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 ]  
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Wednesday 12 December 2018, 10:00 at
IHES,
Centre de conférences Marilyn et James Simons ( Séminaire de Géométrie Arithmétique ParisPékinTokyo )  MATHIHES (TBA)  hepth 



Abstract:  I will show that for any integer N, there are only finitely many cuspidal algebraic automorphic representations of GL_m over Q whose Artin conductor is N and whose "weights" are in the interval {0,...,23} (with m varying). Via the conjectural yoga between geometric Galois representations (or motives) and algebraic automorphic forms, this statement may be viewed as a generalization of the classical HermiteMinkowski theorem in algebraic number theory. I will also discuss variants of these results when the base field Q is replaced by an arbitrary number field. 
Wednesday 12 December 2018, 11:00 at LPT, 114  LPTPHYSMATH (Séminaires de PhysiqueMathématique du LPTOrsay)  hepth 



Abstract:  I will review a method to evaluate correlation functions of certain large N statistical systems via chord diagrams and apply it to compute correlators in doublescaled version of SYK model at all energies, in particular to extract corrections to the maximal chaos exponent. Time permitting, I will comment on the suggested relation of this model to a Hamiltonian reduction of quantum particle moving on a noncompact quantum group SU_q(1,1). 
Thursday 13 December 2018, 11:00 at CPHT, Salle de Conference Louis Michel (Bât.6 CPHT)  SEMCPHT (Séminaire du CPHT)  hepth 



Abstract:  I shall present a class of heterotic compactifications where it is possible to lower the string unification scale down to the GUT scale. I shall illustrate this approach with an explicit example of a fourdimensional chiral heterotic vacuum with N=1 supersymmetry. 
Thursday 13 December 2018, 11:00 at IPN, Salle A201  IPNX (Séminaire commun de physique théorique des particules IPNCPHTX)  hepph 



Abstract:  The amplitudes for the rare decay modes $K^\pm\to\pi^\pm\ell^+\ell^$ and $K_S\to\pi^0\ell^+\ell^$ are studied with the aim of obtaining predictions for them, such as to enable the possibility to search for violations of leptonflavour universality in the kaon sector. 
Thursday 13 December 2018, 11:00 at LPTHE, bibliothèque du LPTHE, tour 1314, 4eme étage  SEMDARBOUX (Séminaire Darboux  physique théorique et mathématiques)  hepth 



Abstract:  Multiple polylogarithms are generalizations of the classical (poly)logarithm functions and play an important role in several areas of mathematics. Their special values at one are wellknown as multiple zeta values and these form the Drinfeld associator. Polylogarithms arise naturally as iterated integrals on the moduli space of genus zero curves, and their integration theory was worked out explicitly by F. Brown and is completely understood. I will define this class of functions, explain some of their properties, and show how they can be used to compute a certain class of integrals. Applications to Feynman integrals, string amplitudes and deformation quantization will be illustrated. 
Thursday 13 December 2018, 14:00 at LPTM, 4.13 St Martin II  SEMLPTMUCP (Seminaires du LPTM , Universite de Cergy Pontoise)  condmat 



Abstract:  A most striking feature of highTc cuprate superconductors is the persistence of a gap above the superconducting transition temperature Tc, where an unconventional metallic phase, known as the pseudogap, sets in. The pseudogap is not well understood yet and it is generally believed to be at the roots of the high Tc superconducting mechanism. By exploiting cluster dynamical mean field theory results on the two dimensional Hubbard Model, we show that the pseudogap and the superconducting gap compete for the same electrons, producing an unconventional form of the superconducting pairing that involves electrons in high energy states. We show that these findings leave visible fingerprints in the Raman response, which displays a characteristic peakdip feature. The good agreement between theoretical and experimental results reveal an unprecedented relationship between the pseudogap and superconducting gap, which eventually boosts up the Tc. 
Thursday 13 December 2018, 16:30 at IHES, Amphithéâtre Léon Motchane  MATHIHES (TBA)  math 



