Le serveur de séminaires SEMPARIS accueille toutes les annonces de séminaires de la région parisienne, dans tous les domaines de la physique et des mathématiques. Il vous permet aussi de recevoir par courrier électronique la sélection de votre choix. Enfin, il permet l'archivage des présentations au format PDF ou Powerpoint. [ Plus d'informations ]
Séminaires à venir  
[ vue agenda ] 
Jeudi 3 Décembre 2020, 15:00 à
IHP,
Zoom ( The zoom link will be posted here one hour before the talk. )  RENCTHEO (Rencontres Théoriciennes)  hepth 



Abstract:  It has long been known that there exist strings with supersymmetry on the world sheet, but not in spacetime. These include the wellknown Type 0 strings, as well as a series of seven heterotic strings, all of which are obtained by imposing unconventional GSO projections. Besides these classic examples though, relatively little is known about the full space of nonSUSY theories. One of the reasons why nonSUSY strings have remained understudied is the fact that nearly all of them have closed string tachyons, and hence do not admit tendimensional flat space as a stable vacuum. The goal of this talk is twofold. First, using recent advances in condensed matter theory, we will reinterpret GSO projections in terms of topological phases of matter, thereby providing a framework for the classification of nonSUSY strings. Having done so, we will show that for all nonSUSY theories in which a tachyon exists, it can be condensed to give a stable lowerdimensional vacuum. In many cases, these stable vacua will be twodimensional string theories already known in the literature. 
Vendredi 4 Décembre 2020, 14:00 à
LPTENS,
GoToMeeting ( https://global.gotomeeting.com/join/819043301 )  STRLPTENSHE (Séminaire commun LPTENS/LPTHE)  hepth 



Abstract:  Modifications to 3+1d general relativity (GR) at high curvatures can eliminate the Big Bang singularity in favor of a bounce. Abstracting away microscopic details of the bounce, the spacetime is simply a GR solution on both sides of a singularity hypersurface, with some theorydependent "singularity scattering map" relating the asymptotic metrics on both sides. The asymptotic metric near a singularity was studied by Belinsky, Khalatnikov and Lifshitz (BKL) and I will explain their finding that the time evolution at different points decouples. Motivated by this ultralocality property, we classify (in the absence of BKL oscillations) all singularity scattering maps that are ultralocal. By matching previous calculations on homogeneous spacetimes in f(R) gravity and in loop quantum cosmology with our classification we obtain a prediction for nonhomogeneous bounces in these theories. Lastly, we construct a class of cyclic spacetimes by solving for the collision of plane gravitational waves (which may create infinitely many successive singularities). (basé sur 2006.08620 ) 
Lundi 7 Décembre 2020, 14:00 à IPHT, Salle Claude Itzykson, Bât. 774  IPHTSTA (Séminaire de Physique Statistique, CEA/Saclay)  condmat 



Abstract:  The most fundamental problem for constructing a science of cities is to understand the hierarchical organization of city population and the statistical occurrence of megacities. This was first thought to be described by a universal principle known since almost a century as Zipfâ€™s law. However, the validity of this model has been challenged by recent empirical studies. In addition, a theoretical model must also be able to explain the rises and falls of cities and civilizations. Despite many attempts that I will briefly review here (the Gibrat and Gabaix models), these fundamental questions have not yet been satisfactorily answered. In this talk, starting from an empirical analysis of recent datasets (for Canada, France, the UK and the USA) I will derive a stochastic equation with multiplicative noises for modelling population growth in cities. This model reveals how rare, but large, interurban migratory shocks dominate city growth and predicts a complex shape for the distribution of city population. It also shows that, owing to finitetime effects, Zipfâ€™s law does not hold in general, implying a more complex organization of cities. It also predicts the existence of multiple temporal variations in the city hierarchy, in agreement with empirical observations. Reference: V. Verbavatz, M. Barthelemy, "The growth equation of cities", Nature 587, 397401 (2020). 
Lundi 7 Décembre 2020, 17:00 à
LPTMC,
Online zoom seminar ( https://zoom.us/j/95348880802 ID de réunion : 953 4888 0802 Code secret : 035303 )  SEMLPTMC (Séminaire du Laboratoire de Physique Théorique de la Matière Condensée)  condmat.meshall 



