Théorie des cordes
Group Directory  Research  Seminars  Job offers 
Coordinator: Marios Petropoulos

Permanent members
Guillaume Bossard
Emilian Dudas
Blaise Goutéraux
Hervé Partouche
Andrea Puhm

PhD students
Mathieu Beauvillain
Gabriele Casagrande
Victor Franken
Clément Supiot

Postdocs
Adrien Fiorucci
Mikel Sanchez Garitaonandia
Group research activities
News
String Theory seminar
Tuesday, October 1 at 11:00
Conference room Louis Michel (CPHT)
Jibril Ben Achour (Munich U.)
Displacement versus velocity memory effects and symmetries in vacuum gravitational plane wave
Abstract: ppwaves are the simplest (and the first obtained) nonlinear radiative solution in general relativity. The realization of memory effects induced by vacuum gravitational plane wave (VGPW) (a specific subset of ppwaves) on a couple of test particles has been extensively study (https://arxiv.org/abs/1704.05997, https://arxiv.org/abs/1705.01378, https://arxiv.org/abs/1803.09640). So far, the work has focused on the velocity memory effect and until recently, it was believed that no displacement memory could be realized in such radiative geometry. In this talk, I will show that both displacement and velocity memories can be induced by a VGPW and I will present the first general classification for the conditions under which this happens. This classification holds for both pulse and step profiles (the latter case being the standard ad hoc model for implementing memory contribution to the waveform profile. See https://arxiv.org/abs/1108.3121) and allows to understand the numerical examples presented recently in https://arxiv.org/abs/2405.12928. Along the way, I will review several key results to analyze the geodesic motion and geodesic deviation equation in VGPW, and in particular the explicit and hidden symmetries of these geometries. I will further show that polarized VGPW enjoys a new type of symmetry under Möbius reparametrization of the null time (which turns out to be a more general property inherent to any null hypersurface). I will also review some theorems relating hidden symmetries (under Killing tensors) and the solution space of the geodesic deviation equation used to identify the memory effects. This talk is based on the recent article: https://inspirehep.net/literature/2796995

String Theory seminar
Tuesday, April 9 at 11:00
Conference room Louis Michel (CPHT)
Alexander Soloviev (U. Ljubljana)
Thermoelectric transport and the spectra of conserved operators in RTA kinetic theory
Abstract: The relaxation time approximation (RTA) of the kinetic Boltzmann equation is one of the simplest windows into the microscopic properties of collective realtime transport. In the context of the RTA with classical massless particles, I will discuss the analytically computed retarded twopoint Green's functions of the energymomentum tensor and a conserved current in thermal states at nonzero density, and in the absence/presence of broken translational symmetry. Furthermore, I will provide an overview of the analytic structure of the different correlators and the transport properties that they imply, such as the thermoelectric conductivities and the quasihydrodynamic dispersion relations.

String Theory seminar
Monday, March 11 at 11:00
Conference room Louis Michel (CPHT)
Olaf Hohm (Hamburg U.)
From Gauge Theory to Gravity via Homotopy Algebras
Abstract: I begin with a selfcontained introduction to Homotopy algebras, which are generalizations of familiar structures such as Lie or associative algebras that in physics emerged in string theory but that more recently have begun to be recognized as the underlying structure of general classical and quantum field theories. This framework allows one, in particular, to formulate two deep connections between gauge theories such as YangMills theory and gravity, as a first step toward a firstprinciple derivation: These are, first, the socalled double copy relations between the scattering amplitudes of gauge theory and of gravity and, second, the holographic or AdS/CFT relation between a gravity theory on AdS and a dual CFT on the boundary. 
String Theory seminar
Wednesday, March 6 at 11:00
Conference room Louis Michel (CPHT)
Carlo Heissenberg (Queen Mary U.)
An eikonal approach to gravitational scattering and waveforms
Abstract: The classical limit of scattering amplitudes offers a convenient strategy to calculate gravitationalwave observables for binary processes in the postMinkowskian (PM) regime, in which the two objects are far apart and interact weakly. In this talk I will discuss how the eikonal exponentiation offers a simple and conceptually transparent framework to exploit this connection and calculate key gravitational observables from amplitudes: the deflection angle for twobody encounters, energy and angular momentum losses, as well as the emitted gravitational waveform itself. The latter emerges in particular from the 2to3 amplitude for the scattering of two massive scalars and the emission of a graviton. I will briefly illustrate the calculation of its oneloop contribution, which is the key ingredient to obtain the first PM correction to the classic result obtained by Kovacs and Thorne in the 70s. Moreover, I will show how the choice of asymptotic BMS frame is crucial in order to compare the resulting amplitudebased waveform with the multipolar postNewtonian (PN) one, in the smallvelocity and soft limits, finding agreement up to 3PN order.

