Local seminars
Academic year 2024-2025
Giorgio Leone (SNS Pisa) | New non-supersymmetric tachyon-free orientifolds in 6d and unitarity constraints | 18 December 2024
The absence of supersymmetry is often accompanied by instabilities emerging from tadpoles or tachyons in the spectrum. While tadpoles are ubiquitous in any non-supersymmetric set-up, few models are known to be tachyon-free. In this seminar, I will focus on orientifold constructions in lower dimensions which do not have tachyons and enjoy Brane Supersymmetry Breaking (BSB). These vacua are characterised by a simultaneous presence of a tree-level supersymmetric closed string sector coupled with a non-supersymmetric open string one, which underlies a non-linear realisation of supersymmetry. After reviewing the original construction in six dimensions built on the T^4/Z_2 orbifold, I will present an almost rigid variation that can only be deformed via overall brane recombination. Afterwards, I will describe the BSB orientifold built upon the T^4/Z_4 orbifold which, in contrast to the previous case, does not admit any continuous deformation, leaving only space for a discrete one encoded into a non-vanishing Kalb-Ramond field. Finally, I will comment on the unitarity constraints arising from 2d defects coupled to the R-R 2-forms required by the Green-Schwarz-Sagnotti mechanism.
Alessandro Tomasiello (U. Milano Bicocca) | Higher spins and Finsler geometry | 3 December 2024
Finsler geometry is a natural generalization of (pseudo-)Riemannian geometry, where the line element is not the square root of a quadratic form but a more general homogeneous function. Parameterizing this in terms of symmetric tensors suggests a possible interpretation in terms of higher-spin fields. We will see here that, at linear level in these fields, the Finsler version of the Ricci tensor leads to the curved-space Fronsdal equation for all spins, plus a Stueckelberg-like coupling. Nonlinear terms can also be systematically analyzed, suggesting a possible interacting structure. No particular choice of spacetime dimension is needed. The Stueckelberg mechanism breaks gauge transformations to a redundancy that does not change the geometry. This creates a serious issue: non-transverse modes are not eliminated, at least for the versions of Finsler dynamics examined in this paper.
Guillaume Bossard (CPHT, Palaiseau) | Higher dual fields and E11 | 26 November 2024
We construct a pseudo-Lagrangian that is invariant under rigid E11 and transforms as a density under E11 generalised diffeomorphisms. The gauge-invariance requires the use of a section condition studied in previous work on E11 exceptional field theory and the inclusion of constrained fields that transform in an indecomposable E11-representation together with the E11 coset fields. We show that, in combination with gauge-invariant and E11-invariant duality equations, this pseudo-Lagrangian reduces to the bosonic sector of non-linear eleven-dimensional supergravity for one choice of solution to the section condition. For another choice, we reobtain the E8 exceptional field theory and conjecture that our pseudo-Lagrangian and duality equations produce all exceptional field theories with maximal supersymmetry in any dimension. We also describe how the theory entails non-linear equations for higher dual fields, including the dual graviton in eleven dimensions. Furthermore, we speculate on the relation to the E10 sigma model.
Saurabh Pant (IISER, Pune) | Supersymmetric Yang-Mills theories without anti-commuting variables | 20 November 2024
In this talk, my main focus will be formulating supersymmetric Yang-Mills theories without anti-commuting variables, thus characterizing them as purely bosonic theories. I will discuss the existence of the transformation (Nicolai map) of the bosonic fields such that the Jacobian determinant of the transformation exactly cancels against the product of the fermion and ghost determinants in critical dimensions where supersymmetric Yang-Mills theories exist. I will discuss various aspects of the map, like gauge dependency and uniqueness, and describe its applications to physics – specifically to scattering amplitudes and correlation functions.
Maximo Banados (Pontificia Universidad Católica de Chile, Santiago) | Weyl symmetry and symmetry breaking | 15 November 2024
We present a Weyl invariant equation for Gravity by gauging the global Weyl invariance of vacuum Einstein equations. The equation is linear in the curvature and a natural generalization of Einstein equations to Weyl geometry. The system has 5 physical polarizations, two tensor modes, two vectors modes and one scalar, associated to the cosmological constant. An exact black hole solution is found and we discuss the dynamics on Friedman backgrounds and the evolution of cosmological perturbations.