Abstract:  Some quantum manybody systems display emergent coarsegrained hydrodynamic behavior. One can make an analogy with the phenomenological derivation of classical fluid dynamics, based on the continuity equations of conserved quantities (such as mass, energy, or momentum). Depending on the intrinsic quantum conservation laws of the system, one expects different kinds of fluidlike behaviors. Remarkably, a class of 1D quantum systems, known as integrable, possesses an infinite number of nontrivial conserved quantities. They can endow them with peculiar transport properties, especially at high temperatures. In this talk, I will present to you how the different hydrodynamic regimes in 1D quantum magnets are classified [1]. I will also discuss a recent theoreticalexperimental collaboration [2] observing with neutrons one of these regimes in a nearlyideal antiferromagnetic spin1/2 Heisenberg chain material (KCuF_3). We found that the spin dynamics belong to the KardarParisiZhang universality class in 1+1 dimensions, confirming the recent theoretical conjecture. [1] M. Dupont and J. E. Moore, Phys. Rev. B 101, 121106(R) (2020) [2] A. Scheie, et al., arXiv:2009.13535 
Mardi 8 Décembre 2020, 14:00 à APC, https://unige.zoom.us/j/93722626572  APC (Séminaires du laboratoire Astroparticule et Cosmologie (APC)) , APCCOLLOQUIUM (Colloquium de l'APC) , APCTH (Seminar of the theory group of APC)  grqc 



Abstract:  In the primordial Universe, neutrino decoupling occurs only slightly before electronpositron annihilations. This leads to an increased neutrino energy density with order 1% spectral distortions compared to the standard instantaneous decoupling approximation. A precise calculation of neutrino evolution is needed to assess its consequences on BBN, structure formation or on the CMB, and requires to take into account multiple effects such as neutrino oscillations, which represents a genuine numerical challenge. In this talk, I will review the progress made in the last three decades towards such an accurate prediction, and present results including for the first time the full collision term which accounts for neutrino mixing. A particular focus will be placed on the physical effect of neutrino oscillations, which allows for an approximate numerical scheme of striking accuracy. 
Mardi 8 Décembre 2020, 16:00 à IHP, Seminar via zoom https://ijclab.zoom.us/j/94900481972  P^3 (Particle Physics in Paris)  astrophhepphhepth 



Abstract:  In the Standard Model of particle physics, a condensate of the Higgs boson determines the range of the weak nuclear force. However, one finds that quantum corrections generically shift this range to a much smaller value than what is observed. This "hierarchy problem'' can be solved by postulating that the Higgs boson is a composite particle, made up of constituents which are tightly bound by a new force. Such a framework necessitates that the closely related top quark is also composite. After briefly discussing the modeling of such a mechanism, I will describe in more detail how we can test this idea in a wide variety of experiments. These signals of Higgs/top compositeness include direct production of the associated new, heavy composite particles at the LHC, as well as modifications of the properties of the Higgs boson and the top quark themselves due to their composite nature. In this model, the idea of grand unification of the fundamental forces works very well and also naturally leads to an exotic particle that may be the dark matter of the universe. Such ambient dark matter can be detected and also produced at colliders in distinctive ways. I will highlight how some of this phenomenological work has triggered the development of novel experimental strategies, which have subsequently found applicability even beyond testing this framework. Time permitting, I will also briefly discuss the cosmological transition from the phase where the relevant degrees of freedom are the constituents of the Higgs boson to the one with bound states. 
Vendredi 11 Décembre 2020, 14:00 à
LPTENS,
GoToMeeting ( Connection details posted here shortly before the talk )  STRLPTENSHE (Séminaire commun LPTENS/LPTHE)  hepth 



Abstract:  Quantum field theories in AdS generate conformal correlation functions on the boundary, and in the limit where AdS is nearly flat one should be able to extract an Smatrix from such correlators. We discuss a particularly simple positionspace procedure to do so. It features a direct map from boundary positions to (onshell) momenta and thereby relates cross ratios to Mandelstam invariants. This recipe succeeds in several examples, includes the momentumconserving delta functions, and can be shown to imply the two proposals in arXiv:1607.06109 based on Mellin space and on the OPE data. Interestingly the procedure does not always work: the Landau singularities of a Feynman diagram are shown to be part of larger regions, to be called `bad regions', where the flatspace limit of the Witten diagram diverges. To capture these divergences we introduce the notion of Landau diagrams in AdS. As in flat space, these describe onshell particles propagating over large distances in a complexified space, with a form of momentum conservation holding at each bulk vertex. As an application we recover the anomalous threshold of the fourpoint triangle diagram at the boundary of a bad region. (basé sur 2007.13745 ) 
Lundi 14 Décembre 2020, 16:00 à
LPTMC,
Online zoom seminar ( https://zoom.us/j/96903025813 ID de réunion : 969 0302 5813 Code secret : 007425 )  SEMLPTMC (Séminaire du Laboratoire de Physique Théorique de la Matière Condensée)  condmat.meshall 



