Wednesday Seminars

Some of the seminars take place remotely on Zoom. If you would like to attend via Zoom but have not subscribed to the mailing list yet you can contact the organizers in order to obtain the passcode.

In addition to the Wednesday seminars there is a biweekly Nordic remote HET seminar.

Organizers: Kays HaddadJacopo Sisti

Fall 2023

Paolo Vallarino (Turin U.)

15 November 2023

Location: TBA

Title: TBA

Abstract: TBA

Elli Heyes (U. of London, LIMS)

25 October 2023

Location: Å80121

Title: TBA

Abstract: TBA

Istvan Szecsenyi (Nordita)

18 October 2023, 13.45

Location: Å80121

Title: TBA

Abstract: TBA

Alessandro Tanzini (SISSA, INFN)

04 October 2023, 13.45

Location: Å80127

Title: Surface defects, Toda equations and BPS spectra

Abstract: We show that the partition functions of 4d supersymmetric gauge theories with 8 supercharges in presence of surface defects obey a non-autonomous version of Toda system and we comment on its M-theory origin. The solution to the corresponding equations provides new recursion relations allowing for instanton counting for all simple groups from A to E. The uplift to 5d is a discrete flow generated by automorphisms of the associated BPS quiver. We show that for a class of theories, the 4d reduction of these discrete flows displays an intriguing new relation with Argyres-Douglas SCFTs.

Giulio Bonelli (SISSA)

Tue 03 October 2023, 13.45

Location: Å80127

Title: Black Hole perturbation theory from 2D CFTs & N=2 D=4 susy gauge theories

Abstract: The study of Black-Hole perturbation theory is a classical problem in General Relativity and crucial to study gravitational waves. Due to the high order of symmetry of the BH gravitational field and the consequent separation of variables at the linear order, the problem reduces to the study of linear ordinary second order differential equations. The resulting ODEs are of Fuchsian type and therefore, as already observed long ago by A.M.Polyakov, can be solved exactly in terms of classical - regular or irregular - Virasoro conformal blocks. By making use of the specific explicit expressions of the latter implied by the AGT dual perspective on the conformal field theory, it is possible to explicitly solve the connection problem of the resulting (confluent)Heun equation and give novel exact and explicit formulas for the grey body factor, quasi-normal modes and Love numbers of diverse black holes. This will be explicitly applied to 4D Kerr and Schwarzschild-(A)de Sitter BHs.

Davide Passaro (Amsterdam U.)

27 September 2023, 13.45

Location: Å80121

Title: Reversing the orientation: new frameworks for the computation of Z-hat invariants for positive Seifert manifolds

Abstract: Z-hat invariants are topological invariants for three manifolds that have deep physical meaning and are intimately related to Cern-Simons theory. Current computational methods only allow for the computation of Z-hat invariants only on a subset of Seifert manifolds which satisfy a negativity property. On such manifolds the Z-hat invariant are Quantum Modular Forms. CP symmetry on Chern-Simons theory suggests a relation between Z-hat invariant on pairs of negative and positive Seifert manifolds related by orientation reversal. A direct application of the Chern-Simons relation on known Z-hat however leads to diverging series. By leveraging the quantum modularity of the Z-hat invariant however it is possible to understand the Chern-Simons relation, and additionaly to predict the specific quantum modular properties of the proposed Z-hat invariant on positive Seifert manifolds. This prediction is known as the False-Mock conjecture. We find that, in specific cases where we can either regularize the diverging series or we can re-express the Z-hat invariant using inverted Habiro series that the relation can be applied the False-Mock conjecture is satisfied.

Franziska Porkert (Bonn U.)

30 August 2023, 13.45

Location: Å80109

Title: Massless Feynman Integrals in 2D & Single-valued Periods

Abstract: In this talk I will show how all massless Feynman integrals in two dimensions with non-integer propagator weights are related to specific single-valued objects. In the framework of intersection theory, those integrals can be evaluated as single-valued periods. At the same time, a sub-class of these integrals, the so-called fishnet integrals, can be considered in the context of Calabi-Yau geometries where they are computed by the Kähler potential or the quantum volume.