Kristiansen Lara (CECs, Valdivia) | Hagedorn Behavior and Log Corrections to Geometrized Integrable Systems | 11 October 2024
The seminal work of Brown and Henneaux [1] initiated the exploration of the various asymptotic faces of three-dimensional General Relativity (GR). One of these, discovered by Pérez, Tempo, and Troncoso, is the duality between integrable systems and AdS$_3$ GR [2]. Specifically, they show that under suitable boundary conditions, along with a specific asymptotic behavior of the Lagrange multipliers, Einstein’s equations reduce to (two independent copies of) the Korteweg-de Vries (KdV) equation. In this talk, we show that under the same boundary conditions but with a different asymptotic behavior, a different integrable hierarchy emerges, known as the Dym hierarchy, whose gravitational configuration corresponds to a black hole whose properties differs from its KdV counterpart: It exhibit constant Hawking temperature (Hagedorn behavior), and therefore, a vanishing angular momentum chemical potential and inverse heat capacity, seemingly leaving no room for thermal fluctuations—i.e., no log corrections to the Bekenstein-Hawking entropy. Following the change of ensemble approach of [3], we address this problem and compute its log corrections. Conclusions and further prospects are discussed.
[1] Brown, Henneaux, Commun. Math. Phys. 104 (1986) 207-226.
[2] Pérez, Tempo, and Troncoso, 1605.04490.
[3] Pathak, Porfyriadis, Strominger, Varela, 1612.04833; Grumiller, Pérez, Tempo, Troncoso, 1705.1060.
Academic year 2023-2024
Viacheslav Krivorol (ITMP MSU, Steklov Mathematical Institute) | First-order GLSM construction in sigma models: complex projective spaces and beyond | 5 September 2024
Sigma models form a class of field theories that play a crucial role in various branches of modern theoretical and mathematical physics. However, studying these models is challenging due to the highly nonlinear nature of their Lagrangians. There are some specific methods, such as the background field method, that can be used to study these models, but they have some limitations.
There is an alternative recently proposed method, called “the first-order GLSM formulation” (or “Gross-Neveu formalism”), for studying sigma models. In this method one cast these models as gauge theories with a finite number of interactions using the idea of symplectic reduction. Instead of presenting general results, however, I will illustrate the ideas using the simplest example, the CP^n sigma model. I will then explain how these ideas can be extended to other target spaces.
Based on joint works with Dmitri Bykov: https://arxiv.org/abs/2306.04555, https://arxiv.org/abs/2407.20423
Subho Chatterjee (UC Davis) | Probabilities and Supergeometry: Measurement theory for dynamical discrete systems | 16 July 2024
Discrete probabilistic systems, like bits on a computer or faces of a coin, abound in nature. We propose a geometric model describing dynamics and measurement theory for such systems. Our approach is covariant with respect to choices of clocks and laboratories. The configuration space is a super phasespacetime modelled by an odd dimensional symplectic supermanifold, observables are superfunctions and states are suitable (star) squares of superfunctions. The data of an odd dimensional symplectic supermanifold canonically incorporates dynamics. We also obtain dynamical probabilities using convex polyhedral cones and find that they obey Markov-like evolution.
Felipe Figueroa (LAPTH, Annecy) | How simple can stringy amplitudes be? Ruling out dual model amplitudes with finitely many trajectories | 2 July 2024
Dual model amplitudes are meromorphic amplitudes involving the exchange of infinitely-many higher spin states. They are the staple of tree-level string theory, with the Veneziano and Virasoro-Shapiro amplitudes being their most famous representatives. They also appear in large N gauge theories as QCD, which becomes a weakly coupled theory of baryons and glueballs in the limit where the number of colors goes to infinity whose interactions are described by this class of objects. Despite being very simple, these amplitudes probe non-perturbative phenomena as confinement and hadronization in QCD, and thus understanding their basic properties is an important question.