Abstract:  Kontsevich's 1997 proof of the formality conjecture provides a universal quantization of every Poisson manifold, by a formal power series whose coefficients are integrals over moduli spaces of marked discs. In joint work with Peter Banks and Brent Pym, we prove that these integrals evaluate to multiple zeta values, which are interesting transcendental numbers known from the Drinfeld associator and as the periods of mixed Tate motives. Our proof is algorithmic and allows for the explicit computation of arbitrary coefficients in the formality morphism, in particular the star product. The essential tools are Francis Brown's theory of polylogarithms on the moduli space of marked genus zero curves, singlevalued integration due to Oliver Schnetz, and an induction over the natural fibrations of moduli spaces. 
Friday 14 December 2018, 11:00 at LPTHE, Bibliothèque  SEMLPTHE (Séminaire du LPTHE)  condmat.statmechhepthmathph 



Abstract:  In this talk I revisit Stokes drift in shallow water from a geometric perspective inspired by conformal field theory. Given a fluid on a circular pool supporting periodic waves, I argue that the displacement of fluid particles after one period involves a Berry phase associated with adiabatic conformal transformations. In particular, the phase shift produced by cnoidal waves can be evaluated in closed form, and may be accessible to experiments or numerical simulations. Conceptually, this relates fluid dynamics, symplectic geometry, conformal field theory and Thomas precession. 
Friday 14 December 2018, 11:00 at APC, 454A (salle Luc Valentin)  APCCOLLOQUIUM (Colloquium de l'APC)  astroph 



Abstract:  The production of the elements heavier than iron in the universe has long been associated with neutroncapture processes. The most neutronrich isotopes are created by rapid (r ) neutroncapture nucleosynthesis in extreme astrophysical environments. Specifics of these environments and the location of the astrophysical sites in which the r process occurs have remained open problems. It has been reported that observations of the gravitational wave event GW170817 and its electromagnetic counterpart suggest that neutron star mergers are a site of rprocess nucleosynthesis. Still many questions remain, such as the nature of the astrophysical conditions within the merger responsible for element synthesis and whether mergers can account for all galactic rprocess production. If we hope to fully understand the connection between this discovery and the origin of rprocess elements, uncertainties in neutrino and nuclear astrophysics must be reduced. I will highlight the role played by neutrinos and nuclear masses. 
Friday 14 December 2018, 14:15 at IPHT, Salle Claude Itzykson, Bât. 774  IPHTMAT (Séminaire de matrices, cordes et géométries aléatoires)  hepth 


Monday 17 December 2018, 10:45 at LPTMC, Jussieu, tower 1312, room 523  SEMLPTMC (Séminaire du Laboratoire de Physique Théorique de la Matière Condensée)  condmat.meshall 



Abstract:  See the attached file.  
Attachments: 
Monday 17 December 2018, 11:00 at IPHT, Salle Claude Itzykson, Bât. 774  IPHTPHM (Séminaire de physique mathématique)  mathph 



Abstract:  In this talk I will introduce a quantum chain generalizing the Zn clock model by keeping nearestneighbour interaction and selfduality, but further requiring U(1) invariance. This model is integrable, and can be related to XXZtype chains at their "roots of unity" points, which present a number of striking features including a rich structure of degeneracies. Such a correspondence allows to revisit these features in an interesting way, in particular I will discuss the physics of the "exact complete strings" underlying degeneracies in the Bethe ansatz language, and, in close connection with the latter, the presence of an underlying Onsager symmetry algebra. 
Monday 17 December 2018, 11:15 at CPHT, Seminar room Aile 0  SEMCPHT (Séminaire du CPHT)  hepth 



Abstract:  We investigate holographic complexity for eternal black hole backgrounds perturbed by shock waves, with both the complexity=action (CA) and complexity=volume (CV) proposals. We consider Vaidya geometries describing a thin shell of null fluid with arbitrary energy falling in from one of the boundaries of a twosided AdSSchwarzschild spacetime. We demonstrate how scrambling and chaos are imprinted in the complexity of formation and in the full time evolution of complexity via the switchback effect for light shocks, as well as analogous properties for heavy ones. 
Tuesday 18 December 2018, 11:00 at LPTMS, LPTMS, salle 201, 2ème étage, Bât 100, Campus d'Orsay  LPTMS (Séminaire du Laboratoire de Physique Théorique et Modèles Statistiques (Orsay))  physics 