Spring 2023

Souvik Banerjee (Würzburg U.)

Tue 13 June 2023, 13.45

Location: Å80121

Title: A universal approach to complexity

Abstract: In this talk, I shall present a general framework in which both the state and the operator complexities can be put on the same footing. This will follow from a generalized definition of complexity based on the violation of the Eigenstate Thermalization Hypothesis. In our formalism, the state complexity is defined in terms of the density matrix of the associated state which, for the operator complexity, lives on a doubled Hilbert space obtained through the channel-state map. I shall discuss how this framework also encompasses nicely, the holographic notions of complexity and explains the universal late-time growth of complexity, followed by saturation. I shall conclude with some open questions.

Paul-Konstantin Oehlmann (Northeastern U.)

Fri 09 June 2023, 13.45

Location: Å80121

Title: The geometry of twisted compactifications in M-theory

Abstract: Geometric Engineering is an important tool to engineer interesting and non-Lagrangian theories in various dimensions. If a theory admits a discrete 0-form symmetry, there is the possibility to twist by it when performing a circle reduction. Since F-theory is related to M-theory by circle reductions, this begs the question which geometries capture such twisted compactifications, what their algebraic description may be and how their moduli spaces look like. In this talk I will show that so called genus-one fibrations capture the respective twisted reductions in M-theory. I will explore the moduli space by relating twisted to untwisted compactifications via Higgs transitions of KK-charged multiplets in five dimensions. I close by outlining various applications. This talk is based on upcoming work with L. Anderson and J. Gray.

Piotr Tourkine (Annecy, LAPTh)

Thu 08 June 2023, 13.45

Location: Å80121

Title: Scattering amplitudes from dispersive iterations of unitarity

Abstract: In the late 60s, D. Atkinson proved in a series of papers the existence of functions satisfying rigorously the constraints of the S-matrix bootstrap for the 2-to-2 S-matrix of scalar, gapped theories, following an approach suggested by Mandelstam. Beyond the mathematical results themselves, the proof, based on establishing the existence of a fixed point of a certain map, also suggests a procedure to be implemented numerically and which would produce fully consistent S-matrix functions via iterating dispersion relations, and using as an input a quantity related to the inelasticity of a given scattering process. In this talk, I will present the results of a recent paper in collaboration with A. Zhiboedov, about the first implementation of this scheme. I will first review some basic concepts of the S-matrix program, and state our working assumptions. I will then present our numerical non-perturbative S-matrices, and discuss some of their properties. They correspond to scalar, massive phi^4-like S-matrices in 3 and 4 dimensions, and have interesting and non-trivial high energy and near-threshold behaviour. They also allow to make contact with the running of the coupling constant. I will also compare to other approaches to the S-matrix bootstrap in the literature.

Stijn van Tongeren (Humboldt U.)

31 May 2023, 13.45

Location: Zoom

Title: Gauge theory duals of deformed integrable strings

Abstract: Integrable examples of the AdS/CFT correspondence admit numerous deformations on the AdS side, preserving this integrability. These can be viewed as AdS analogues of the real-beta Lunin-Maldacena deformation of (AdS5 x) S5. I will discuss how we can similarly deform the CFT side, in the setting of noncommutative field theory, to (hopefully) give duals of these deformed strings. The resulting noncommutative gauge theories admit a planar equivalence theorem for their Feynman diagrams, and have twisted superconformal symmetry, providing the first hints of their likely planar integrability.

