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SEMPARIS
Séminaires des laboratoires de la DR4
CPHT's Seminars:
Séminaire de Physique des Particules CPhTEcole Polytechnique et Groupe Théorie IPN Orsay 
Mardi 20 février 2018 à 11h00 CPhT, Ecole Polytechnique, Salle de Conférence Louis Michel (Bât. 6) 
Kasper Larsen (Southampton U.) 
Algebraic geometry applied to multiloop scattering amplitudes/span>
The computation of twoloop amplitudes forms a current bottleneck to computing precisionlevel cross sections for LHC phenomenology. In this talk I will discuss several new methods for analytic evaluation of twoloop amplitudes, drawing inspiration from algebraic geometry and modern unitarity. More specifically, the methods involve efficient determination of a basis of integrals in terms of which the amplitude is decomposed; derivation of the integral identities needed to perform the decomposition; and evaluation of the basis integrals via differential equations. The first step of determining a basis of integrals has been implemented in the publicly available code Azurite which I will discuss. I will also discuss future implementations of the methods and their application to computing the twoloop fivegluon QCD amplitud

Journée des thèses du CPHT 
Mardi 6 février 2018 à partir de 10h30 CPHT, Ecole Polytechnique, Salle de Conférence Louis Michel 
Séminaire de Physique des Particules CPHTEcole Polytechnique et Groupe Théorie IPN Orsay 
Vendredi 15 décembre 2017 à 14h00 CPHT, Ecole Polytechnique, Salle de Conférence Louis Michel 
Cédric Mezrag (INFN. Roma) 
Exploring the nucleon structure
Understanding the emergence of the nucleon structures in terms of QCD degrees of freedom, i.e. quarks and gluons, is one of the major challenges of hadron physics. Two main types of nonperturbative techniques have been developed to tackle this challenge, discretised ones using large computer simulations (LQCD), and continuum ones relying on DysonSchwinger equations (DSEs). The latter suggest for instance that dynamical diquarks are generated inside the nucleon, creating eective degrees of freedom. Since both LQCD and DSEs techniques can be used in order to compute local matrix elements related to the structures of the Nucleon through for instance Distributions Amplitudes (DAs), we will explore in this talk the possibility of understanding the nucleon structure in terms of dynamical degrees of freedom.

PhD Defense at CPHT: Alain Marx "Two major steps for XTOR code: enhanced parallelization and free boundary geometry" Thursday, November 23th at 2:00pm, Room conference "Curie", Ecole Polytechnique Abstract: The XTOR2F code simulates the 3D dynamics of full bifluid MHD instabilities in tokamak plasmas. The first part of the thesis was dedicated to the parallelisation of XTOR2F code. The code has been parallelised significantly despite the numerical profile of the problem solved, i.e. a discretisation with pseudospectral representations in all angular directions, the stiffness of the twofluid stability problem in tokamaks, and the use of a direct LU decomposition to invert the physical preconditioner. The execution time of the parallelised version is an order of magnitude smaller than the sequential one for lowresolution cases, with an increasing speedup when the discretisation mesh is refined. Moreover, it allows to perform simulations with higher resolutions, previously forbidden because of memory limitations. The second part of the thesis was dedicated to the development of free boundary condition. The original fixed boundary computational domain of the code was generalised to a freeboundary one, thus approaching closely the geometry of today’s and future large experimental devices. The initial conditions are given by the CHEASE equilibrium code inside the plasma. Outside the plasma, fitting the magnetic potential at the CHEASE computation domain boundary with a set of external poloidal magnetic coils extends the solution. The boundary conditions use Green functions to construct a response matrix matching the normal and tangential components of the outside magnetic field with the inside solution. A thin resistive wall can be added to the computational domain. This new numerical setup generalises the investigation field from internal MHD instabilities towards external instabilities. The code linear behaviour is validated with two families of instabilities, n = 0 axisymmetric modes and n = 1 / m = 2 external kinks. In order to validate the nonlinear behaviour, nonlinear resistive MHD simulations of tearing modes at zero beta evolving to a stationary state have been performed. 