Abstract:  I will discuss a new connection between two different problems: the counting of equilibria of a directed polymer in a random medium (DPRM) and the problem of Anderson localisation for the 1D Schrödinger equation. Using the KacRice formula, it is possible to express the mean number of equilibria of a DPRM in terms of functional determinants. In the onedimensional situation, these functional determinants can be calculated thanks to the GelfandYaglom method, showing that the mean number of equilibria of the DPRM growth exponentially with the length of the polymer, with a rate controlled by the generalized Lyapunov exponent (GLE) of the localisation problem (cumulant generating function of the log of the wave function). The GLE is solution of a spectral problem studied by combining numerical approaches and WKBlike approximation. Furthermore, the formalism can be extended in order to obtain the number of equilibria at fixed energy, providing the (annealed) distribution of the energy density of the line over the equilibria. Reference: Yan V. Fyodorov, Pierre Le Doussal, Alberto Rosso and Christophe Texier, Exponential number of equilibria and depinning threshold for a directed polymer in a random potential, Annals of Physics 397, 164 (2018) 
Tuesday 18 December 2018, 14:00 at APC, 646 A  Mondrian  APCTH (Seminar of the theory group of APC)  hepth 



Abstract:  Vacuum decay is a prominent example of strongly nonlinear effects in quantum field theory with potentially important implications for cosmology, relating to phase transitions in the early universe or the supposed metastability of the current Higgs vacuum. Although a general theoretical description was laid out in the 80s by Sidney Coleman and his collaborators, fundamental questions pertaining to the backreaction of true vacuum bubbles on spacetime curvature and their correlations remain so far unanswered, calling for different approaches to the problem. In this talk, after a brief review of Coleman's theory emphasizing its genericness and limitations, I will present a recentlyproposed coldatoms model in which some of these ideas could be tested in laboratory experiments. I will discuss the mathematical correspondence between the two problems and focus on how a localized defect changes the decay rate, taking the example of a vortex in a BoseEinstein condensate and comparing with the effect of a black hole in a relativistic theory. 
Tuesday 18 December 2018, 16:00 at IPHT, Salle Claude Itzykson, Bât. 774  IPHTHEP (Séminaire de physique des particules et de cosmologie)  hepph 



Abstract:  The astrophysical background of gravitational waves (AGWB) is made up by the incoherent superposition of gravitational wave signals emitted by a large number of resolved and unresolved astrophysical sources from the onset of stellar activity until today. I present a theoretical framework to fully characterize the AGWB in terms of energy density anisotropies which is based on the resolution of Olbers 
Wednesday 19 December 2018, 11:00 at LPTMC, Jussieu, tower 1312, room 523  SEMLPTMC (Séminaire du Laboratoire de Physique Théorique de la Matière Condensée)  condmat.meshall 



Abstract:  Great efforts are currently devoted to the engineering of topological Bloch bands in ultracold atomic gases. Recent achievements in this direction, together with the possibility of tuning interparticle interactions, suggest that stronglycorrelated states reminiscent of fractional quantum Hall (FQH) liquids could soon be generated in these systems. In this experimental framework, where transport measurements are limited, identifying unambiguous signatures of FQHlike states constitutes a challenge on its own. Here, we demonstrate that the fractional nature of the quantized Hall conductance, a fundamental characteristic of FQH states, could be detected in ultracold gases through a circulardichroic measurement, namely, by monitoring the energy absorbed by the atomic cloud upon a circular drive. We validate this approach by comparing the circulardichroic signal to the manybody Chern number, and discuss how such measurements could be performed to distinguish FQHtype states from competing states. Our scheme offers a practical tool for the detection of topologicallyordered states in quantumengineered systems, with potential applications in solid state. 
Thursday 20 December 2018, 10:00 at IHP, 201  RENCTHEO (Rencontres Théoriciennes)  hepth 


Thursday 20 December 2018, 11:40 at IHP, 201  RENCTHEO (Rencontres Théoriciennes)  hepth 



Abstract:  In this talk I will briefly review the conformal bootstrap approach in two dimensions. I will focus in particular on the Liouville field theory, that plays a major role among the solutions of the bootstrap equations. I will discuss some new insights of this conformal field theory, concerning in particular its extensions via the analytic continuation of the central charge and/or of the spectrum. I will mention recent applications of these results to critical percolation and to the logRandom Energy Models. 
Thursday 20 December 2018, 14:00 at LPTHE, Library  LPTHEPPH (Particle Physics at LPTHE)  hepph 



Abstract:  Minimal selfconsistent scenarios with light (subGeV) dark matter are usually accompanied by a correspondingly light "dark sector". The presence of the latter leads to bright detection prospects at fixed target experiments and colliders and potentially strong astrophysical bounds. We will illustrate this point by exploring explicitly a typical simple fermion dark matter setup, examining in particular the key role of the dark sector in the interplay between accelerator searches and astrophysical signatures. 
Friday 21 December 2018, 10:30 at IPHT, Salle Claude Itzykson, Bât. 774  IPHTPHM (Séminaire de physique mathématique)  mathph 


