and Dark Energy have their precursors in this era. A whole unifying picture for the Universe epochs and their quantum precursors emerges with the cosmological constant as the vacuum energy, entropy and temperature of the Universe, clarifying the so called cosmological constant problem which once more in its rich history needed to be revised. The consequences for the deep universe surveys, and missions like Euclid will be outlined. 
Monday 25 March 2019, 10:45 at LPTMC, Jussieu tower 1312 5th floor 523  SEMLPTMC (Séminaire du Laboratoire de Physique Théorique de la Matière Condensée)  condmat.meshall 



Abstract:  Les métamatériaux sont des milieux artificiels qui offrent des propriétés de propagation exotiques, comme par exemple des effets de réfraction négative. Ces effets sont la conséquence de l’interaction des ondes avec la cellule unité composant ces métamatériaux, celleci étant beaucoup plus petite que la longueur d'onde de travail. Dans le domaine des microondes, un exemple de cellule unité est une simple tige métallique placée sur un plan de masse, et dans le cas de l’acoustique une simple canette de soda jouant le rôle de résonateur de Helmholtz. Dans un milieu composé de nombreuses cellules unités, la propagation de l'onde est décrite simplement avec le modèle du polariton, c'estàdire par une relation de dispersion résultant de l'hybridation de l'onde et de la résonance individuelle de chaque résonateur. Dans cet exposé, nous montrerons comment aller audelà de cette description habituelle permet d’introduire des propriétés topologiques à la propagation des ondes. 
Monday 25 March 2019, 11:00 at IAP, Evry Schatzman  SEMEXCEP (Séminaire exceptionel)  astroph 



Abstract:  The search for cosmic particle accelerators capable of reaching the PeV ($10^{15}$ eV) range, PeVatrons, is a crucial science target of the veryhighenergy (gammaray domain $\sim$ TeV= $10^{12}$ eV) community. Such accelerators are essential in the context of the problem of the origin of Galactic cosmic rays, and more generally, in order to understand the physical mechanisms involved in the production of PeV particles. Subsequent to the acceleration of PeV particles, the production of gamma rays in the 100 TeV range is expected. Explorations in this energy domain are thus natural in the search for PeVatrons, where gamma—ray instruments are already accumulating data. Relying on Monte Carlo simulations, we explore the capabilities of current instruments to identify the most promising candidates. 
Monday 25 March 2019, 11:00 at IPHT, Salle Claude Itzykson, Bât. 774  IPHTPHM (Séminaire de physique mathématique)  mathph 


Monday 25 March 2019, 14:00 at
IAP,
Henri Mineur ( Series of 5 lectures )  COURS (Cours)  astroph 


Tuesday 26 March 2019, 14:00 at LPTHE, library  LPTHEPPH (Particle Physics at LPTHE)  hepphhepth 



Abstract:  Twoloop scattering amplitudes are a vital component in highprecision cross section calculations at the LHC. At higher multiplicities, the increased number of scales can make the algebra of analytic amplitude calculations intractable. In this talk, we discuss how to apply numerical techniques to sidestep this intermediate complexity and combine them with physical properties of the amplitude to efficiently determine its analytic form. In particular we focus on planar fivepoint amplitudes in QCD. 
Tuesday 26 March 2019, 15:00 at CPHT, Old Ecole Polytechnique, Amph. Poincaré  25 rue de la Montagne SainteGeneviève  SEMEXCEP (Séminaire exceptionel)  hepth 



Abstract:  15.0016.30 session 1  Gabriele Veneziano: Spontaneous CP breaking in QCD and the axion potential  Prasanta Tripathy: The nonAbelian Tdual of KlebanovWitten background and its Penrose limits  Rob Leigh 17:00  18:30 session 2  Amihay Hanany: Magnetic quivers  Jnan Maharana: Is the scattering amplitude analytic in field theory with a compact spatial coordinate?  JeanPierre Derendinger: N = 2 to N = 1 with one hypermultiplet 
Wednesday 27 March 2019, 12:00 at LPENS, Conf IV  FORUMENS (Forum de Physique Statistique @ ENS)  condmat.statmech 


Thursday 28 March 2019, 10:00 at IHP, 201  RENCTHEO (Rencontres Théoriciennes)  hepth 



