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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Monday 19 March 2018, 10:30 at IHES, Amphithéâtre Léon Motchane  MATHIHES (TBA)  hepth 


Monday 19 March 2018, 10:45 at LPTMC, Jussieu, tower 1312, 5th floor, room 523  SEMLPTMC (Séminaire du Laboratoire de Physique Théorique de la Matière Condensée)  condmat.meshall 



Abstract:  Quantum Chromodynamics (QCD)  the theory of strong nuclear forces  has baffled the physics community and remains one of the poorly understood parts of the standard model. Its quintessential property: the confinement of quarks into protons, neutrons and mesons, while verified both experimentally and numerically, remains an elusive theoretical problem. The various cousins of QCD are however possible to understand to varying degrees and precision. In some of these theories the vacuum state is degenerate, and hence allows for domain walls  a surface excitation which interpolates between two vacua of the theory. These domain walls have a remarkable property that quarks become liberated on them, and the domain wall excitation spectrum is very different from that of the bulk. Such QCD cousins are, unfortunately, not the physical theory, and they do not occur in nature. QCD however has another unlikely cousin: the Valence Bond Solid (VBS) state of the quantum antiferromagnet, where spin 1/2 excitations (or spinons) are bound into spin 1 excitations by a mechanism very similar to confinement of quarks. Perhaps surprisingly the low energy theory describing the behavior of the VBS phase is virtually identical to its QCD cousins under certain conditions. Further the VBS phase may have multiple vacua, and thus support domain walls, which in turn support liberated spinon excitations absent in the bulk. This has been verified numerically in the socalled JQ model. These domain wall modes can in fact be seen as edge modes akin to those of the symmetry protected topological state. A multidisciplinary effort is slowly emerging to understand such phenomena, from the theoretical aspects of fundamental and condensed matter physics, to the numerical efforts in trying to understand QCD and quantum magnets. 
Monday 19 March 2018, 11:00 at IPHT, Salle Claude Itzykson, Bât. 774  IPHTPHM (Séminaire de physique mathématique)  mathph 



Abstract:  The Feynman diagram expansion of scattering amplitudes in perturbative superstring theory can be written as a series of integrals over compactified moduli spaces of Riemann surfaces with marked points, indexed by the genus. In genus zero one finds that open and closed string amplitudes can be described in terms of special values of multivalued and singlevalued polylogarithms, respectively, and are related by a map called singlevalued projection. In this talk I want to report on recent evidence that an analogous statement can be made in genus one in terms of elliptic polylogarithms. 
Monday 19 March 2018, 14:00 at IPHT, Salle Claude Itzykson, Bât. 774  IPHTSTA (Séminaire de Physique Statistique, CEA/Saclay)  condmat 



Abstract:  Controlling the selfassembly of supramolecular structures is vital for living cells, and a central challenge for engineering at the nano and microscales. Nevertheless, even particles without optimized shapes can robustly form welldefined morphologies. This is the case in numerous medical conditions where normally soluble proteins aggregate into fibers. Beyond the diversity of molecular mechanisms involved, we propose that fibers generically arise from the aggregation of irregular particles with shortrange interactions. Using minimal models of frustrated aggregating particles, we demonstrate robust fiber formation for a variety of particle shapes and aggregation conditions. Geometrical frustration plays a crucial role in this process, and accounts for the range of parameters in which fibers form as well as for their metastable, yet longlived character. 
Monday 19 March 2018, 14:30 at
IHES,
Amphithéâtre Léon Motchane ( Séminaire Géométrie et groupes discrets )  MATHIHES (TBA)  hepth 



Abstract:  Degenerations of maximal representations of a surface group may be seen as maximal representations in Sp(2n,F) for some nonArchimedean real closed field F. We associate to every such maximal representation a geodesic current whose intersection number is the length function of the representation for the L^1 norm. When the current is a measured lamination, we reconstruct an equivariant isometric embedding of the dual real tree in the BruhatTits building of Sp(2n,F). This involves a general construction of an intersection current associated to a non necessarily continuous positive crossratio. This is joint work with Marc Burger, Alessandra Iozzi, and Beatrice Pozzetti. 
Monday 19 March 2018, 16:00 at IPN, Salle A201  IPNX (Séminaire commun de physique théorique des particules IPNCPHTX)  hepph 



Abstract:  The quark/gluon 3dimensional (3D) content of the proton in momentum space is encoded in the socalled Transverse Momentum Dependent functions (TMDs). In this talk I will give an overview of the TMD factorization and resummation formalism, paying special attention to gluon TMDs. 
Monday 19 March 2018, 16:30 at
IHES,
Amphithéâtre Léon Motchane ( Séminaire Géométrie et groupes discrets )  MATHIHES (TBA)  hepth 