PhD Defense at CPHT: Alexander Efremov "Renormalization of SU(2) YangMills theory with flow equations" Wednesday, September 27th at 3:00pm, Room conference, CPHT, Building 6 Abstract: The problem of perturbative renormalization of SU(2) YangMills theory is studied in four dimensional Euclidean space with Lorenz gauge fixing. The analysis is based on the renormalization group flow equations. This is a unified approach which permits to study a large class of field theories without recourse to Feynman diagrams. It is remarkable that the flow equations allow us to construct all vertex functions of the theory using only the renormalization conditions. An important part of the work consists in establishing upper bounds in momentum space on all vertex functions at all loop orders. These bounds have a very natural graphical interpretation in form of trees. In order to properly define the vertex functions one must introduce ultraviolet and infrared cutoffs. But this regularization breaks BRST invariance. Thus it is essential to prove at all loop orders that the construction can be accomplished in such a way that BRST invariance is restored when the UV cutoff goes to infinity. Furthermore, substantial effort is made to provide a physical renormalization scheme which is explicitly independent of the flow parameter. 
Séminaire de Physique des Particules CPHTEcole Polytechnique et Groupe Théorie IPN Orsay 
Jeudi 28 Septembre 2017 CPHT, Ecole Polytechnique, Salle de Conférence du bât. 6 
Rainer Stiele (Lyon, IPN et Frnakfurt U.) 
Phase structure and thermodynamics of stronglyinteracting matter
Currently, large effort is undertaken, experimentally in heavy ion collisions and theoretically doing simulations on supercomputers, to explore the state of matter under the extreme conditions of the largest temperatures and densities in the universe. Effective models, such as Polyakovloopextended chiral models, which capture the main properties of the strong interaction, i.e. the creation of con stituent quark masses and connement, play the role of an important guidance that allow access at all temperature and density domains. Results of such a model, the Polyakovloopextended QuarkMeson truncation of QCD, for the phase diagram of stronglyinteracting matter are presented and it will be discussed how constraints from the highenergy domain (lattice calculations) as well as low energy domain (astrophysics measurements on compact stars) can contribute to pin it down more precisely. The importance of including the quark backreaction on the gauge eld dynamics is demonstrated and its impact on the structure of the phase diagram and on the surface tension for nucleation in a rst order transition region discussed. Finally, the necessity to improve the description of the gauge part of the strong interaction with phenomenological Polyakovloop potentials will be motivated and current ways to improve will be outlined.

PhD Defense at CPHT: Pascal Delange "Manyelectron effects in transition metal and rare earth compounds: Electronic structure, magnetic properties and point defects from first principles" Friday, september 29th 11:00 AM, Amphi Becquerel, Ecole Polytechnique Abstrac : The structural, spectral, magnetic, and many other properties of materials are mostly determined by the behavior of the electrons that glue the atoms together. Some of the most important technological advances of the 20th century were made in the field of semiconductors, where electrons behave like a dilute gas of weakly interacting particles. In contrast, many of the most promising materials for technological applications today are oxides. In transition metal or rare earth oxides, but also in some pure metals or alloys, the Coulomb repulsion between the electrons can give rise to exotic properties and phase transitions. In this thesis, we developed and applied firstprinciples (i.e. based on fundamental laws of physics) methods to evaluate the behavior of electrons interacting with each other and the physical properties stemming from it. We use these tools to study several materials of fundamental and technological interest, in particular iron, vanadium dioxide and hard magnets. 
Soutenance de thèse au CPHT : Pierre Chopin "Modélisation à grande échelle pour les phénomènes éruptifs" Vendredi 29 septembre, 14h, Amphi Bécquerel, Ecole Polytechnique Résumé de la thèse : Cette thèse a pour objet la modélisation du champ magnétique de la couronne solaire à l’aide du code de reconstruction nonlinéaire XTRAPOLS, avec une attention particulière pour les phénomènes éruptifs. Le caractère novateur des études menées porte sur l’aspect sphérique global de la méthode. 