Abstract:  I will review recent developments on soft gravitational radiation from ultrarelativistic collisions. Calculations based on recent developments in the eikonal approach and in softgraviton theorems can be successfully compared in their common region of applicability. Combining the results of both approaches leads to a knee in the spectrum at a typical "Hawking frequency” and to a bump at wavelengths comparable to the impact parameter of the collision. 
Thursday 28 March 2019, 11:40 at IHP, 201  RENCTHEO (Rencontres Théoriciennes)  hepth 


Thursday 28 March 2019, 14:00 at LPTM, 4.13 St Martin II  SEMLPTMUCP (Seminaires du LPTM , Universite de Cergy Pontoise)  mathph 



Abstract:  The computation of form factors for the spin chains has always been one of the most important problems of the theory of quantum integrable systems. It gained even more importance when it was shown that the dynamical structure factors can be computed using explicit analytic representations for the form factors and these numerical results give extremely good predictions for neutron scattering experiments. It was also recently demonstrated that the form factor analysis leads to a very powerful method of asymptotic computation of the correlation functions and dynamical structure factors. In this talk I’ll review these recent developments and introduce the lqtest results on the form factors in the thermodynamic limit of the spin chains in zero magnetic field. 
Thursday 28 March 2019, 14:30 at
IHES,
Amphithéâtre Léon Motchane ( Cours de l'IHES )  MATHIHES (TBA)  hepth 



Abstract:  Bridgeland stability structure/condition on a triangulated category is a vast generalization of the notion of an ample line bunlde (or polarization) in algebraic geometry. The origin of the notion lies in string theory, and is applicable to derived categories of coherent sheaves, quiver representations and Fukaya categories. In a category with Bridgeland stability every objects carries a canonical filtration with semistable pieces, an analog of HarderNarasimhan filtration. It is expected that for categories over complex numbers Bridgeland stability structures often admit analytic enhancements, similar to the relation between ample bundles and usual Kaehler metrics. In a sense, this should be a generalization DonaldsonUhlenbeckYau theorem which syas that a holomorphic vector bundle over compact Kaehler manifold is polystable if and only if it admits a metrization satisfying hermitean YangMills equation. In my course I will talk about a nonarchimedean analog of analytic Bridgeland stability. I will show several examples, results and conjectures. In particular, I'll introduce nonarchimedean moment map equations, generalized honeycomb diagrams, and hypothetical stability on derived categories of coherent sheaves on maximally degenerating varieties over nonarchimedean fields. 
Thursday 28 March 2019, 16:00 at LPT, 114  LPTPTH (Particle Theory Seminar of LPT Orsay)  hepph 



Abstract:  There is a growing interest in the High Energy community in searching for new physics with longlived particles which decay at displaced vertices. Many models exhibit this feature. I will describe one of such models : the electroweakscale nonsterile righthanded neutrino model. In this model, nonsterile righthanded neutrinos as well as other "mirror" quarks and leptons can be produced at the LHC and their decay products in the form of standard model particles appear at displaced vertices. A deep connection between the thetaangle of strong CP and neutrino masses will be presented. 
Friday 29 March 2019, 11:00 at LPTHE, Bibliothèque  SEMLPTHE (Séminaire du LPTHE)  condmat.statmechhepthmathmathphmath.AG 



Abstract:  The dimer model is a model from statistical mechanics corresponding to random perfect matchings on graphs. Circle patterns are a class of embeddings of planar graphs such that every face admits a circumcircle. In this talk I describe a correspondence between dimer models on planar bipartite graphs and circle pattern embeddings of these graphs. As special cases of this correspondence we recover the Tutte embeddings for resistor networks and the sembeddings for Ising models. This correspondence is also the key for studying Miquel dynamics, a discrete integrable system on circle patterns. 
Friday 29 March 2019, 11:00 at APC, Amphitheatre Pierre Gilles de Gennes  APCCOLLOQUIUM (Colloquium de l'APC)  astroph 



Abstract:  Axion–like particles (ALPs) can convert into photons in several different ways: in an external magnetic field (known as the Primakoff effect), by decay into two photons, or through parametric amplification of incoming radio photons in an oscillating ALP field within a narrow frequency range. For non–relativistic ALPs all three effects can give rise to radio lines, in particular from astrophysical objects with strong magnetic fields or from regions with ALP over–densities, such as ALP stars. I will give an overview over estimated intensities and prospects for detectability of such radio lines. 

[ English version ] 