Abstract:  A classical result of Myers and Steenrod states that the isometry group of a compact Riemannian manifold is a compact Lie transformation group. Also classical is the fact that this compactness property fails for general pseudoRiemannian manifolds, allowing interesting dynamics for the group of isometries. In this talk, we will be interested by the topological, and dynamical consequences of the noncompactness of the isometry group. We will especially focus on the case of Lorentz manifolds, and we will present a complete topological classification of 3dimensional closed Lorentz manifolds having a noncompact isometry group. 
Tuesday 20 March 2018, 14:00 at APC, 483A  Malevitch  APCTH (Seminar of the theory group of APC)  hepth 



Abstract:  We show how to translate into tensorial language the ChernWeil theorem for the Lorentz symmetry, which equates the difference of the Euler densities of two manifolds to the exterior derivative of a transgression form. For doing so we need to introduce an auxiliary, hybrid, manifold whose geometry we construct explicitely. This allows us to find the vector density, constructed out of spacetime quantities only, whose divergence is the exterior derivative of the transgression form. As a consequence we can show how the EinsteinHilbert, GaussBonnet and, in general, the Euler scalar densities can be written as the divergences of genuine vector densities in the critical dimensions D=2,4, etc. As Lovelock gravity is a dimensional continuation of Euler densities, these results are of relevance for GaussBonnet and, in general, Lovelock gravity. Indeed, these vectors which can be called generalized Katz vectors ensure, in particular, a wellposed Dirichlet variational principle. 
Tuesday 20 March 2018, 17:15 at DPTPHYSENS, room Jean Jaurès (29 rue d'Ulm)  SEMPHYSENS (Colloquium du Département de Physique de l'ENS)  condmat.strel 



Abstract:  The standard model of solids, grounded on FermiLiquid theory and powerful computational techniques, provides an accurate description of many materials of great technological significance. Correlated electron systems are materials which fall outside the standard model of solidstate physics. They display remarkable emergent phenomena as for example metal to insulator transitions and unconventional high temperature superconductivity. The most recent example provided by the ironbased hightemperature superconductors. From a theoretical perspective correlated electrons pose a most challenging nonperturbative problem in physics. In this Colloquium, I will give an elementary introduction to the field of strongly correlated electron systems and Dynamical Mean Field Theory (DMFT) a nonperturbative method which provided a zeroth order picture of the strong correlation phenomena in close analogy with the Weiss meanfield theory in statistical mechanics. Applications to materials containing f and d electrons will be presented to show how the anomalous properties of correlated materials emerge from their atomic constituents. Different roads for the formation of strongly correlated states, will be traced to Mott Hubbard and Hunds physics. I will conclude with an outlook of the challenges ahead and the perspectives for rational material design using stronglycorrelated materials. 
Wednesday 21 March 2018, 13:45 at DPTPHYSENS, ENS, salle tba  SEMPHYSENS (Colloquium du Département de Physique de l'ENS)  quantph 



Abstract:  I will present my recent work made in IQOQI. The main part of the talk is devoted to the quantum scanning microscope arXiv:1709.01530 (to be published in PRL) We propose and analyze a scanning microscope to monitor `live' the quantum dynamics of cold atoms in a Cavity QED setup. The microscope measures the atomic density with subwavelength resolution via dispersive couplings to a cavity and homodyne detection within the framework of continuous measurement theory. We analyze two modes of operation. First, for a fixed focal point the microscope records the wave packet dynamics of atoms with time resolution set by the cavity lifetime. Second, a spatial scan of the microscope acts to map out the spatial density of stationary quantum states. Remarkably, in the latter case, for a good cavity limit, the microscope becomes an effective quantum nondemolition (QND) device, such that the spatial distribution of motional eigenstates can be measured backaction free in single scans, as an emergent QND measurement. In the final part of the talk I will present an overview of our ongoing work involving cold Rydberg atoms in regular arrays forming an optical antenna arXiv:1802.05592 We describe the design of an artificial `free space' 1Datom for quantum optics, where we implement an effective twolevel atom in a 3D optical environment with a chiral lightatom interface, i.e. absorption and spontaneous emission of light is essentially unidirectional. This is achieved by coupling the atom of interest in a laserassisted process to a `fewatom' array of emitters with subwavelength spacing, which acts as a phasedarray optical antenna. We develop a general quantum optical model based on WignerWeisskopf theory, and quantify the directionality of spontaneous emission in terms of a Purcell $\beta$factor for a given Gaussian (paraxial) mode of the radiation field, predicting values rapidly approaching unity for `fewatom' antennas in bi and multilayer configurations. Our setup has for neutral atoms a natural implementation with laserassisted Rydberg interactions, and we present a study of directionality of emission from a string of trapped ions with superwavelength spacing. 
Wednesday 21 March 2018, 14:00 at LPTENS, LPTENS library  JOURCLUB (Journal Club)  hepth 