Friday 21 December 2018, 11:00 at LPTHE, Bibliothèque  SEMLPTHE (Séminaire du LPTHE)  condmat.statmechhepthmathph 



Abstract:  We compute the exact partition function of the onedimensional su(1m) supersymmetric tJ model with longrange interactions, in the presence of an external magnetic field and a charge chemical potential. To this end, we use the equivalence of this model to a suitable modification of the su(1m) HaldaneShastry spin chain with chemical potential terms. In this way, we also obtain a complete analytical description of the spectrum of the longrange tJ model in terms of the supersymmetric version of Haldane's motifs and their related skew Young tableaux and skew Schur polynomials. As an application, we give a complete description of the different ground state phases, characterized by their spin content  i.e., su(12), su(11) and su(02), apart from the trivial phases consisting only of holes or fermions of one type. 
Friday 21 December 2018, 13:00 at IPHT, Salle Claude Itzykson, Bât. 774  SOUTENHDR (Soutenance d'Habilitation à Diriger des Recherches)  hepth 



Abstract:  In this habilitation defense, I give a subjective overview of some recent progress in solving twodimensional conformal field theories. I discuss what solving means and which techniques can be used. I insist that there is much to discover about Virasorobased CFTs, i.e. CFTs that have no symmetries beyond conformal symmetry. I claim that we should start with CFTs that exist for generic central charges, because they are simpler than CFTs at rational central charges, and can nevertheless include them as special cases or limits. Finally, I argue that in addition to writing research articles, we should use various other media, in particular Wikipedia. \\ \\ Members of the jury: Denis Bernard, Matthias Gaberdiel, Jesper Jacobsen, Vyacheslav Rychkov, Véronique Terras, Gérard Watts, JeanBernard Zuber. 
Friday 21 December 2018, 14:00 at LPTM, 4.13 St Martin II  SEMLPTMUCP (Seminaires du LPTM , Universite de Cergy Pontoise)  condmat 



Abstract:  Chiral objects are ubiquitous in science and pose fundamental challenges, such as the importance of chiral molecules in commercial drugs or the dominance of matter over antimatter in the universe. Magnetic materials lacking inversion symmetry, called chiral magnets, constitute a unique platform for the exploration and control of chiral objects. In these systems, typically comprising multilayers of transition metal ferromagnets and heavy metals (W, Pt, Ta, Bi2Se3 etc.), interfacial spinorbit coupling adopts a peculiar form, called Rashbalike interaction [1,2]. This interaction promotes a wealth of physical phenomena, among which the emergence of magnetic skyrmions – topological magnetic textures –, spinorbit torques – an efficient means to electrically control magnetization dynamics –, as well as chiral magnetic damping – energy dissipation that depends on the texture chirality. Over the years, my group has developed several theoretical approaches to describe spinorbit physics at interfaces, from minimal models to tight binding and first principle methods, in order to provide guidelines to better understand these effects. In this seminar, I will present various aspects of the interplay between spin transport and magnetization dynamics mediated by spinorbit coupling in chiral magnets. I will first discuss the nature of interfacial spinorbit coupling in magnetic multilayers and examine how it facilitates the onset of chiral magnetic textures [3]. I will then present the physics of spinorbit torques [4,5], their general features in metals and specific characteristics in topological insulators [6]. Finally, the novel concept of chiral damping [7], i.e. the idea that energy dissipation depends on the chirality, will be introduced. In conclusion, I will show how such effects can be exploited to excite and control antiferromagnets and other frustrated magnets [8]. 
Tuesday 8 January 2019, 11:00 at LPTMS, LPTMS, salle 201, 2ème étage, Bât 100, Campus d'Orsay  LPTMS (Séminaire du Laboratoire de Physique Théorique et Modèles Statistiques (Orsay))  physics 