Colloquium CPHT  Ecole polytechnique 
Mardi 12 septembre à 14h CPHT, Ecole Polytechnique, Salle de Conférence du bât. 6 
Eric Akkermans, Universite Technion, Israel https://phys.technion.ac.il/en/people/faculty?view=person&id=45 
Observing a Scale Anomaly in Graphene : a Universal Quantum PhaseTransition Abstract : 
PhD Defense at CPHT: 
The defence will be held in English Friday, September 8th at 02:00pm at EcolePolytechnique in amphi Becquerel. 
Loïc Herviou "Topological phases and Majorana fermions" 
Abstract of the thesis: This work was done both at Centre de Physique Théorique and at LaboratoirePierre Aigrain (ENS) under the joint supervision of Karyn Le Hur and Christophe Mora. 
Séminaire de Physique des Particules CPHTEcole Polytechnique et Groupe Théorie IPN Orsay 
Jeudi 6 juillet 2017 
BoWen Xiao (CCNU, Wuhan and HuaZhong Normal U.) 
Inclusive dijet productions in pp and AA collisions
In this talk, I will rst discuss the systematic calculation of dijet angular correlation and dijet asymmetries in pp collisions. Then, using the above pp results as a baseline, we can further put in the quarkgluon plasma medium effect and extract the corresponding medium transport coefficient at RHIC and the LHC. The dijet processes can serve as direct probes of QGP medium at NLO accuracy and play important roles at RHIC and the LHC. In the end, I will also discuss the further development of jet physics in heavy ion physics.

Séance de présentation des stages de master 
Mercredi 21 juin 2017 
Ecole Polytechnique, CPHT, Aile zéro, Salle de Conférence 
Le laboratoire organise une séance de présentation des stages de master le mercredi 21 juin, à 14h, dans la salle de conférence de l’aile 0. Chaque étudiant disposera de dix minutes pour présenter son sujet de stage de manière non technique et compréhensible par des non spécialistes. Cette séance sera suivie d’une collation vers 16h30. 
Programme :  R. CAMPOS DE CARVALHO (C. Lorcé)  B. DAGALLIER (J.R. Chazottes)  J. DOUÇOT (M. Petropoulos)  C. FLEMING et T. COUDARCHET (H. Partouche)  G. GIACALONE (C. Marquet)  M. HUMBERT (M. Petropoulos)  T. LESAFFRE (S. Billard)  A. LI (C. Lorcé)  S. POEGEL (G. Bossard)  C. SHI (G. Bossard)  F. YANG (Karyn Le Hur)  H. YAO (Laurent SANCHEZPALENCIA) 
Séminaire de Physique des Particules CPHTEcole Polytechnique et Groupe Théorie IPN Orsay 
Jeudi 22 juin2017 
IPN, Orsay, Bât. 100, Salle A201 
Shayan Ghosh (Indian Institute of Science, Bangalore) 
Analytic solutions to massive twoloop sunset diagrams,
In this talk, we begin by showing how it is possible to obtain full analytic results for the twoloop three distinct mass scale sunset diagrams that appear in the expressions for the masses and decay constants of the pion, kaon and eta particles in twoloop SU(3) chiral perturbation theory. We then talk about two applications of these results: a) how it allows one to obtain small quark mass ratios to arbitrarily high powers, and b) how it allows one to make ts to lattice data to extract greater information about low energy constants. We then change track and discuss in more detail the Mellin Barnes method used in our calculations, and demonstrate how this method can be extended using analytic continuation to obtain results for Feynman diagrams whose mass value parameters are such that it is impossible to use a direct calculation. We show how such methods of analytic continuation can also be helpful in contexts where a direct calculation does yield results, and thus set the stage for more widespread application of this technique.