String Theory seminar
Thursday, February 1 at 14:00
Conference room Louis Michel (CPHT)
Javier G. Subils (Utrecht University) Confining superfluids and deconfined ferromagnets Abstract: We present the phase diagram of a confining threedimensional gauge theory with holographic dual, corresponding to a known string theory solution. The theory possesses a global U(1) baryonic symmetry, and we discovered two different phases at finite temperature and baryonic chemical potential, separated by a line of first order phase transitions. I will explain how we found this result by means of a dual description in terms of monopoles, and argue that the phases that are realised correspond to a confined superfluid and a deconfined ferromagnet. 
String Theory seminar
Wednesday, December 13 at 15:00
Conference room Louis Michel (CPHT)
Celine Zwikel (Perimeter Institute) Leaky boundary conditions for JT gravity
Abstract: Leaky boundary conditions are unavoidable when considering radiation escaping at the asymptotic boundary of spacetimes. In this talk I describe leaky boundary conditions for JT gravity with conformal matter. I then discuss the implications on the path integral measure and on the Gauss law.

String Theory seminar
Wednesday, November 8 at 11:00
Conference room Louis Michel (CPHT)
Timotej Lemut (Ljubljana University)
Reconstructing the quasinormal spectrum from poleskipping
Abstract: The holographic gauge/gravity duality provides an explicit reduction of quantum field theory (QFT) calculations in the semiclassical largeN limit to sets of `gravitational' differential equations whose analysis can reveal all details of the spectra of thermal QFT correlators. We argue that in certain cases, a complete reconstruction of the spectrum and of the corresponding correlator is possible from only the knowledge of an infinite, discrete set of poleskipping points traversed by a single (hydrodynamic) mode. Conceptually, this reduces the computation of a QFT correlator spectrum to performing a set of purely algebraic manipulations.