Abstract:  We reconsider the study of the eigenvectors of a random matrix, to better understand the relation between localization and eigenvalue statistics. Traditionally, the requirement of base invariance has lead to the conclusion that invariant models describe only extended (conductive) systems. We show that deviations of the eigenvalue statistics from the WignerDyson universality reflects itself on the eigenvector distribution. In particular, gaps in the eigenvalue density spontaneously break the U(N) symmetry to a smaller one, hence rendering the system not anymore ergodic. Models with lognormal weights, recently considered also in string theory models such as ABJM theories, show a critical eigenvalue distribution which would indicate a critical breaking of the U(N) symmetry, supposedly resulting into a multifractal eigenvector statistics. These results pave the way to the exploration of localization problems using random matrices via the study of new classes of observables and potentially to novel, interdisciplinary, applications of matrix models. 
Tuesday 8 January 2019, 16:00 at IPHT, Salle Claude Itzykson, Bât. 774  IPHTHEP (Séminaire de physique des particules et de cosmologie)  hepph 



Abstract:  The fundamental nature of Dark Matter (DM) and Dark Energy (DE) has not been established. Indeed, beyond its gravitational effects, both components remain undetected by present experiments. In this situation, it is reasonable to wonder if other alternatives can effectively explain the observations usually associated with their existence. The modification of the gravitational interaction has been studied in this context from many different approaches. However, the large amount of different astrophysical evidences makes difficult to think that modified gravity can account for all these observations. On the other hand, if such a modification introduces new degrees of freedom, they may work as DM or DE candidates. We will summarize the phenomenology of these gravitational candidates focusing in the DM question. 
Wednesday 9 January 2019, 11:30 at
IPN,
Bâtiment 100, Salle A015 ( $ $ )  IPNTHEO (Séminaire du groupe de physique théorique de l'IPN Orsay)  nuclth 



Abstract:  Microscopic calculations of the electromagnetic response of light and mediummass nuclei are now feasible thanks to the availability of realistic nuclear interactions with accurate saturation and spectroscopic properties, and the development of largescale computing methods for manybody physics. In my talk I will present calculations of isovector dipole electromagnetic response and related quantities, i.e. integrated dipole cross section and polarizability, and compare with data from photoabsorption and Coulomb excitation experiments. The manybody formalism adopted is the SelfConsistent Green's Functions formalism, and special emphasis will be given to the methods used to include nonperturbatively the nuclear correlations. The nuclei considered are selected isotopes in the Oxygen, Nickel and Calcium chains, with the aim of investigating the evolution of computed quantities towards the neutron dripline.  
Attachments: 
Thursday 10 January 2019, 10:00 at IHP, 314  RENCTHEO (Rencontres Théoriciennes)  hepth 


Thursday 10 January 2019, 11:40 at IHP, 314  RENCTHEO (Rencontres Théoriciennes)  hepth 


Thursday 10 January 2019, 14:00 at LPTM, 4.13 St Martin II  SEMLPTMUCP (Seminaires du LPTM , Universite de Cergy Pontoise)  condmat 



Abstract:  We study the role of zeropoint quantum fluctuations in magnetic states which on the classical level are close to spinaligned ferromagnets. These include Skyrmion textures which arise in the context of nonzero topological charge solutions of nonlinear sigmamodels, and topologicallytrivial spirals generated by a competition of Heisenberg and DzyaloshinskiiMoriya interactions. We show that the degeneracy of the BogomolnyPrasadSommerfield (BPS) manifold is not lifted by quantum fluctuations in the case of general nonlinear sigmamodels with the target space given by Kähler manifolds presenting a physicallyimportant example of the case of Grassmanian manifold relevant to quantum Hall effect in graphene. Further, we show that the phenomenon of vanishing zeropoint motion can appear more generally in slowlytwisted almost ferromagnets. From a broader perspective, beyond the implications to exotic magnets, we suggest that this work provides two interesting angles on longstanding interesting issues in statistical physics and field theory. One is the existence of undressed states generally. The other item is the behaviour of ‘nonuniversal’ quantities, i.e. those involving information from the lattice scale, in the ‘universal’ continuum limit. Here, the Casimir energy of zeropoint fluctuations vanishes in the continuum limit, but is nonzero for any lattice discretisation. This Casimir energy, however, does play a physical role, e.g. in the lifting of groundstate degeneracies in a process known as quantum order by disorder. Our results obtained for nonlinear sigma models without supersymmetry about the absence of zeropoint fluctuations in BPS manifolds may either be a feature entirely unrelated to the more familiar instances arising in relativistic field theory from the cancellation of fluctuations in bosonic and fermionic sectors, or they may be more pedestrian and perhaps intuitively accessible instances of the same physics. 

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