Abstract:  (based on 1608.05499 ) 
Thursday 22 March 2018, 10:00 at IHP, 314  RENCTHEO (Rencontres Théoriciennes)  hepth 



Abstract:  I will discuss the onedimensional complex conformal manifold that controls the infrared dynamics of a threedimensional N=2 supersymmetric theory of three chiral superfields with a cubic superpotential. Two special points on this conformal manifold are the XYZ model and three decoupled copies of the critical WessZumino model. The conformal manifold enjoys a discrete duality group and can be thought of as an orbifold of CP1. I will describe how to compute the spectrum of low lying operators and their OPE coefficients as a function of the coordinates on the conformal manifold using the epsilonexpansion and the numerical conformal bootstrap. 
Thursday 22 March 2018, 10:00 at
IHES,
Amphi. Léon Motchane ( Cours de l'IHES )  MATHIHES (TBA)  hepth 


Thursday 22 March 2018, 11:00 at IPN, Salle des Conseils  IPNX (Séminaire commun de physique théorique des particules IPNCPHTX)  hepph 



Abstract:  Transverse momentum dependent parton distribution and fragmentation functions (TMDs) are fundamental objects that encode information on the intrinsic motion of quarks and gluons inside hadrons. As such, they provide a threedimensional picture of hadrons in momentum space. Here we present the first extraction of unpolarized quark TMDs, inside unpolarized protons, through a simultaneous analysis of semiinclusive deepinelastic scattering, DrellYan and Z boson hadroproduction processes. 
Thursday 22 March 2018, 11:45 at IHP, 314  RENCTHEO (Rencontres Théoriciennes)  hepth 



Abstract:  In string theory, black hole microstates at finite coupling give rise to horizonscale structures that play an important role in addressing the information paradox. These are based on configurations of branes puffed up by the supertube effect. I will show that such supertubes admit an exact worldsheet treatment as gauged WessZuminoWitten models. This description includes both BPS and nonBPS threecharge microstates. These constructions reveal stringy structures that support precursors of the long strings which hold most of the entropy in the dual CFT. 
Thursday 22 March 2018, 14:00 at LPT, 110  LPTCOSM (Seminar of the Gravity and cosmology group of LPT)  grqc 



Abstract:  Thin vortices in fluids and superfluids admit an effective field theory (EFT) description in which they can be treated as onedimensional extended objects, or strings. In this theory, vortex lines can interact among each other as well as with the “bulk” fluid by the exchange of phonons, whose dynamics can be described by a simple scalar effective theory. The coupling between vortices and sound is however more complicated, as it requires switching to a magnetictype dual formulation in which the phonon field is encoded by a gauge twoform. After a review of the theory and its applications, I will discuss recent work where we show that the EFT can be recast into the simpler scalar field language, in a description that is valid for closed string configurations and for bulk modes of wavelength much longer than the typical string size. This is essentially a systematic expansion of the original EFT in which the string is regarded as a pointlike source with degrees of freedom given by the string’s multipole moments. As an illustration we compute the sound emitted by an oscillating vortex ring, a calculation that in our field theory language reduces to a simple Feynman diagram to leading order. 
Thursday 22 March 2018, 16:00 at LPT, 114  LPTPTH (Particle Theory Seminar of LPT Orsay)  hepph 



Abstract:  It is possible that measurements of vector boson scattering (VBS) processes at the LHC will reveal disagreement with Standard Model predictions, but no new particles will be observed directly. The task is then to learn as much as possible about the new physics from a VBS analysis carried within the framework of the Effective Field Theory (EFT). I will discuss issues related to the correct usage of the EFT when the WW invariant mass is not directly accessible experimentally, as in purely leptonic W decay channels. The strategies for future data analyses in case such scenario indeed occurs will also be proposed. 
Friday 23 March 2018, 10:00 at
IPHT,
Salle Claude Itzykson, Bât. 774 ( https://courses.ipht.cnrs.fr/?q=fr/node/196 )  COURS (Cours)  hepth 