Séminaire de Physique des Particules CPHTEcole Polytechnique et Groupe Théorie IPN Orsay 
Vendredi 28 avril 2017 (Attention : Jour inhabituel!) 
Ecole Polytechnique, CPHT, bât. 6, Salle de Conférence 
Zsolt Szép (Eötvös University, Budapest) 
Thermodynamics of the vector meson extended quarkmeson model We investigate the effects of (axial)vector mesons on the chiral phase transition of the model containing 2+1 constituent quarks and Polyakovloop degrees of freedom. A x2 minimization procedure is used to parameterize the model based on treelevel decay widths and vacuum scalar and pseudoscalar curvature masses which includes the vacuum and thermal contribution of the constituent quarks. The pressure and the derived thermodynamical observables determined from it at nite temperature T and chemical potential using a simple approximation for the grand potential are compared to lattice results. The best parametrization of the model allows for the existence of the critical end point (CEP) of the T phase diagram. 
Colloquium du CPHT, Ecole Polytechnique 
Mercredi 15 mars 2017 à 15h00 
Salle de séminaire du CPHT, bâtiment 6 
Sylvain Carrozza (Perimeter Institute, Waterloo Canada) 
Quantum fields with tensorial locality Abstract: In recent years, generalizations of matrix models known as Tensor Models and Group Field Theories have been developed into a consistent formalism. The common feature of these field theories is an abstract notion of locality, known as tensorial locality, which encodes the combinatorial structure of the elementary field interactions. It has initially been introduced in the context of quantum gravity, where indeed the absence of a nondynamical background spacetime renders the standard notion of locality inoperative. I will provide an overview of this approach, focusing on general features of the phase diagrams of tensorial theories, and of their possible applications to quantum gravity and statistical physics. I will also discuss a new range of applications at the crossroad between condensed matter and the AdS/CFT correspondence: following a recent insight from Witten, tensorial techniques are being taken advantage of to investigate the properties of socalled SachdevYeKitaev models of randomly interacting fermions. 
Colloquium du CPHT, Ecole Polytechnique 
Vendredi 10 mars 2017 à 14h 
Salle de séminaire du CPHT, bâtiment 6 
Leonardo Mazza (Ecole Normale Supérieure, Paris) 
Towards the quantumHalleffect with ultracold atoms and synthetic dimensions Abstract: Synthetic ladders pierced by a magnetic field realized with onedimensional alkalineearth(like) gases represent a promising environment for the investigation of manybody quantum physics with ultracold atoms. A fundamental question is whether these setups can give access to pristine twodimensional phenomena, such as the fractional quantum Hall effect, and how. I discuss the existence of a hierarchy of fractional insulating and conducting states by means of both analytical techniques (bosonization) and numerical methods (densitymatrix renormalization group algorithm). I show that such states can be exploited for constructing a topological Thouless pump where the charge transported after one cycle is quantized to fractional values and demonstrate this behavior with a full manybody timedependent calculation. I conclude by presenting numerical signatures of bosonic and fermionic Laughlinlike states that can be observed in these setups. 1  S. Barbarino, L. Taddia, D. Rossini, L. Mazza, R. Fazio, Nat. Commun. 6, 8134 (2015) 
Colloquium du CPHT, Ecole Polytechnique 
Vendredi 3 mars 2017 à 11h 
Salle de séminaire du CPHT, bâtiment 6 
Charles Grenier
(Ecole Normale Supérieure de Lyon) 
Mesoscopic transport with ultracold atoms
Abstract: The last years have seen the emergence of a new trend in cold atoms experiments towards the simulation of quantum transport.
In this talk, I will show how one can use cold gases to simulate mesoscopic devices, and how adding a cold atom flavour to a transport
setup can help pushing further our understanding of low dimensional structures. In particular here, I will focus on the evolution of particle
and spin conductances as functions of the interaction strength.