In this talk I will show some recent progress in this direction and explain how dual model amplitudes require an infinite number of Regge trajectories to be consistent. After a pedagogical motivation/introduction to dual model amplitudes, I will explain the different ingredients of the proof and discuss the implications and limitations of this result.
Mirian Tsulaia (OIST, Okinawa) | Massive Higher Spins and Black Hole Interactions | 28 May 2024
We shall discuss an effective field theoretic approach to description of scattering of two Kerr Black Holes, emitting gravitational waves. In this approach one considers massive higher spin fields interacting with gravitational and electromagnetic fields. We use a BRST formalism, similar to the Open String Field Theory, to construct a cubic action for two massive higher spin fields and one massless spin two (or spin one) field. This action reproduces the cubic amplitudes that correspond to the interactions between Kerr Black Holes.
Kristiansen Lara (CECs, Valdivia) | Integrable Systems and spacetime dynamics | 26 October 2023
In this talk, we show that the Ablowitz-Kaup-Newell-Segur (AKNS) integrable hierarchy can be obtained as the dynamical equations of three-dimensional general relativity with a negative cosmological constant. This geometrization of the AKNS system is possible by constructing novel boundary conditions for the gravitational field. These are invariant under an asymptotic symmetry group characterized by infinite AKNS commuting conserved charges. Gravitational configurations are studied through conjugacy classes. Conical singularities and black hole solutions are included in the boundary conditions.
Anders Bengtsson (University of Borås) | Archaeology of Higher Spin: A research program in itself | 26 October 2023
Higher spin theory is entering its 10th decade. There are two clear dividing lines: (i) in the mid 80’s as regards progress on interactions. (ii) before and after the millennium — a surge of dedicated interest. First Vasiliev and covariant Minkowski, and then light-front. Archaeology of higher spin: What’s in the old papers, actually? Ideas, motivations, methods, results, … Excavations already done: (a) Vasiliev original papers from 1986-1992 (Volume 2, Chapter 8 of my book). (b) Mechanical models — Directly interacting particles (Volume 2, Chapter 2). Excavations to be done: (c) Fronsdal’s work mid 50’s to mid 80’s. (d) Dirac (of course!)
Athanasios Chatzistavrakidis (Boskovic Institute, Zagreb) | Graded Generalised Geometry for Gauge and Gravity theories | 9 October 2023
After a brief motivational introduction to graded and generalised geometry, in this talk we will discuss some applications of bidifferential bigraded manifolds. We will see how kinetic, mass and healthy higher derivative interaction terms for mixed symmetry tensor fields can be accommodated under the same roof and how various dualities can be described in a universal way. Generalisations of this approach to include multiple fields, generalised theta terms and certain nonlinear theories will be briefly mentioned, as well as the relation to generalised global symmetries. Finally, we will single out one particular gravitational theta term and motivate potential physical consequences within the model of axion gravitodynamics.
Academic year 2022-2023
Euihun Joung (Kyung Hee University) | Manifestly covariant worldline actions from coadjoint orbits | 6 September 2023
I will demonstrate how one can derive a manifestly covariant worldline action starting from Poincare and (A)dS algebra. Starting from a coadjoint orbit of the latter algebra and using the Kostant-Kirillov-Souriau symplectic structure on it, we first derive the unconstrained Hamiltonian action on a “curved” phase space, whose quantization would lead to a unitary irreducible representation of the starting Lie algebra. We then reformulate this action as a constrained Hamiltonian action on a “flat” embedding phase space. The set of constraints is in general a mixture of the first and second class constraints, and it defines a new coadjoint orbit of a “dual” symmetry. Upon quantization, this construction provides the reductive dual pair correspondence. I will also briefly comment about this correspondence, a very powerful tool to handle a large class of representations.
Jakob Palmkvist (Orebro University) | Extended geometry and restricted associativity | 24 April 2023
Tensor hierarchy algebras are infinite-dimensional generalisations of Lie superalgebras of Cartan type, which have proven useful in the description of certain gauge structures. In particular, they have turned out to play a crucial role in the framework of extended geometry, where gauge transformations are unified with diffeomorphisms. In my talk, I will present step towards a new construction of the tensor hierarchy algebras, where the brackets originate from the commutator in a generalised Clifford algebra which is not associative, but satisfies a weaker version of associativity.