3rd CARROLL WORKSHOP THESSALONIKI  02 ▸ 06 OCTOBER 2023
String Theory seminar
Wednesday, March 29 at 11:00
Conference room Louis Michel (CPHT)
Diego Liška (Amsterdam University)
A Principle of Maximum Ignorance for Semiclassical Gravity
Abstract: Recent findings involving Euclidean and replica wormholes have demonstrated that the path integral of semiclassical gravity in AdS offers a "coarsegrained" description of its holographic CFT. In this seminar, I will discuss a statistical interpretation of these results, examining various notions of coarsegraining and state averaging within a unified framework. I will establish a probability distribution on the space of density matrices that remains maximally ignorant about the UV while still reproducing lowenergy observables. This approach resembles the eigenstate thermalization hypothesis and can be characterized with a simple stateaveraging ansatz. Finally, I will show how this ansatz reproduces several wormhole amplitudes in three and higher dimensions. The ideas presented in this seminar are based on forthcoming work with Jan de Boer, Boris Post, and Martin Sasieta. 
String theory seminar Tuesday, February 7 at 11:00 at CPHT, Conference room Louis Michel Matthew Dodelson (CERN) Orbits, scars, and supersymmetric instantons Abstract: I will describe recent work on the boundary interpretation of orbits around an AdS black hole. When the orbits are far away from the black hole, these orbits describe heavylight doubletwist operators in the boundary CFT. I will discuss how the dimensions of these operators can be computed exactly in terms of quasinormal modes in the bulk, using techniques from supersymmetric gauge theory. I will also explain how these results are related to the concept of quantum scars, which are eigenstates that do not obey the eigenstate thermalisation hypothesis. 
AdS/CMT seminar Monday, January 16 at 11:00 at CPHT, Conference room Louis Michel Nicolas Chagnet (Leiden University) Ionic lattices in holography and hydrodynamics Abstract: We use inhomogeneous holographic backgrounds to model the transport of strongly correlated electrons in the presence of an ionic lattice. Such holographic systems prove to have similar transport properties as those observed in the strange metallic phase of real world cuprates. We focus on the small lattice limit of such models and use hydrodynamics in the presence of a periodic chemical potential to study the dynamics and pole structure of these holographic models. We briefly mention the incoherent metal regime of strong lattices. 
String theory seminar on Tuesday January 10th at 11am at CPHT, Conference room Louis Michel Andrea Guerrieri (U. Padova and Perimeter Institute) "Where is Mtheory?" Abstract: I will use the Smatrix Bootstrap to carve out the space of unitary, crossing symmetric and supersymmetric graviton scattering amplitudes in nine, ten, and eleven dimensions. I will introduce a novel extension of positivity for massless particles dubbed “positivity in the sky” that will help to significantly improve the convergence of the bootstrap algorithm. I will show the bounds on the first leading correction to maximal supergravity, study the physics of the extremal amplitudes and compare them with the String and Mtheory results . 
AdS/CMT seminar on Monday September 26th at 11am at CPHT, Conference room Louis Michel Daniel Arean (Universidad Autonoma de Madrid) "NonHermitian Holography" Abstract: The formulation and study of nonHermitian PTsymmetric quantum theories has been the focus of both theoretical and experimental activity in recent years. In this talk I will present a minimal gravity dual of a nonHermitian QFT, discuss its main features, and hopefully open up a discussion on future nonHermitian directions.

AdS/CMT seminar on Tuesday September 20th at 11am at CPHT, Conference room Louis Michel Benjamin Withers (University of Southampton) "Relativistic hydrodynamics: a singular perspective" Abstract: As an effective theory of conserved currents near equilibrium, relativistic hydrodynamics is of wide applicability. I will give an overview of recent developments understanding the largeorder behavior of the hydrodynamic gradient expansion and present evidence that it is a divergent series in general. I will then introduce singulants as a way to gain analytic control over this largeorder behavior. Singulants obey simple equations of motion and govern the spacetime dependence of the largeorder behavior of onshell constitutive relations.

AdS/CMT seminar Monday May 23rd, 2022 at 11am at CPHT, room Louis Michel Kamran Behnia (ESPCI, France) "On the dynamic distinguishability of nodal quasiparticles in overdoped cuprates" Abstract: La$_{1.67}$Sr$_{0.33}$CuO$_4$ is not a superconductor and its resistivity follows a purely T$^2$ temperature dependence at very low temperatures. La$_{1.71}$Sr$_{0.29}$CuO$_4$, on the other hand, has a superconducting ground state together with a TLinear term in its resistivity. The concomitant emergence of these two features below a critical doping is mystifying. Here, I begin by noticing that the electronelectron collision rate in the Fermi liquid above the doping threshold is unusually large. Therefore, the scattering time of nodal quasiparticles is close to the threshold for dynamic indistinguishibality, which is documented in liquid $^3$He at its zerotemperature melting pressure. Failing this requirement of FermiDirac statistics will exclude nodal electrons from the Fermi sea. Becoming classical, 
String theory seminar on Wednesday 18th 2022 at 14pm at CPHT, Conference room Jean Lascoux (Wing 0) Marc Geille (ANS Lyon) "Relaxing the Bondi gauge in 3d and 4d gravity" Abstract: The Bondi gauge is a convenient choice of coordinates and line element to analyze the asymptotic structure of general relativity and aspects of gravitational radiation. Over the recent years several relaxations of the boundary conditions have been proposed, with the aim of understanding how general the asymptotic structure should be considered, and how large the asymptotic symmetry algebra can become. After presenting some motivations related to the understanding of charges, integrability, and flux in the covariant phase space formalism, I will review some recently proposed relaxations of the Bondi gauge in 3d and 4d gravity.