Francesco Galvagno (ETH)

24 May 2023, 13.45

Location: Å80101

Title: Holographic perspectives for N=2 SCFTs in the tensionless limit

Abstract: In this talk we discuss the holographic properties of some 4d planar gauge theories in the zero coupling limit, corresponding to the tensionless limit from the string dual perspective. In particular, we consider a special family of theories, arising as orbifolds of N=4 SYM, which realize a circular quiver theory preserving N=2 superconformal invariance. Following the Gaberdiel-Gopakumar derivation, we propose a free field worldsheet theory in the tensionless string limit which is dual to the orbifold gauge theory at the free theory point. In particular, after imposing some specific gauge constraints on the worldsheet degrees of freedom, the spectrally flowed worldsheet spectrum is in one-to-one correspondence with the single trace operators of the 4d theory. Finally, we discuss possible generalizations of this approach, mentioning N=2 SCQCD as the main example.

Subhajit Mazumdar (Seoul Natl. U.)

10 May 2023, 13.45

Location: Å80121

Title: Kite and Triangle diagrams through Symmetries of Feynman Integrals

Abstract: The Symmetries of Feynman Integrals (SFI) is a method for evaluating Feynman Integrals which exposes a novel continuous group associated with the diagram which depends only on its topology and acts on its parameters. Using this method we study the kite diagram (a two-loop diagram with two external legs) and the most general triangle diagram (one-loop diagram with three external legs) with arbitrary masses and space-time dimensions. Generically, this method reduces a Feynman integral into a line integral over simpler diagrams. We identify the locus/loci in parameter space where the integrals further reduce to a mere linear combination of simpler diagrams. We generalize and revisit some known results.

Eric D'Hoker (UCLA)

26 April 2023, 13.45

Location: Å80101

Title: Cascade flow from N=2 to adjoint QCD

Abstract: The renormalization group flow is considered of N=2 super Yang Mills theory in the presence of a supersymmetry soft breaking operator, with the help of the Seiberg-Witten solution. An Abelian Higgs model is proposed as the magnetic dual to this flow near a multi-monopole point on the Coulomb branch and argued to lead from the Coulomb phase to the maximal Higgs phase through a cascade of mixed phases.

Nikolay Bobev (KU Leuven)

19 April 2023, 13.45

Location: Å4001

Title: Large N Partition Functions, Holography, and Black Holes

Abstract: I will discuss the large N behavior of partition functions of the ABJM theory on compact Euclidean manifolds. I will pay particular attention to the S^3 free energy and the topologically twisted index for which I will present closed form expressions valid to all orders in the large N expansion. These results have important implications for holography and the microscopic entropy counting of AdS_4 black holes which I will discuss. I will also briefly discuss generalizations to other SCFTs arising from M2-branes.

Irene Valenzuela (CERN and IFT Madrid)

29 March 2023, 13.45

Location: Zoom

Title: Where do we live in the string landscape?

Abstract: In this talk, I will discuss the possibility that our universe lies near the boundary of the field space in string theory, including the theoretical challenges and the exciting phenomenological implications. These boundaries share some universal properties imposed by quantum gravity (sometimes promoted to Swampland constraints) that resemble our universe, like weak couplings, approximate global symmetries or small (time-dependent) vacuum energy. However, it remains as an open challenge to get an accelerated cosmology. We study whether the runaway behaviour of stringy scalar potentials towards infinite distance can produce an accelerated expanding cosmology a la quintessence, finding some potential examples in F-theory flux compactifications. I will discuss the caveats of these examples and the comparison to Swampland bounds. Furthermore, a universal feature of these regions is that there is a light infinite tower of states which is correlated to the value of the vacuum energy. I will show how experimental constraints force this tower to correspond to a KK tower (of mass of order neutrino scale) of a single extra mesoscopic dimension of order 10-6 m, which we denote as the Dark Dimension.