Abstract:  In this talk I show how perturbation theory can be turned into a viable computational approach for physical systems afflicted by the fermionic sign problem. This is accomplished by designing new numerical approaches to reach arbitraryhigh orders for the bare [1] and the renormalized [2] expansion. I discuss the results obtained for the doped squarelattice Hubbard model in the pseudogap regime, and in frustrated lattices. Finally, I present the first unbiased diagrammatic computation in a brokensymmetry phase by discussing the swave superfluid transition in the spinpolarized cubiclattice attractive Hubbard model. [1] RR, PRL 119, 045701 (2017) [2] RR, Simkovic, Ferrero, EPL 132, 11001 (2020) 
Mardi 12 Janvier 2021, 17:00 à IHP, Seminar via zoom https://ijclab.zoom.us/j/94900481972  P^3 (Particle Physics in Paris)  hepth 


Vendredi 15 Janvier 2021, 14:00 à LPTENS, GoToMeeting  STRLPTENSHE (Séminaire commun LPTENS/LPTHE)  hepth 



Abstract:  We review unitarity and crossing constraints on scattering amplitudes for particles with spin in four dimensional quantum field theories. As an application we study two to two scattering of neutral spin 1/2 fermions in detail. Assuming Mandelstam analyticity of its scattering amplitude, we use the numerical Smatrix bootstrap method to estimate various nonperturbative bounds on quartic and cubic (Yukawa) couplings. (basé sur 2011.11708 ) 
Jeudi 21 Janvier 2021, 11:00 à IHP, (video)  RENCTHEO (Rencontres Théoriciennes)  hepth 


Mardi 26 Janvier 2021, 16:00 à IHP, Seminar via zoom https://ijclab.zoom.us/j/94900481972  P^3 (Particle Physics in Paris)  astrophhepph 



Abstract:  In this talk I will highlight some special experimental capabilities of future liquid argon time projection chamber (LArTPCs) neutrino experiments like the Fermilab’s Short Baseline Neutrino Program and DUNE. I will give a broad overview of what could be gained with these capabilities. Then, I will provide concrete physics opportunities that are enabled by LArTPCs, including measuring the CP violation phase with atmospheric neutrinos, searching for light dark matter, and performing precision electroweak physics measurements. 
Jeudi 28 Janvier 2021, 11:00 à IHP, (video)  RENCTHEO (Rencontres Théoriciennes)  hepth 


Lundi 1 Fevrier 2021, 10:45 à
LPTMC,
online zoom seminar? ( https://zoom.us/j/95015087598 ID de réunion : 950 1508 7598 Code secret : 599922 )  SEMLPTMC (Séminaire du Laboratoire de Physique Théorique de la Matière Condensée)  condmat.meshall 



Abstract:  Our modern understanding of universality, namely the independence of the critical properties of a system from its microscopic details, is based on the ideas put forward by K. Wilson and formalised within the framework of the renormalization group (RG). Despite the centrality of the subject in modern days theoretical Physics and the many decades passed since Wilson's original works, the classification of universality classes is to date largely unsolved. In recent years, however, the functional reformulation of the perturbative RG provided a simple tool to approach such a classification, extending significantly our knowledge of universality classes entailed by discrete symmetry groups in scalar field theories. In this talk, I will first introduce the technique, elucidate its keypoint features and explain why it is so effective in charting the theory space. In passing, I briefly review the theory of invariant polynomials paying particular attention to the socalled Hilbert (or Molien) series. I will then focus on three particular cases: the scalar field theories endowed with the symmetry group of regular polytopes, the randomly diluted Ising models and the more general hypercubic scalar field theories. For each of these three cases, I review the main results and suggest possible lines of future investigations. References: https://arxiv.org/abs/1902.05328 https://arxiv.org/abs/2006.12808 
Jeudi 4 Fevrier 2021, 11:00 à APC, bibliothèque du LPTHE, tour 1314, 4eme étage  SEMDARBOUX (Séminaire Darboux  physique théorique et mathématiques)  hepth 


Mardi 9 Fevrier 2021, 16:00 à
IHP,
Seminar via zoom https://ijclab.zoom.us/j/94900481972 ( This seminar will be followed by the one of Constantinos Skordis (University of Cyprus and Czech Academy of Sciences) at 16:30 )  P^3 (Particle Physics in Paris)  astrophhepphhepth 


Mardi 9 Fevrier 2021, 16:30 à
IHP,
Seminar via zoom https://ijclab.zoom.us/j/94900481972 ( This seminar follows the one at 16:00 by David Spergel (Princeton University) )  P^3 (Particle Physics in Paris)  astrophhepphhepth 


Mardi 23 Fevrier 2021, 16:00 à IHP, Seminar via zoom https://ijclab.zoom.us/j/94900481972  P^3 (Particle Physics in Paris)  hepphhepth 


Vendredi 5 Mars 2021, 11:00 à APC, https://uparis.zoom.us/j/84776224570?pwd=ZWNLMEQ0bThsWGJuL0IvbnUxTEpxUT09  APCCOLLOQUIUM (Colloquium de l'APC)  hepth 



Abstract:  TBA 
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