Thursday 21 September 2023, 10:00 at IHP, Grisvard (314)	RENC-THEO (Rencontres Théoriciennes)	hep-th
Dalimil Mazac ( IPhT )	Conformal measure spaces
Abstract:	The conformal bootstrap equations in any dimension are an infinite set of coupled non-linear equations in infinitely many variables. According to the lore, the solutions of the full set of equations correspond to physical CFTs. At the same time, the only solutions truly known to exist above two dimensions are mean field theories. In this talk, I will discuss conformal measure spaces, which are objects guaranteed to produce solutions of the conformal bootstrap in any dimension. I will review why hyperbolic manifolds give rise to a particular class of conformal measure spaces, and thus to solutions of the complete set of the conformal bootstrap equations. I will then use the bootstrap equations to prove new bounds on the Laplace spectra of hyperbolic manifolds in two and three dimensions. Finally, I will discuss the similarities and differences between these solutions, and those that are believed to arise in physical CFTs.

Thursday 21 September 2023, 11:45 at IHP, Grisvard (314)	RENC-THEO (Rencontres Théoriciennes)	hep-th
Anthony Ashmore ( LPTHE )	Machine learning for geometry and string compactifications
Abstract:	Understanding Calabi-Yau metrics and hermitian Yang-Mills connections has long been a challenge in mathematics and theoretical physics. These geometric objects play a crucial role in constructing realistic models of particle physics in string theory. However, with no closed-form expressions for them, we are unable to compute basic quantities in top-down string models, such as particle masses and couplings. Breakthroughs in machine learning have opened a new path to tackle this problem. After recalling the relationship between these geometric ingredients and 4d effective field theory, I will review recent progress in using machine learning to calculate these metrics and connections numerically. Finally, I will highlight how this newly available geometric data can be used, including studying the spectrum of Laplace-type operators on a Calabi-Yau in the presence of a background gauge field.
Attachments:	Ashmore_slidesIHP.pdf (5466661 bytes)

Monday 25 September 2023, 10:45 at LPTMC, campus Jussieu, couloir 12-13, 5ème, salle 5-23	SEM-LPTMC (Séminaire du Laboratoire de Physique Théorique de la Matière Condensée)	cond-mat
Hadrien Kurkjian ( LPT Toulouse )	The normal Fermi gas: a Fermi liquid?
Abstract:	Landau's Fermi liquid theory provides an effective description of a low-temperature fermionic system in the form of a dilute gas of quasiparticles confined to the Fermi level. It remarkably applies to systems whose microscopic physics is poorly understood, such as Helium-3, and successfully describes their long wavelength collective modes, in particular the phononic ones. However, due to the difficulty of solving exactly the quasiparticle transport equation in the case of arbitrary interactions, Landau's theory is generally used only in the hydrodynamic or collisionless limiting cases. In the case of a two-component ultracold Fermi gas in its normal phase, the simplicity of contact interactions has allowed us to go further and describe the entire transition from the hydrodynamic to the collisionless regime. In the weakly-interacting limit, our results are in excellent quantitative agreement with density-density response measurements performed by the Yale group, where the resonance corresponding to the first sound emerges from the Lindhard function of the non-interacting gas. In time-of-flight images, which enable tomography of the Fermi liquid, this evolution corresponds to a drastic change in the distribution of quasiparticles on the Fermi sphere. While the density response thus seems to be very well described by Landau's theory, I will show that non-Fermi liquid properties appear in the pairing susceptibility, near the superfluid critical temperature.

Monday 25 September 2023, 14:00 at IPHT, Salle Claude Itzykson, Bât. 774	IPHT-STA (Séminaire de Physique Statistique, CEA/Saclay)	cond-mat
Andrea Zoia ( CEA )	TBA

Tuesday 26 September 2023, 10:00 at IPHT, Salle Claude Itzykson, Bât. 774	COURS (Cours)	physics
Jules Lamers (ipht) ( IPhT )	Introduction to quantum integrability
Abstract:	This course will give a pedagogical introduction to (quantum) integrability, a topic in mathematical physics with applications ranging from experiments in condensed-matter physics to high-energy theory. The aim is to show some highlights of the field, with a glimpse of the underlying algebraic structures, while keeping technicalities to a minimum. \\ \\ The provisional plan of the course is as follows