Pedro Liendo (DESY)

22 March 2023, 15.15

Location: Å80127

Title: Bootstrapping line defects with O(2) global symmetry

Abstract: We discuss 1d conformal line defects with O(2) symmetry using the numerical bootstrap. We start with an agnostic approach and perform a systematic bootstrap study of correlation functions of  two canonical defect operators: the displacement and the tilt, without making any assumptions regarding the spectrum of the theory. We then move on to study a specific model: a localized magnetic field line defect, where we use analytic input coming from an epsilon-expansion. The interplay between analytical and numerical techniques gives intriguing results which we explore. We also comment on the generalization of our analysis to fermionic CFTs.

Giulia Isabella (IPhT, Saclay and IJCLab, Orsay)

15 March 2023, 13.15

Location: Å80115

Title: Quantum and Classical Eikonal Scattering

Abstract: I will discuss the eikonal scattering of two gravitationally interacting bodies, showing that exponentiation of the scattering phase matrix is a direct consequence of the group contraction $SU(2) \rightarrow ISO(2)$, in the large angular momentum limit. The emergence of the classical limit is understood in terms of the continuous-spin representations admitted by $ISO(2)$. We will compare the competing classical and quantum corrections to the leading classical eikonal scattering in the transplanckian regime and discuss how observables are extracted from the scattering phase matrix.

Alexander Monin (EPFL, USC)

08 March 2023, 13.45

Location: Zoom

Title: Going beyond perturbative for large quantum numbers

Abstract: It is generally expected that states with large quantum numbers can be described semiclassically. In the context of CFT with additional $U(1)$ symmetry semiclassical methods allow to find the spectrum of large charge primary operators in a class of models. I will explain the methodology with an example of a scalar theory at Wilson Fisher fixed point in $d=3-\epsilon$ dimensions and show how CFT data (scaling dimensions and fusion coefficients) can be obtained systematically as the inverse charge power series. I will also present a construction allowing to identify all spinning primary operators with number of derivatives bounded by the charge in a free 3d scalar theory.

Souvik Banerjee (Würzburg U.) -- POSTPONED

01 March 2023, 13.45

Location: Å80121

Title: TBA

Abstract: TBA

Shruti Paranjape (UC Davis, QMAP)

25 January 2023, 15.15

Location: Å80121

Title: Non-Planar Geometries from Tree-Level Supersymmetric Yang-Mills

Abstract: The special structure of interactions in maximally supersymmetric Yang-Mills (SYM) allows us to construct certain scattering amplitudes on-shell. Most known geometric constructions work for N=4 SYM in the planar limit. In this talk, we make a first step towards non-planar SYM. To do this, we express tree-level amplitudes as a sum of “non-planar” contributions. These non-planar puzzle pieces appear on unitarity cuts of non-planar loop integrands. We give a nice geometric interpretation of these terms and discuss the extension of these non-planar geometries to more general cases, including a possible extension to theories of gravity.

Fall 2022

Marko Berghoff (HU Berlin)

07 December 2022, 13.45

Location: Å80109

Title: The geometry of scattering amplitudes

Abstract: Scattering amplitudes can be studied from many viewpoints, connecting them with (a priori) very different areas of mathematics. One example is the work of Bloch, Brown, Esnault, Kreimer (and others) who uncovered the (algebraic) geometry behind Feynman periods. Generalizing this approach to Feynman amplitudes poses some serious difficulties, as then the geometry depends on parameters (kinematics): Instead of single periods, we now need to understand families of periods, or rather the respective geometries. While the complete answer remains a mystery, a lot of information can already be gained from studying where and how the geometry degenerates. This “singularity theory” approach allows to determine the poles and branch points of amplitudes (the Landau variety) and make some qualitative statements about their nature. In particular, it produces vanishing criteria for iterated variations (e.g. (extended) Steinmann relations). This is joint work with Erik Panzer (U Oxford).

Ling Lin (Oxford)

30 November 2022, 13.45

Location: Å4003

Title: Aspects of decomposition

Abstract: A d-dimensional quantum field theory with (d-1)-form symmetry is conjectured to “decompose” into individual quantum field theories. After a review of this phenomenon, I will discuss two instances where decomposition occurs in a subtle way: the fusion rules of condensation defects in 3d gauge theories, and in string / M-theory compactifications with frozen singularities.