AdS/CMT seminar Monday May 16th, 2022 at 11am at CPHT, room Louis Michel Saso Grozdanov (University of Edinburgh, UK) "Univalence bounds on transport and effective field theories" Abstract: Bounds on transport represent a way of understanding allowable regimes of quantum and classical dynamics. Numerous such bounds have been proposed, either for classes of theories or (by using general heuristic arguments) universally for all theories. Few are exact and inviolable. In this talk, I will present new methods for deriving exact, rigorous, and sharp bounds on all coefficients of hydrodynamic dispersion relations, including diffusivity and the speed of sound. These general techniques combine analytic properties of hydrodynamics and the theory of univalent (complex holomorphic and injective) functions. Concrete examples will include bounds that relate transport to quantum chaos through 'poleskipping' as well as bounds without relation to chaos, such as the conformal bound on the speed of sound. I will also outline a set of general observations regarding the univalence properties of diffusion and sound in holographic models. Finally, I will discuss how these ideas could be generally applicable to constraining any effective field theory, not only hydrodynamics. 
AdS/CMT seminar Monday May 9th, 2022 at 11am at CPHT, room Louis Michel Nick Poovuttikul (Durham University, UK) "Anomaly “induce” transport in 2+1 dimensional QFT" Abstract: It is wellknown that anomaly in 3+1 d(or other even dimensional spacetime) has profound effect on the transport properties in the hydrodynamic limit of a QFT (such as the chiral magnetic effect etc). I will discuss anomaly related phenomena that can also occur in odd spacetime dimensions (in particular 2+1 d). These include anomaly induce transport involving higherform symmetry, highergroup symmetry and 't Hooft anomaly captured in characteristic class beyond ChernSimons term. 
Les Séminaires des Cordes 11 a.m.  11:45 12h  
Aspects of gravity, mathematics and physics This conference is organized by the Laurent Schwartz Mathematics Center and the Polytechnique Center for Theoretical Physics. Speakers Nicolas Besset (Paris Saclay) Guillaume Bossard (Polytechnique) Organizers Dietrich Häfner (UGA), Cécile Huneau (Polytechnique), Karim Noui (Paris Saclay), Marios Petropoulos (Polytechnique) Mandatory registration here Link to poster here 
AdS/CMT seminar on Tuesday, December 14, 2021 at 11 a.m. at the CPHT, Louis Michel conference room Richard Davison (HeriotWatt University, Scotland) "Chaos and poleskipping in rotating black holes" Abstract: Outoftimeordered correlators of local operators (OTOCs) provide a way of characterizing scrambling and chaos in quantum field theory states. For states with a classical, static black hole description, these correlators decay in a universal manner that is quantitatively related to a 'poleskipping' property of the Green's function of the energy density operator. This is consistent with a hydrodynamic effective theory for chaos in these states. In this talk I will discuss how these results generalize to the rotating, thermal state dual to the KerrAdS black hole. In the limit of slow rotation I will present an explicit form of the OTOC and show how its form can be used to obtain exact constraints on the dispersion relations of the collective modes that carry energy. 
AdS/CMT seminar on Friday December 10, 2021 at 11 a.m. at the CPHT, Louis Michel conference room Sebastian Grieninger (IFT Madrid) "(Pseudo)Spontaneous U(1) Symmetry Breaking in Hydrodynamics and Holography" Abstract: We investigate the lowenergy dynamics of systems with (pseudo)spontaneously broken U(1) symmetry. First, we consider the purely spontaneous case which corresponds to a superfluid where we compute the support of the hydrodynamic modes on the different field theory operators across the phase diagram. In the pseudospontaneous case, we construct a hydrodynamic framework and consider two generalizations of the standard holographic superfluid model to incorporate explicit breaking. In all cases, we find agreement between hydrodynamics and holography. Furthermore, we verify that phase relaxation arises only due to the breaking of the inherent Goldstone shift symmetry. The interplay of a weak explicit breaking of the U(1) and phase relaxation renders the DC electric conductivity finite but does not result in a Drudelike peak. 
String seminar on Thursday, December 9, 2021 at 4:00 p.m. on Zoom Anomalies and Emergent Symmetries on the Lattice and in the Continuum We consider various standard and exotic lattice models, and deform them using an approach generalizing the Villain formulation. The new lattice models exhibit many properties that were thought to be exclusive to the continuum field theories. These include exact winding/magnetic symmetries, anomalies, and new dualities. We apply this approach to the standard XY model, ZN clock model, gauge theories, and exotic models related to fractons. 
String seminar on Monday, November 22, 2021 at 11 a.m. at the CPHT, Louis Michel conference room ChernSimons supergravity and black holes with unbroken supersymmetries We study particular supersymmetric extension of EinsteinGaussBonnet AdS gravity in five dimensions. The theory admits black hole solutions with nontrivial topological properties, which are also BPS states that leave some of the supersymmetries unbroken. We analyze physical properties of these solutions, such as their conserved charges and topological numbers. We find many similarities of these solutions with standard supergravity black holes, and also with threedimensional BTZ geometries.
Conformal renormalization in AdS gravity We present evidence that the renormalization of EinsteinAdS gravity can be derived from conformal structures in the bulk. In point of fact, by applying a holographic mechanism which reduces Conformal Gravity to its Einstein sector in 4 and 6 dimensions, the EinsteinAdS action is naturally endowed with the correct counterterms. 
Séminaire des cordes le vendredi 8 octobre 2021 à 15h00 au CPHT, Salle de conférence Louis Michel Anastasios Petkou "On scale, conformal and Weyl invariance in QFT" Abstract: Scale, conformal and Weyl invariance play a central role in Quantum Field Theories, and they are omnipresent in AdS/CFT correspondence. Through various examples I will review various issues and possible misconceptions regarding the interrelationships between these three important notions and connect them to some recent investigations. 
Andrea Puhm sur France Culture le 16 février 2021 Andrea Puhm, chargée de recherche au Centre de Physique Théorique a été invitée à parler du paradoxe de l'information des trous noirs dans l'émission " La méthode scientifique" sur France ulture le 16 février 2021. En savoir plus ici. 
Postdoctoral position(s) ERC "Hydrodynamics, Holography and stronglycoupled Quantum Matter" at the Center for Theoretical Physics, Ecole Polytechnique Contact : Blaise Goutéraux Dates 
Postdoctoral Position(s) ERC Starting Grant 'Information Encoding in Quantum Gravity and the Black Hole Information Paradox' Contact : Andrea Puhm Dates 
Gravity and Strings Conference Organized by Luca Ciambelli
Speakers :
Carlo Angelantonj
Contact : ciambelli.lucca@gmail.com