Tuesday 26 September 2023, 11:00 at LPTMS, Salle des séminaires du FAST et du LPTMS, bâtiment Pascal n°530	LPTMS (Séminaire du Laboratoire de Physique Théorique et Modèles Statistiques (Orsay))	cond-mat.stat-mech
Laure Dumaz ( DMA, ENS Paris )	Some aspects of the Anderson Hamiltonian in 1D
Abstract:	In this talk, I will present several results on the Anderson Hamiltonian with white noise potential in dimension 1. This operator formally writes « – Laplacian + white noise ». It arises as the scaling limit of various discrete models and its explicit potential allows for a detailed description of its spectrum. We will discuss localization of its eigenfunctions as well as the behavior of the local statistics of its eigenvalues. Around large energies, we will see that the eigenfunctions are delocalized and follow a universal shape given by the exponential of a Brownian motion plus a drift, a behavior already observed by Rifkind and Virag in tridiagonal matrix models. Based on joint works with Cyril Labbé.

Tuesday 26 September 2023, 14:00 at IPHT, Salle Claude Itzykson, Bât. 774	IPHT-HEP (Séminaire de physique des particules et de cosmologie)	hep-ph
William Derocco ( UC Santa Cruz )	Searching for Sub-nHz Gravitational Waves with Pulsar Parameter Drift
Abstract:	Gravitational waves with frequencies below 1 nHz are notoriously difficult to detect. In this talk, I will present a new means of probing this regime by using secular drifts in observed pulsar timing parameters. I will show the results of searches for both continuous and stochastic signals in this regime and will discuss what future observations may reveal about the signal recently discovered by pulsar timing collaborations at frequencies above a nanohertz.

Thursday 28 September 2023, 11:00 at IHES, Amphithéâtre Léon Mochane	MATH-IHES (TBA)	math
Shuddhodan Kadattur Vasudevan ( IHES )	Perverse Filtration via the Brylinski-Radon Transformation
Abstract:	In 2010, de Cataldo-Miglorini used generic flags to compute the perverse filtration on the cohomology of an affine variety with values in a constructible sheaf. In this talk, I shall introduce the Brylinski-Radon transformation, discuss its properties and derive consequences for the perverse filtration. We shall also discuss some arithmetic applications of our results. This is joint work with Ankit Rai.

Thursday 28 September 2023, 11:00 at LPENS, Salle Dussane, 45 rue d'Ulm	LPENS-MDQ (Séminaire Matériaux et Dispositifs Quantiques du LPENS)	cond-mat
Adrian Bachtold ( ICFO Barcelona )	The sound of tiny guitars approaching the quantum regime
Abstract:	Mechanics has historically played a pivotal role in science by providing the basis for classical physics. Today, with the advent of nanoscale mechanical devices combined with quantum electronic devices, we are witnessing a renaissance in the field of mechanics. After an introductory introduction on the mesoscopic physics of nanomechanical resonators, I will discuss our recent advances on mechanical resonators based on carbon nanotubes. The nanotube in these devices vibrates as a guitar string. Single-electron tunneling enables coupling the mechanical vibrations to electrons by a large amount. I will show how to use this coupling to create a nonlinear mechanical oscillator approaching the quantum regime, where the resulting quantum energy levels of the mechanical oscillator are no longer evenly spaced [1]. Using mechanical nanotubes hosting multiple quantum dots, we expect that our approach may enable the realization of a mechanical qubit [2] and a quantum simulator of quantum matters featuring strong electron-phonon correlations [3,4]. [1] C Samanta, SL De Bonis, CB Møller, R Tormo-Queralt, W Yang, C Urgell, B Stamenic, B Thibeault, Y Jin, DA Czaplewski, F Pistolesi, A Bachtold, (arXiv:2211.07632) Nature Physics (2023). [2] F. Pistolesi, A. N. Cleland, and A. Bachtold, Phys. Rev. X 11, 031027 (2021) [3] U Bhattacharya, T Grass, A Bachtold, M Lewenstein, F Pistolesi, Nano Lett. 21, 9661 (2021) [4] Lin Zhang, Utso Bhattacharya, Adrian Bachtold, Stefan Forstner, Maciej Lewenstein, Fabio Pistolesi, Tobias Grass, npj Quantum Information 9, 7 (2023)