Alfredo Guevara (Harvard)

Thu 24 November 2022, 15.15

Location: Å80121

Title: Gravity amplitudes, W_{1+\inf} and color-kinematics

Abstract: W-algebras are ubiquitous extended symmetries of a vast class of CFTs, with deep connections to quantum groups and integrability. Recently, through the techniques of celestial holography, the W_{1+inf} algebra was realized explicitly in celestial correlation functions associated to 4d gravitational scattering amplitudes, but the connection to other realizations of the symmetry, and in particular integrability, was vaguely understood. In this talk I will attempt to shed some light on this issue, arguing that the emergence of the symmetry is directly connected to the color-kinematics duality relating gravity and gauge theory amplitudes, at the same time unveiling an associative structure in collinear singularities of the tree-level S-Matrix.

Giulio Salvatori (Brown U.)

02 November 2022, 13.45

Location: Å80109

Title: Positive Geometries and Scattering Amplitudes

Abstract: In recent years it has been appreciated how the physics of scattering amplitudes can be described in terms of Positive Geometry. While this was first understood in the context of scattering in maximally super symmetric Yang-Mills theory, whose planar S-matrix is tied to a geometry called Amplituhedron, recent developments have shown how a positive geometric picture holds more generally: bosonic string theory, non supersymmetric field theories beyond the planar limit and even individual Feynman diagrams all admit a geometry describing some of their most salient features. In my talk I will review these recent developments in a self contained fashion, without assuming any prior knowledge on the topic.

Alessandro Pini (Turin U.)

Thu 13 October 2022, 13.45

Location: Zoom

Title: Localization vs holography in some 4d N=2 SCFTs

Abstract: We analyse two distinct 4d N=2 SCFTs. The first theory has gauge group SU(N) and matter in the symmetric plus anti-symmetric representation. The second is a quiver gauge theory obtained with an orbifold projection from N=4 SYM. For both theories we compute the 2-point and 3-point correlation functions among chiral/anti-chiral single trace operators and the corresponding structure constants. In the first part of the seminar, using supersymmetric localization, we map the computation to an interacting matrix model and obtain expressions for the structure constants that are valid for any value of the 't Hooft coupling in the planar limit of the theory. In particular, at strong coupling, these expressions simplify and allow us to extract the leading behaviour in an analytic way. We successfully compare these predictions to a direct Monte Carlo numerical evaluation of the matrix integral and to Padé resummation derived from very long perturbative series. Finally, using the AdS/CFT correspondence, we compute the structure constants from the dual supergravity theory and obtain results that perfectly match the strong-coupling predictions from localization.

Anindita Maiti (IAIFI, Cambridge U.)

05 October 2022, 13.45

Location: Zoom

Title: A Study of Neural Network Field Theories

Abstract: The backbones of modern-day Deep Learning, Neural Networks (NN), define field theories on Euclidean background through their architectures, where field interaction strengths depend on the choice of NN architecture width and stochastic parameters. Infinite width limit of NN architectures, combined with independently distributed stochastic parameters, lead to generalized free field theories by the Central Limit Theorem (CLT). Small and large deviations from the CLT, due to finite architecture width and/or correlated stochastic parameters, respectively give rise to weakly coupled field theories and non-perturbative non-Lagrangian field theories in Neural Networks. I will present a systematic exploration of Neural Network field theories via a dual framework of NN parameters: non-Gaussianity, locality by cluster decomposition, and symmetries are studied without necessitating the knowledge of an action. Such a dual description to statistical or quantum field theories in Neural Networks can have potential implications for physics.

Fridrik Gautason (Iceland U.)

