# Local seminars

##### Academic year 2023-2024

**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.