Thursday 28 September 2023, 11:00 at IHP, Amphi Darboux (unusual location !)	SEM-DARBOUX (Séminaire Darboux - physique théorique et mathématiques)	hep-th
Sabin Cautis ( University of British Columbia )	The categorical structure of Coulomb branches
Abstract:	Coulomb branches have recently been given a rigorous mathematical definition by the work of Braverman-Finkelberg-Nakajima. We will discuss their geometric and categorical structure based on recent work with Harold Williams. The Grothendieck groups of these categories recover previously studied algebras such as double affine Hecke algebras (DAHAs), certain open Richardson varieties in affine flag manifolds, multiplicative Nakajima quiver varieties etc. One of our main results is that these Coulomb categories carry a natural t-structure consisting of what we call Koszul-perverse coherent sheaves. The classes of such simple sheaves give a canonical basis of these algebras in a uniform way. These Coulomb categories also carry (conjecturally) a cluster structure. We will survey some of these results as time (and interest) permits. (based on 2306.03023 )

Thursday 28 September 2023, 14:00 at LPENS, E244 (24 Rue Lhomond)	LPTENS-HE (Séminaire commun LPTENS-LPTHE)	hep-th
Hareram Swain ( Indian Inst. Tech., Madras )	A semi-holographic approach to strange metallic behaviour
Abstract:	"Strange metals" exhibit an anomalous temperature dependence of the low temperature resistivity and the measurement of their spectral function via ARPES indicates the breakdown of the conventional quasi-particle picture. In my talk, I will explain how we construct a semi-holographic description for such behaviours where we propose an effective theory in which the electron of a two-dimensional band hybridizes with a fermionic operator of a critical holographic sector, while also interacting with other bands that preserve quasiparticle characteristics. Besides the scaling dimension $\nu$ of the fermionic operator in the holographic sector, the effective theory has two dimensionless couplings $\alpha$ and $\gamma$ determining the holographic and Fermi-liquid-type contributions to the self-energy respectively. In the case of DC conductivity that irrespective of the choice of the holographic critical sector, there exists a ratio of the effective couplings for which we obtain linear-in-T resistivity for a wide range of temperatures. This scaling persists to arbitrarily low temperatures when $\nu$ approaches unity in which limit we obtain a marginal Fermi liquid with a specific temperature dependence of the self-energy. Interestingly, we explain the origin of the linear-in-T resistivity and strange metallic behavior as a consequence of the emergence of a universal form of the spectral function which is independent of the model parameters when the ratio of the two couplings takes optimal values determined only by the critical exponent. This universal form fits well with photoemission data of copper oxide samples for under/optimal/over-doping with a fixed exponent over a wide range of temperatures. We further obtain a refined Planckian dissipation scenario.

Thursday 28 September 2023, 17:00 at CPHT, Amphitheatre Faure (Ecole Polytechnique) ( Friedmann Colloquium Physics Department Ecole Polytechnique )	SEM-EXCEP (Séminaire exceptionel)	hep-th
Slava Mukhanov ( Ludwig-Maximilians-Universitat Munchen )	How predictive are cosmological theories?
Abstract:	I will review the progress achieved in theoretical and observational cosmology over the past 40 years and explain how the theoretical predictions for the temperature fluctuations of the Cosmic Microwave Background, made in 1980, have subsequently been fully confirmed in precision measurements for the last 30 years.

Friday 29 September 2023, 10:00 at IPHT, Salle Claude Itzykson, Bât. 774	COURS (Cours)	physics
Raffaele Tito D`agnolo (ipht) ( IPhT )	Naturalness in Quantum Field Theory after the Large Hadron Collider
Abstract:	The two parameters of the Standard Model that have the strongest impact on physics at low en- ergy are also the most sensitive to the details of the theory at high energies. They are the cos- mological constant and the Higgs boson mass. Experimentally they are both associated to energy scales that we can measure. Theoretically they force us to ask questions about physics at higher energies, possibly all the way to the Planck scale. \\ \\ At the moment it is very hard to understand their measured values. The difficulty has the same origin for both parameters and can be traced to the symmetries of fundamental interactions. A prediction based on symmetry gives results that are 120 orders of magnitude too large for the cosmological constant and 34 orders of magnitude too large for the Higgs mass. The course starts with a brief review that makes the meaning of these estimates precise and discusses to what ex- tent we have a problem in the theory of fundamental interactions. In the first two lectures I will show that the resolution of this apparent failure of symmetry is guaranteed to teach us something qualitatively new about Nature. \\ \\ I will then discuss theories that can successfully predict the value of the Higgs boson mass, includ- ing traditional attempts based on supersymmetry and scale invariance. However, the emphasis will be on modern ideas that involve the evolution of the Universe. I will also review some incom- plete, but stimulating proposals based on more radical departures from standard effective field theory arguments. This includes possible UV/IR mixing effects, inspired by string theory, or the role of non-invertible symmetries. The broader aim of this second part of the course is to give a comprehensive overview of all conceptually distinct possibilities that are still viable experimen- tally. In the process I will comment on which (very small) subset of these ideas can explain also the value of the cosmological constant.