Matthieu Vilatte (CPHT, École Polytechnique) | Some features and applications of Carrollian physics | 27 March 2023
I will give an overview of Carrollian physics which is the physics naturally appearing at null infinity of asymptotically flat spacetimes. Starting by a review of the geometry of Carrollian spaces in a frame where the decoupling of time and space is apparent, I will show how to construct Carrollian momenta whose conservation equations follow from the invariance under Carrollian diffeomorphisms. The way to deal with Killing vectors and (non)-conserved charges will also be discussed with an accent on the example of the scalar field. Then, I will apply this theoretical framework within the flat fluid/gravity paradigm, first by discussing the gauge in which the expansion is performed. Finally, I will go through the conditions for the expansion to be resummable and present how one can, in the case of time independent Ricci-flat spacetimes, recover the gravitational multipoles from a pure Carrollian boundary viewpoint.
Harold Steinacker (University of Vienna) | Emergent 3+1-dimensional gravity from the IKKT matrix model | 28 February 2023
A mechanism for a 3+1-dimensional gravity on quantized branes in the IKKT matrix model is discussed. The Einstein-Hilbert action arises in a well-defined way as a quantum effect on suitable backgrounds, as part of a higher-spin extended gauge theory. This can be seen as emergent gravity arising from the open string sector.
Lorenzo Kuchler (Université libre de Bruxelles) | Waveforms from inspiral, transition and plunge in compact binaries | 23 February 2023
Within general relativity, the planar motion of a stellar-mass compact object around a supermassive black hole admits a quasi-circular inspiral motion followed by a transition across the innermost stable circular orbit (ISCO) and a final plunge behind the event horizon. I will present the modelling of the motion in these three regions using gravitational self-force theory and compare the waveforms generated in this framework with numerical relativity simulations.
Per Sundell (Universidad del Bío-Bío) | Fractional spins, real-time holography and conformal higher spin gravity | 8 December 2022
We are accommodating Vasiliev’s holography proposal anno 2013 within an AKSZ functor creating boundary states containing 4d HSG and 3d CHSG defects.
Stefan Fredenhagen (University of Vienna) | Fusion of interfaces in Landau-Ginzburg models in a functorial approach | 28 September 2022
Interfaces between two-dimensional field theories provide an interesting algebraic structure because of the possibility to fuse them. I discuss interfaces in N=(2,2) supersymmetric Landau-Ginzburg models where B-type interfaces can be represented as matrix factorisations and their fusion by a graded tensor product. Fusing a fixed interface to any other gives rise to a functor on the category of matrix factorisations. In many important cases this functor can be lifted to a functor on the category of ring modules. Such fusion functors are on the one hand efficient tools to actually compute fusion of interfaces in examples, and on the other hand provide an alternative way of representing interfaces in which the algebraic structure of fusion becomes more apparent.
Academic year 2021-2022
Michel Pannier (University of Jena) | Probing Flat-Space Holography in 3D | 8 June 2022
The Holographic Principle, though its best studied application being the AdS/CFT duality, is expected to hold in rather general circumstances. One may thus try to test and extend its applicability on different examples, such as asymptotically de Sitter or flat space-times. The latter is the idea of the talk, in particular focusing on the introduction of propagating degrees of freedom to an otherwise purely topological three-dimensional theory of gravity. I will present a candidate for a possible linear scalar coupling equation, its relation to a certain higher-spin algebra and the construction of Wilson lines as holographic probes.
Eric Bergshoeff (University of Groningen) | Non-relativistic Quantum Gravity: a Status Report | 6 May 2022
In this talk I will discuss the geometry underlying non-relativistic (super-) string theory. Next, I will discuss the low energy effective action together with some of it’s basic half-supersymmetric brane solutions. The notion of non-relativistic T-duality (and S-duality) will play an important role in this discussion.