28 September 2022, 13.45

Location: Å80127

Title: Wilson loops and non-conformal Holography

Abstract: I will discuss two examples of non-conformal holographic theories. First is the maximally supersymmetric Yang-Mills theory in five dimensions, and the second is a mass deformation of N=4 SYM in four dimensions commonly known as N=2^*. Using supersymmetric localization, the expectation value of  BPS Wilson loop operator can be computed exactly at large rank and strong coupling in both of these theories. In the talk I will review these results and explain how to reproduce them in string theory.

Martin Cederwall (Gothenburg U.)

21 September 2022, 13.45

Location: Å80101

Title: Supersymmetry and Koszul duality

Abstract: Pure spinor superfield theory provides a way to describe essentially any supermultiplet as the cohomology of a nilpotent operator Q=\lambda^\alpha D_\alpha, the contraction of a spinorial covariant superspace derivative with a spinor, which is constrained to obey (\lambda\gamma^a\lambda)=0. I will give a very brief review of how this comes about, and how it can be used to obtain Batalin-Vilkovisky actions for D=10 super-Yang-Mills theory and D=11 supergravity with manifest supersymmetry. Then I will talk about the constrained object \lambda itself, in particular how the very simple quadratic constraint (without any reference to space-time) encodes the full information about the supermultiplet in question. The tool used is the Koszul dual superalgebra – in an appropriate sense, which I will explain – to the associative algebra generated by \lambda. I will maybe also comment on some future prospects and applications. This is work in progress in collaboration with S. Jonsson, J. Palmkvist and I. Saberi.

Ulf Lindström (Middle East Tech. U., Ankara and Uppsala U.)

Tue 20 September 2022, 15.15

Location: Å80109

Title: Yano F-structures and extended supersymmetry

Abstract: Extended supersymmetry realised directly on the (2,2) semichiral superfields of a symplectic sigma model gives rise to a geometry on the doubled tangent bundle consisting of two Yano F structures on an almost para-hermitian manifold. Closure of the algebra and invariance of the action is discussed in this framework and integrability of the F structures is defined and shown to hold. The reduction to the usual (1, 1) sigma model description and identification with the bi-quaternionic set of complex structures and their properties is elucidated.

Victor Pozsgay (Imperial College)

Mon 12 September 2022, 13.45

Location: Å101166

Title: Low-energy EFT causality bounds

Abstract: In this talk, I will present a new tool to constrain low-energy Wilson coefficients in a scalar EFT (scalar for simplicity's sake but the range of applicability is much wider) based on the requirement that such theories should respect causality. Causality will be defined in the sense that no low-energy observer should be able to measure any resolvable time-advance resulting from a scattering event. I will show that these so-called causality bounds are in remarkable agreement with previously derived positivity bounds (where low energy constraints on the 4-point amplitude make use of physical assumptions of the UV completion of the EFT), while being considerably simpler and a better candidate to get cosmological and black hole gravitational bounds.

Felipe Diaz-Jaramillo (Humbolt U.)

7 September 2022, 15.15

Location: Å80127

Title: Double field theory as the double copy of Yang-Mills theory

Abstract: The Bern-Carrasco-Johansson (BCJ) double copy in scattering amplitudes states that exchanging colour information by kinematic information in Yang-Mills (YM) amplitudes leads to gravity amplitudes. In this talk, based on the BCJ double copy, I will establish a relation at the level of the action and gauge structure between YM and a reformulation of the Graviton-B-field-Dilaton theory on a doubled spacetime, called Double Field Theory (DFT). I will present two approaches to do this: the first one is a simple colour-kinematic substitution in the YM action that yields the gauge invariant action of DFT at quadratic order and a gauge-fixed version at cubic order. The second approach is based on Lie homotopy algebras. These mathematical structures encode field theories including their symmetries and, in particular, the algebra of YM exhibits a manifest colour-kinematic split. I will show that, in the spirit of the BCJ construction, replacing colour by kinematics at this algebraic level reproduces the gauge invariant DFT action up to cubic order.

Last modified: 2023-09-27