Friday 29 September 2023, 11:00 at CPHT, Salle de conférences du LLR aile 5 (Ecole Polytechnique) ( Friedmann biseminar CPHT-LLR )	SEM-EXCEP (Séminaire exceptionel)	hep-th
Oleg Lebedev ( University of Helsinki )	Gravity, dark matter and its detection
Abstract:	I will discuss gravitational dark matter production in the Early Universe and how it spoils predictivity of most non-thermal dark matter models. I will give an example of a class of models that are free from such uncertainties known as « stronger coupling freeze-in » and discuss observational prospects for this framework.

Friday 29 September 2023, 14:30 at CPHT, Salle de conférences du LLR aile 5 (Ecole Polytechnique)	SEM-EXCEP (Séminaire exceptionel)	hep-th
Slava Mukhanov ( Ludwig-Maximilians-Universitat Munchen )	False vacuum decay: thick wall approximation
Abstract:	I will consider the case where the potential for the scalar field is either unbounded from below or the second minimum corresponding to the true va- cuum has a depth exceeding the height of the potential barrier. In this case, the materialized bubbles dominating the vacuum decay naturally have a thick wall and the thin-wall approximation is not applicable. In such a case the main contribution to the action determining the decay probability of the false va- cuum comes from the part of the solution for which the potential term in the equation for instantons can be neglected compared to the friction term. I show that the developed approximation exactly reproduces the leading order results for the few known exactly solvable potentials.

Monday 2 October 2023, 10:30 at IHES, Amphithéâtre Léon Motchane	MATH-IHES (TBA)	math
Shrawan Kumar ( University of North Carolina, Chapel Hill & IHES )	Verlinde Dimension Formula for the Space of Conformal Blocks and the moduli of G-bundles
Abstract:	Let G be a simply-connected complex simple algebraic group and let C be a smooth projective curve of any genus. Then, the moduli space of semistable G-bundles on C admits so called determinant line bundles. E. Verlinde conjectured a remarkable formula to calculate the dimension of the space of generalized theta functions, which is by definition the space of global sections of a determinant line bundle. This space is also identified with the space of conformal blocks arising in Conformal Field Theory, which is by definition the space of coinvariants in integrable highest weight modules of affine Kac-Moody Lie algebras. Various works notably by Tsuchiya-Ueno-Yamada, Kumar-Narasimhan-Ramanathan, Faltings, Beauville-Laszlo, Sorger and Teleman culminated into a proof of the Verlinde formula. The main aim of this talk is to give a basic outline of the proof of this formula derived from the Propogation of Vacua and the Factorization Theorem among others. The proof requires techniques from algebraic geometry, geometric invariant theory, representation theory of affine Kac-Moody Lie algebras, topology, and Lie algebra cohomology. Some basic knowledge of algebraic geometry and representation theory of semisimple Lie algebras will be helpful; but not required. This lecture should be suitable for any one interested in interaction between algebraic geometry, representation theory, topology and mathematical physics.