Abstract:  It is known since the work of Emmy Noether that continuous global symmetries lead to local conserved currents. In gauge theories such as electromagnetism or gravity, the currents associated to gauge transformations vanish because gauge symmetries are not physical symmetries, but rather redundancies of description. However, if the manifold on which the gauge theory is defined has a boundary and the gauge parameter does not vanish on it, then the associated conserved charge can be nonzero. Such gauge transformations that do not vanish at infinity are known as asymptotic symmetries and they act nontrivially on the space of states. \par Asymptotic symmetries make a natural appearance in holography, the proposed equivalence between a theory of quantum gravity on a given manifold and a field theory living on its boundary. More precisely, they should correspond to global symmetries of the dual field theory. Despite the huge success of the AdS/CFT holographic correspondence, holography for most spacetimes of interest remains very poorly understood. One may then hope that the study of asymptotic symmetries, especially when there is an infinite number of them, can shed light on the nature of the dual field theory. \par In this course, we will study several examples of gravitational backgrounds for which the group of asymptotic symmetries is infinitedimensional, such as AdS$_3$, the near horizon of extreme black holes, and flat space. In the latter case, a parallel development relates asymptotic symmetries to the soft behaviour of scattering amplitudes, providing a new perspective on these wellknown results. The black holes will always be lurking in the background. \\ \\ 1. Introduction to the covariant phase space formalism and construction of the conserved charges. \\ 2. Asymptotic symmetries of AdS$_3$ gravity and generalization to extreme Kerr black holes. \\ 3. Asymptotic symmetries of flat space: supertranslations, superrotations and the memory effect. \\ 4. Symmetries of scattering in flat space and relation to soft theorems.  
Attachments: 
Friday 23 March 2018, 11:00 at
IPN,
Bâtiment 100, Salle A018 (Bibliothèque) ( $ $ )  IPNTHEO (Séminaire du groupe de physique théorique de l'IPN Orsay)  nuclexnuclth 



Abstract:  Compact stars such as neutron stars and supernovae are unique laboratories to probe matter in extreme conditions. Nuclear physics experiments, in tandem with astrophysical observations, can give valuable insight in to the properties of dense matter encountered in these stellar objects. In this seminar, I will discuss the importance of some nuclear physics inputs in the description of these compact stars. In particular, the impact of nuclear masses of neutronrich nuclei for the compactstar modelling will be discussed. Moreover the dense matter found in neutron stars and supernovae being very neutron rich, a correct description of the symmetryenergy contribution to the nuclear energy is of great importance. The role of the symmetry energy on some properties of the neutronstar structure and composition will be addressed.  
Attachments: 
Friday 23 March 2018, 14:00 at LPTENS, LPTENS library  STRLPTENSHE (Séminaire commun LPTENS/LPTHE)  hepth 



Abstract:  Title: Computational complexity of cosmology in string theory Abstract: We describe a new approach for quantum cosmology based on computational complexity. By defining a cosmology as a spacetime containing a vacuum with specified properties (for example small cosmological constant) together with rules for how time evolution will produce the vacuum, we can associate global time in a multiverse with clock time on a supercomputer which simulates it. We argue for a principle of ``limited computational complexity" governing early universe dynamics as simulated by this supercomputer, which translates to a global measure for regulating the infinities of eternal inflation. We also give various definitions of the computational complexity of a cosmology, and argue that there are only a few natural complexity classes. (based on 1706.06430 ) 
Monday 26 March 2018, 09:45 at
LPTM,
Maison internationale de la recherche, NeuvillesurOise. ( See: https://perso.ucergy.fr/~atorcini/IEAday.html )  WORKCONF (Workshop or Conference)  nlin 



Abstract:  The meeting will address subjects in areas ranging from statistical physics to biology, from applied mathematics to computational neuroscience. In particular, the speakers will present specific talks on active matter (e.g. on cell migration and on dynamics of bacterial suspensions), as well as on neural systems (on seizure propagation in the brain and on synchronization of neural networks) and on transport in oscillator chains and in random graphs. The presentation of the subjects will allow participants from different disciplines to profit of the proposed talks. 
Monday 26 March 2018, 10:45 at LPTMC, tower 1312 room 523  SEMLPTMC (Séminaire du Laboratoire de Physique Théorique de la Matière Condensée)  condmat.meshall 



Abstract:  I will first start with a general introduction on theoretical ecology, stressing the reasons that make connections with statistical physics interesting and timely. I will then focus on LotkaVolterra equations, which provide a general model to study large assemblies of strongly interacting degrees of freedom in many different fields: biology, economy and in particular ecology. I will present our analysis of LotkaVolterra equations as model of ecosystems formed by a large number of species and show the different phases that emerge. Two of them are particularly interesting: when interactions are symmetric we find a regime characterised by an exponential number of multiple equilibria, all poised at the edge of stability for a large number of species. For non symmetric interactions, this phase is replaced by a chaotic one. I will then conclude discussing relationships with experiments and general consequences of our works. 
Tuesday 27 March 2018, 10:30 at
IHES,
Amphi. Léon Motchane ( Cours des Professeurs Permanents de l'IHES )  MATHIHES (TBA)  hepth 



