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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
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Monday 30 November 2020, 10:45 at LPTMC, On-line zoom seminar
( ID de réunion : 947 4225 4686 Code secret : 769824 )
SEM-LPTMC (Séminaire du Laboratoire de Physique Théorique de la Matière Condensée) cond-mat.mes-hall
Owen Benton ( MPIPKS Dresden ) Exactly solvable spin-1/2 models with highly-degenerate, partially ordered, ground states
Abstract: Magnetic ground states tend to be divided into ordered and spin liquid phases. Ordered phases have spontaneous symmetry breaking and sharp excitations, while spin liquids are symmetric, and have excitation continua. Increasingly, however, it is being realized that the sharp division between these two types of magnetic matter may make sometimes break down. This is particularly the case in certain classical magnets which exhibit the phenomenon of “moment fragmentation” in which antiferromagnetic order coexists with a type of spin liquid. It is natural to ask whether similar physics, mixing ordered and spin liquid-like behaviour can occur in a quantum magnet? And, if so, are there simple, tractable models in which this occurs? In this talk, I will introduce a family of exactly solvable S=1/2 anisotropic exchange models, with highly degenerate, partially ordered, ground states. This set of models involves only nearest neighbour interactions of a simple “XYZ” form. The ground states have a degeneracy which grows exponentially with system size, and are partially ordered, in the sense that infinite-range correlations of some spin components coexist with a macroscopic number of undetermined degrees of freedom. This new family of models thus establishes a starting point for the investigation of quantum systems in between spin liquids and magnetic order.

Monday 30 November 2020, 11:00 at IAP, Webinar (contact for details) SEM-GRECO (Séminaire du GReCO : groupe de GRavitation et COsmologie à l'IAP) astro-ph
Aurélien Pascal ( LUTH, Observatoire de Paris ) The role of neutrinos in supernovae and neutron star formation
Abstract: If the main features of the core-collapse supernova mechanism are now understood, some important details about the microphysics are still subject to a lot of uncertainties. In particular, neutrinos are playing a key role in the core-collapse mechanism but interactions rates between neutrinos and dense matter are still poorly understood. In this talk, I will start by reviewing what we actually know about core-collapse supernovae and neutron star formation by focusing on the role of neutrinos. Then I will present a code for proto-neutron star evolution and some studies we have been conducting on the influence of neutrino - dense matter interactions on core-collapse and (proto-)neutron star cooling.

Thursday 3 December 2020, 15:00 at IHP, Zoom
( The zoom link will be posted here one hour before the talk. )
RENC-THEO (Rencontres Théoriciennes) hep-th
Justin Kaidi ( Simons Center for Geometry snd Physics ) Adventures in Non-Supersymmetric String Theory
Abstract: It has long been known that there exist strings with supersymmetry on the world sheet, but not in spacetime. These include the well-known 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 non-SUSY theories. One of the reasons why non-SUSY strings have remained understudied is the fact that nearly all of them have closed string tachyons, and hence do not admit ten-dimensional flat space as a stable vacuum. The goal of this talk is two-fold. 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 non-SUSY strings. Having done so, we will show that for all non-SUSY theories in which a tachyon exists, it can be condensed to give a stable lower-dimensional vacuum. In many cases, these stable vacua will be two-dimensional string theories already known in the literature.

Friday 4 December 2020, 14:00 at LPTENS, GoToMeeting
( Connection details posted somewhat soon )
STR-LPT-ENS-HE (Séminaire commun LPTENS/LPTHE) hep-th
Bruno Le Floch ( Institut Philippe Meyer ) Universal scattering laws for quiescent bouncing cosmology
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 theory-dependent "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 non-homogeneous 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). (based on 2006.08620 )

Monday 7 December 2020, 14:00 at IPHT, Salle Claude Itzykson, Bât. 774 IPHT-STA (Séminaire de Physique Statistique, CEA/Saclay) cond-mat
Marc Barthelemy ( IPhT ) Modeling the growth of cities
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 Zipf’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 finite-time effects, Zipf’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, 397-401 (2020).

Monday 7 December 2020, 17:00 at LPTMC, On-line zoom seminar
( ID de réunion : 953 4888 0802 Code secret : 035303 )
SEM-LPTMC (Séminaire du Laboratoire de Physique Théorique de la Matière Condensée) cond-mat.mes-hall
Maxime Dupont ( U. C. Berkeley ) Hydrodynamics in Quantum Magnets
Abstract: Some quantum many-body systems display emergent coarse-grained 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 fluid-like 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 theoretical-experimental collaboration [2] observing with neutrons one of these regimes in a nearly-ideal antiferromagnetic spin-1/2 Heisenberg chain material (KCuF_3). We found that the spin dynamics belong to the Kardar-Parisi-Zhang 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

Tuesday 8 December 2020, 16:00 at IHP, Seminar via zoom P^3 (Particle Physics in Paris) astro-ph|hep-ph|hep-th
Kaustubh Agashe ( University of Maryland ) Is the Higgs boson composite?
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.

Friday 11 December 2020, 14:00 at LPTENS, GoToMeeting
( Connection details posted here shortly before the talk )
STR-LPT-ENS-HE (Séminaire commun LPTENS/LPTHE) hep-th
Miguel Paulos ( ENS, CNRS ) Landau diagrams in AdS and S-matrices from conformal correlators
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 S-matrix from such correlators. We discuss a particularly simple position-space procedure to do so. It features a direct map from boundary positions to (on-shell) momenta and thereby relates cross ratios to Mandelstam invariants. This recipe succeeds in several examples, includes the momentum-conserving 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 flat-space 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 on-shell 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 four-point triangle diagram at the boundary of a bad region. (based on 2007.13745 )

Monday 14 December 2020, 16:00 at LPTMC, On-line zoom seminar
( ID de réunion : 969 0302 5813 Code secret : 007425 )
SEM-LPTMC (Séminaire du Laboratoire de Physique Théorique de la Matière Condensée) cond-mat.mes-hall
Riccardo Rossi ( Simons Foundation, Flatiron, New-York ) TBA

Tuesday 12 January 2021, 17:00 at IHP, Seminar via zoom P^3 (Particle Physics in Paris) hep-th
John Terning ( UC Davis ) TBA

Thursday 21 January 2021, 11:00 at IHP, (video) RENC-THEO (Rencontres Théoriciennes) hep-th
Rajesh Gopakumar ( ICTS Bangalore ) TBA

Tuesday 26 January 2021, 16:00 at IHP, Seminar via zoom P^3 (Particle Physics in Paris) astro-ph|hep-ph
Pedro Machado ( Fermilab ) New ideas and opportunities at future liquid argon neutrino experiment
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.

Thursday 28 January 2021, 11:00 at IHP, (video) RENC-THEO (Rencontres Théoriciennes) hep-th
Eric Perlmutter ( Caltech & IPhT ) TBA

Thursday 4 February 2021, 11:00 at APC, bibliothèque du LPTHE, tour 13-14, 4eme étage SEM-DARBOUX (Séminaire Darboux - physique théorique et mathématiques) hep-th
Johannes Kellendonk ( Institut Camille Jordan, Université Claude Bernard Lyon 1 ) TBA

Friday 5 March 2021, 11:00 at APC, APC-COLLOQUIUM (Colloquium de l'APC) hep-th
Mikhail Shaposhnikov ( EPFL Lausanne ) TBA
Abstract: TBA

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