Tuesday 3 October 2023, 10:00 at IPHT, Salle Claude Itzykson, Bât. 774	COURS (Cours)	physics
Raffaele Tito D`agnolo (ipht) ( IPhT )	Naturalness in Quantum Field Theory after the Large Hadron Collider
Abstract:	The two parameters of the Standard Model that have the strongest impact on physics at low en- ergy are also the most sensitive to the details of the theory at high energies. They are the cos- mological constant and the Higgs boson mass. Experimentally they are both associated to energy scales that we can measure. Theoretically they force us to ask questions about physics at higher energies, possibly all the way to the Planck scale. \\ \\ At the moment it is very hard to understand their measured values. The difficulty has the same origin for both parameters and can be traced to the symmetries of fundamental interactions. A prediction based on symmetry gives results that are 120 orders of magnitude too large for the cosmological constant and 34 orders of magnitude too large for the Higgs mass. The course starts with a brief review that makes the meaning of these estimates precise and discusses to what ex- tent we have a problem in the theory of fundamental interactions. In the first two lectures I will show that the resolution of this apparent failure of symmetry is guaranteed to teach us something qualitatively new about Nature. \\ \\ I will then discuss theories that can successfully predict the value of the Higgs boson mass, includ- ing traditional attempts based on supersymmetry and scale invariance. However, the emphasis will be on modern ideas that involve the evolution of the Universe. I will also review some incom- plete, but stimulating proposals based on more radical departures from standard effective field theory arguments. This includes possible UV/IR mixing effects, inspired by string theory, or the role of non-invertible symmetries. The broader aim of this second part of the course is to give a comprehensive overview of all conceptually distinct possibilities that are still viable experimen- tally. In the process I will comment on which (very small) subset of these ideas can explain also the value of the cosmological constant.

Tuesday 3 October 2023, 11:00 at LPTMS, Salle des séminaires du FAST et du LPTMS, bâtiment Pascal n°530	LPTMS (Séminaire du Laboratoire de Physique Théorique et Modèles Statistiques (Orsay))	cond-mat.stat-mech
Denis Boyer ( Universidad Nacional Autónoma de México )	TBA

Tuesday 3 October 2023, 14:00 at IHES, Amphithéâtre Léon Motchane	PT-IHES (Séminaire de physique théorique de l'IHES)	hep-th
Thomas Scaffidi ( University of California Irvine )	Complex CFT in the O(n) Loop Model: Numerical Evidence for Loss of Conformality in the Large-n Limit
Abstract:	The presence of nearby complex conformal field theories (CCFTs) hidden in the complex plane of the tuning parameter was recently proposed as an elegant explanation for the ubiquity of "weakly first-order" transitions in condensed matter and high-energy systems. Recently, we have numerically confirmed the presence of such a CCFT in a loop model which derives from a high-temperature formulation of the O(n) model. Surprisingly, we found that the CCFT only survives until n=12.34, beyond which the transfer matrix acquires a gap. In this talk, I will discuss ongoing work in trying to explain this loss of complex conformality at large n, using a mapping to a hard hexagon model for n going to infinity. I will also discuss the connection between the original O(n) model and its loop version and the consequences for CCFTs in these models.

Wednesday 4 October 2023, 11:00 at LKB, Amphi 25 – Sorbonne université – 4 place Jussieu – 75005 Paris	SEM-LKB (Séminaire du Laboratoire Kastler Brossel)	quant-ph
Andrea Cavalleri ( Max Planck Institute for the Structure and Dynamics of Matter, Hamburg, DE )	Recent Progress in Driven Superconductivity
Abstract:	I will discuss how irradiation of certain quantum materials with coherent electromagnetic radiation at Tera-Hertz and mid-infrared frequencies can induce transient high temperature states that have striking similarities with equilibrium superconductors. These phases are observed at base temperatures as high as room temperature, underscoring the ability to impress coherence with light. This talk will also cover our search for new experimental methods that enable the characterization of these transient phases, to measure optical, structural, electrical and magnetic properties at very fast speeds. (https://qcmd.mpsd.mpg.de)

Wednesday 4 October 2023, 12:45 at LPENS, Seminar room, 3 rue d’Ulm, College de France	FORUM-ENS (Forum de Physique Statistique @ ENS)	cond-mat.stat-mech
Thomas Scaffidi ( UC Irvine, University of Toronto )	Solvable models for 2D quantum critical points: Loop soups of 1+1D conformal field theories
Abstract:	For gapped topological systems, there exist several approaches to construct higher-dimensional models by combining lower- dimensional ones (e.g. coupled wires or decorated domain wall constructions). In this talk, we will propose a generalization of this approach to gapless systems. We construct a class of solvable models for 2+1D quantum critical points by attaching 1+1D conformal field theories (CFTs) to fluctuating domain walls. Our local Hamiltonian attaches gapless spin chains to the domain walls of a triangular lattice Ising antiferromagnet. The macroscopic degeneracy between antiferromagnetic configurations is only split by the Casimir energy of each decorating CFT, which is usually thought to be a universal function of the central charge $E_{Cas} = - (\pi c)/(3 L)$. Remarkably, we found several examples of 1D Hamiltonians realizing CFTs for which the Casimir energy is positive (i.e. $c<0$ in the last formula, making it favorable for domain walls to condense into a single self-avoiding random walk (or "snake") visiting every site of the 2D lattice. Since the snake is macroscopically long, the CFT living on it has a vanishing gap, and the resulting 2+1D theory is thus gapless. We obtain predictions for critical exponents and for entanglement by combining results about 1+1D CFTs and about the statistical fluctuations of the snake (which are described by the $O(n=0)$ fully-packed loop model). We show that the area law for entanglement is restored for the 2D state (despite the log term for the entanglement of the 1+1D CFTs) but that it is non-local in nature. Finally, we provide a verification of our predictions based on Monte Carlo calculations.