Abstract:  The course will focus on rigorous results for the selfavoiding walk model on lattices, with a special emphasis on lowdimensional ones. The model is defined by choosing uniformly at random among random walk paths starting from the origin and without selfintersections. Despite its simple definition, the selfavoiding walk is difficult to comprehend in a mathematically rigorous fashion, and many of the most important problems illustrating standard challenges of critical phenomena remain unsolved. The model is combinatorial in nature but many questions about the stochastic properties of these random paths can be solved by combining nice combinatorial features with probabilistic techniques. In the course, we will describe some of the recent techniques developed in the area, including the use of discrete holomorphicity to understand the model on the hexagonal lattice. 
Tuesday 27 March 2018, 11:00 at LPTHE, Library  LPTHEPPH (Particle Physics at LPTHE)  hepph 



Abstract:  Stable string states arise in string models due to Wilson line breaking of GUT symmetries. I will describe a new class of such Wilsonian dark matter states that emerge in string derived Z' model. The string construction utilises the spinorvector duality symmetry to obtain a viable low scale Z' model. I will describe the nature of this duality and its role in the construction of the string model. I will describe the systematic classification tools, developed with Kounnas and Rizos, to analyse large classes of free fermionic string vacua. These methods were instrumental in the discovery of the spinorvector duality, as well as in the trawling of the string Z' model. Other dark matter candidates that arise in string model will be briefly discussed. 
Tuesday 27 March 2018, 11:30 at LPTENS, LPTENS library  STRLPTENSHE (Séminaire commun LPTENS/LPTHE)  hepth 



Abstract:  TBA 
Tuesday 27 March 2018, 14:00 at APC, 483 A  Malevitch  APCTH (Seminar of the theory group of APC)  hepth 


Thursday 29 March 2018, 10:00 at
IHES,
Amphi. Léon Motchane ( Cours de l'IHES )  MATHIHES (TBA)  hepth 


Thursday 29 March 2018, 11:00 at LPTM, 4.13 St Martin II  SEMLPTMUCP (Seminaires du LPTM , Universite de Cergy Pontoise)  mathph 



Abstract:  We study the dynamics of a quantum spin chain (S=1/2 XXZ model), which is prepared at t=0 in a domainwall initial sate, where the spins are initially pointing up on the left halfline and down on the right halfline. Using extensive timedependent DMRG simulations we analyze the evolution of the magnetization profile, as a function of the anisotropy parameter (Delta) the Hamiltonian. These numerical results are compared with the predictions of a recently developed hydrodynamics approach [CastroAlvaredo et al., PRX 2016 and Bertini et al. PRL 2016], which generalizes the conventional hydrodynamics to integrable onedimensional quantum systems. For this domain wall problem the situation of the isotropic Heisenberg model (Delta=1) is particularly interesting and not yet fully understood, and at this point our data suggest a diffusive behavior [Misguich, Mallick & Krapivksy, PRB 2017]. 
Thursday 29 March 2018, 11:00 at LPTHE, bibliothèque  SEMDARBOUX (Séminaire Darboux  physique théorique et mathématiques)  mathmath.MP 


Thursday 29 March 2018, 15:00 at IPHT, Amphi Claude Bloch, Bât. 774  SCOPI (Séminaire SCOPI ParisSaclay)  physics 



Abstract:  MICROSCOPE is a CNES/ONERA space mission that aims to test the Weak Equivalence Principle (WEP) at the $10^15$ level, i.e. two orders of magnitude better than the best onground tests to date. The WEP is the cornerstone of General Relativity, the postulate that led Einstein to establish his theory: it states that all bodies fall at the same rate, independently of their mass and composition. \par Alternative theories of gravity, like those developed to overcome such conundrums as dark energy or the unification of gravity with the forces of the standard model of particle physics, generically predict a small violation of the WEP. \par As a consequence, not only does MICROSCOPE test the very foundation of General Relativity, but it also provides new constraints on theories beyond Einstein's. \par The MICROSCOPE satellite was launched on April 25, 2016. It has since then provided highquality data. The first results (using only a small part of the total data) were announced last December, together with updated constraints on new longrange forces and on the characteristics of string theory's dilaton. In this talk, I will first introduce the MICROSCOPE mission, in particular its scientific goals and measurement principles. I will then present the exploitation phase up to now. Finally, I will discuss its first results. 

[ English version ] 