Andrea Puhm sur France Culture le 10 septembre 2019 Andrea Puhm, chargée de recherche au Centre de Physique Théorique a été invitée à parler de supergravité dans l'émission " La méthode scientifique" sur France ulture le 10 septembre 2019. En savoir plus ici. 
Andrea Puhm, lauréate de l’ERC Starting Grant 2019 Chargée de recherche du CNRS au Centre de physique théorique de l’École polytechnique, Andrea Puhm a été distinguée par une bourse ERC Starting Grant qui va lui permettre de lancer un groupe de recherche sur la thématique «Codage d'information en gravité quantique et le paradoxe d'information des trous noirs». En savoir plus ici 
Séminaire des cordes le lundi 22 novembre 2021 à 11h00 au CPHT, Salle de conférence Louis Michel ChernSimons supergravity and black holes with unbroken supersymmetries We study particular supersymmetric extension of EinsteinGaussBonnet AdS gravity in five dimensions. The theory admits black hole solutions with nontrivial topological properties, which are also BPS states that leave some of the supersymmetries unbroken. We analyze physical properties of these solutions, such as their conserved charges and topological numbers. We find many similarities of these solutions with standard supergravity black holes, and also with threedimensional BTZ geometries.
Conformal renormalization in AdS gravity We present evidence that the renormalization of EinsteinAdS gravity can be derived from conformal structures in the bulk. In point of fact, by applying a holographic mechanism which reduces Conformal Gravity to its Einstein sector in 4 and 6 dimensions, the EinsteinAdS action is naturally endowed with the correct counterterms. 