Thursday 5 October 2023, 14:00 at LPTMC, Jussieu, towers 12-13, 5th floor, LPTMC seminar room	SEM-EXCEP (Séminaire exceptionel)	cond-mat
Leonardo Mazza ( LPTMS Orsay )	A spin-statistics relation for the anyonic quasiparticles of the quantum-Hall effect
Abstract:	The spin-statistics relation is a pillar of our description of the world. In this seminar, I will show that it is possible to introduce a measurable spin also for the quasiparticles of the quantum Hall effect, and that this spin satisfies a spin-statistics relation. I will discuss this idea in several explicit cases (including the case of Laughlin's quasielectron) and lay down a sketch of the proof. [1,2] Finally, I will conclude the seminar discussing the entanglement properties of FQHE wavefunctions using a Bisognano-Wichmann Hamiltonian (see Ref. [3] for a recent experiment in the non-interacting case). References: [1] Nardin, Ardonne, LM, Spin-statistics relation for quantum Hall states, https://arxiv.org/abs/2211.07788 [2] Nardin, LM, Laughlin's quasielectron is a non-local composite fermion, https://arxiv.org/abs/2306.13972 [3] Redon, Liu, Bouhiron, Mittal, Fabre, Lopes, Nascimbene, Realizing the entanglement Hamiltonian of a topological quantum Hall system, https://arxiv.org/abs/2307.06251

Thursday 5 October 2023, 14:00 at IJCLAB, Salle A018 (IJCLab)	NUC-THEO (Séminaire de physique nucléaire théorique)	nucl-th
Sławomir Wycech ( NCBJ Warsaw )	Pionisation: a method to study nuclear surface
Abstract:	CERN experiment - PUMA - is going to form anti-protonic atoms of unstable nuclei. The purpose is to detect $\pi$-mesonic decays of such atoms and extract the neutron excess at nuclear surfaces
Attachments:	Abstract.pdf (142597 bytes)

Monday 9 October 2023, 14:00 at IHES, Bâtiment Alix et Marwan Lahoud	PT-IHES (Séminaire de physique théorique de l'IHES)	hep-th
Balt Van Rees ( Ecole polytechnique )	Perturbative RG Flows in AdS
Abstract:	We discuss general properties of perturbative RG flows in AdS with a focus on the treatment of boundary conditions and infrared divergences. In contrast with flat-space boundary QFT, general covariance in AdS implies the absence of independent boundary flows. We illustrate how boundary correlation functions remain conformally covariant even if the bulk QFT has a scale. We apply our general discussion to the RG flow between consecutive unitary diagonal minimal models which is triggered by the φ(1,3) operator. For these theories we conjecture a flow diagram whose form is significantly simpler than that in flat-space boundary QFT.

Monday 9 October 2023, 14:00 at IHES, Amphithéâtre Léon Motchane ( Séminaire Géométrie et groupes discrets )	MATH-IHES (TBA)	math
François Labourie ( Université Côte d'Azur )	Positivity, Cross-ratios and the Collar Lemma
Abstract:	I will explain why positive representations of fundamental groups of surfaces satisfy a "collar lemma" similar to the classical collar lemma for hyperbolic geometry, and have associated positive cross-ratios. As a consequence, I will deduce that positive representations form closed subsets of the representation variety. I will spend some time recalling what a positive representation is, what the associated cross-ratios are, and explain the main new object that we shall use and that we call "photons". This is joint work with Jonas Beyrer, Olivier Guichard, Beatrice Pozzetti and Anna Wienhard.