S. Krinner et al., Proceedings of the National Academy of Sciences 113 (29), 81448149 
Séminaire de Physique des Particules CPHTEcole Polytechnique et Groupe Théorie IPN Orsay 
Mardi 28 février 2017 à 11h00 (Attention : Jour inhabituel!) 
Ecole Polytechnique, CPhT, bât. 6, Salle de Conférence 
Daniel Kroff (IFTUnesp, Sao Paulo) 
The Linde problem on R^{2} x S^{1} x S^{1} Thermal eld theory provides the natural framework to describe the thermodynamic properties and to study phase transitions of systems describe by quantum eld theories, in particular, the quarkgluon plasma. However, its perturbative realization faces important technical difficulties whenever massless bosons are considered, due to divergences in the IR sector. After briefly reviewing these IR divergences in thermal eld theory, and especially the Linde problem, we analyze the IR behavior of YangMills theory in a Torus  R^{2} x S^{1} x S^{1}  and show that, in this geometry, any perturbative calculation is expected to breakdown already at O(g2). 
Colloquium du CPHT, Ecole Polytechnique 
Jeudi 23 février 2017 à 11h 
Salle de séminaire du CPHT, bâtiment 6 
Jamal JalilianMarian "Visiting Professor" au Centre de Physique Théorique, programme "Jean d'Alembert" de l'Université ParisSaclay 
An introduction to high energy scattering in QCD Abstract: After a brief introduction to Quantum ChromoDynamics (QCD), the fundamental theory of strong interactions, we consider scattering of hadrons (protons and neutrons) and nuclei at very high energies. It is shown that scattering cross sections at high energy are dominated by components of the hadron/nucleus wave function that contain a large number of soft partons (gluons and sea quarks carrying a small fraction x of the hadron energy). An effective action, known as Color Glass Condensate, is proposed that describes the dynamics of this many body system of gluons at small x. Applications to particle production at the Large Hadron Collider (LHC), Relativistic Heavy Ion Collider (RHIC) and the proposed Electron Ion Collider (EIC) will be discussed in detail and connections to other branches of physics, cosmic rays, high energy neutrinos, Quark Gluon Plasma and statistical physics will be elucidated. 
Colloquium du CPHT, Ecole Polytechnique 
Jeudi 2 février 2017 à 11h 
Salle de séminaire du CPHT, bâtiment 6 
Jeffrey Brown Université de Louisiane, Baton Rouge et Centre de Physique Théorique, Ecole polytechnique 
Modeling ultrashort laser pulses in gases Abstract: The interaction of short, intense (10^14 W/cm^2) laser pulses with atomic and molecular gases produces a wide range of interesting nonlinear optical phenomena. Accurately modeling how microscopic quantum processes give rise to macroscopic optical properties in a computationally feasible manner is challenging. For determining which "ingredients" are necessary in a model to capture the most dominant quantum processes, we investigated the utility of exactly solvable, 
Colloquium du CPHT, Ecole Polytechnique 
Jeudi 12 janvier 2017 à 11h 
Salle de séminaire du CPHT, bâtiment 6 
Timothée Nicolas SPCEPFL, Lausanne, Suisse 
MHD instabilities : from tokamaks to stellarators Abstract: We will explore different aspects of magnetohydrodynamic (MHD) instabilities in tokamaks and stellarators. Both devices require a helical magnetic field to efficiently confine a hot fusion plasma in a torus. External coils generate a main field of the order of 24 T. Tokamaks generate the second component of the field using a large electric current in the plasma itself. Stellarators however generate all the field using only external coils. This gives rise to different kinds of instabilities in both devices, mainly current driven in tokamaks, mainly pressure driven (RayleighTaylor like) in stellarators. We will review some theoretical issues related to the MHD stability in both tokamaks and stellarators. We will see why they are crucial in practice, in view of future experiments such as ITER. We will also try to give a taste of the numerical challenge faced by whoever tries to simulate the complex nonlinear behavior of such plasmas. 