Monday 9 October 2023, 16:00 at IHES, Amphithéâtre Léon Motchane ( Séminaire Géométrie et groupes discrets )	MATH-IHES (TBA)	math
Julien Marché ( IMJ-PRG )	Signatures of Quantum Representations and 2-bridge Knots
Abstract:	Quantum representations are families of finite-dimensional representations of mapping class groups satisfying strong compatibility conditions. One of the most well-known (the so-called SO(3)-TQFT) depends on a parameter q which is a root of unity of order 2r (r odd). These representations preserve a Hermitian form: recently, with B. Deroin, we explained how to compute its signature (among other things). More recently, I observed that this computation is related to the trace field of the 2-bridge knot K(r,s) where q=exp(iπs/r). During the talk, I will explain this relation and the objects involved in it.

Tuesday 10 October 2023, 11:00 at LPTMS, Salle des séminaires du FAST et du LPTMS, bâtiment Pascal n°530	LPTMS (Séminaire du Laboratoire de Physique Théorique et Modèles Statistiques (Orsay))	cond-mat.stat-mech
Alessandra Carbone ( IBPS Sorbonne )	TBA

Wednesday 11 October 2023, 10:00 at IPHT, Salle Claude Itzykson, Bât. 774	COURS (Cours)	physics
Raffaele Tito D`agnolo (ipht) ( IPhT )	Naturalness in Quantum Field Theory after the Large Hadron Collider
Abstract:	The two parameters of the Standard Model that have the strongest impact on physics at low en- ergy are also the most sensitive to the details of the theory at high energies. They are the cos- mological constant and the Higgs boson mass. Experimentally they are both associated to energy scales that we can measure. Theoretically they force us to ask questions about physics at higher energies, possibly all the way to the Planck scale. \\ \\ At the moment it is very hard to understand their measured values. The difficulty has the same origin for both parameters and can be traced to the symmetries of fundamental interactions. A prediction based on symmetry gives results that are 120 orders of magnitude too large for the cosmological constant and 34 orders of magnitude too large for the Higgs mass. The course starts with a brief review that makes the meaning of these estimates precise and discusses to what ex- tent we have a problem in the theory of fundamental interactions. In the first two lectures I will show that the resolution of this apparent failure of symmetry is guaranteed to teach us something qualitatively new about Nature. \\ \\ I will then discuss theories that can successfully predict the value of the Higgs boson mass, includ- ing traditional attempts based on supersymmetry and scale invariance. However, the emphasis will be on modern ideas that involve the evolution of the Universe. I will also review some incom- plete, but stimulating proposals based on more radical departures from standard effective field theory arguments. This includes possible UV/IR mixing effects, inspired by string theory, or the role of non-invertible symmetries. The broader aim of this second part of the course is to give a comprehensive overview of all conceptually distinct possibilities that are still viable experimen- tally. In the process I will comment on which (very small) subset of these ideas can explain also the value of the cosmological constant.

Wednesday 11 October 2023, 13:30 at DPT-PHYS-ENS, ConfIV (E244) - Dépt de Physique de l'ENS - 24 rue Lhomond 75005 PARIS	COLLOQUIUM-ENS (Colloquium of the Physics Department of ENS)	physics
Yiwen Chu ( ETH Zurich )	The quantum world of massive mechanical objects
Abstract:	One of the first model systems we encounter in quantum mechanics class is a mass on a spring. However, in practice, it is not easy to observe a massive mechanical object exhibiting the quantum properties of a harmonic oscillator, such as zero-point fluctuations, energy quantization, or quantum superpositions. Nevertheless, in recent years, it has become possible to control and the measure the quantum states of the motion of macroscopic mechanical objects. I will present our recent experiments on creating “Schrödinger cat” states in a bulk acoustic wave resonator by coupling it to a superconducting circuit, and how we can observe the quantum ground state of a several hundred microgram mechanical mode using light. I will also discuss the applications of these systems in quantum information, quantum sensing, and explorations of fundamental physics.

Thursday 12 October 2023, 14:00 at LPTMC, Jussieu, towers 12-13, 5th floor, LPTMC seminar room	SEM-LPTMC (Séminaire du Laboratoire de Physique Théorique de la Matière Condensée)	cond-mat
Clément Delcamp ( Université de Gand )	TBA
Abstract:	TBA