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: Luca Cassia (, Nicolò Piazzalunga (

Spring 2022

Mauricio Romo (Tsinghua U.)

18 August 2022, 13.45

Location: TBA

Title: TBA

Abstract: TBA

Marcus Berg (Karlstad U.)

Mo 20 June 2022, 13.45

Location: Å90101

Title: One-loop N<max string amplitudes

Abstract: I will review some work in progress on field theory limits of one-loop four-point string amplitudes in less-than-maximal supersymmetry. This is a continuation of 1603.05262.

Marco David (ENS Paris & Uppsala U.)

08 June 2022, 13.45

Location: Å90101 or Zoom

Title: QED. The Quest to Formally Verify Mathematics

Abstract: Mathematics prides itself as the most fundamental of sciences, constructed as a pyramid of proven truths. Yet, the proof style of most mathematicians is strikingly different from an ideal, fully rigorous argument. At the same time, mathematical proofs are becoming increasingly sophisticated, stretching the boundaries of what is humanly comprehensible. Inevitably, mistakes will go unnoticed, putting the unique status of mathematics as exact science into peril. Now may be the time to reconcile the intuitive with the correct through the advent of “computer proof assistants”. For the most time, this has been a topic for experts in specialized communities. More recently, major theorems in mathematics have been computer-verified by people from outside of these communities, even by students. This talk presents a journey through the field of interactive theorem proving with particular emphasis on mathematics. Challenging well-known preconceptions and exploring new perspectives, we give an outlook on future applications, including in mathematical physics.

Mariana Carrillo Gonzalez (Imperial College)

01 June 2022, 13.45

Location: Å90101

Title: Constructing Massive Double Copies

Abstract: In this talk, I will give a short introduction to the double copy relation that gives gravity as the “square” of Yang-Mills. I will focus on explaining how to extend this relation to the case of massive mediators and explain the issues that arise in this situation. I will mention some standard ways of solving these issues as well as two novel theories that can have well-defined double copies. One consists of a cubic scalar with an additional infinite-dimensional symmetry and the other one consists of topologically massive theories. Time permitting, I will also mention a classical double copy relation that arises for the latter.

Vasco Gonçalves (Porto U.)

Thu 26 May 2022, 13.45

Location: Zoom

Title: The Wilson Loop – Large Spin OPE Dictionary

Abstract: In this talk I will work out the map between null polygonal hexagonal Wilson loops and spinning three point functions in large N conformal gauge theories by mapping the variables describing the two different physical quantities and by working out the precise normalization factors entering this duality. By fixing all the kinematics we open the ground for a precise study of the dynamics underlying these dualities – most notably through integrability in the case of planar maximally supersymmetric Yang-Mills theory.

Lionel Mason (Oxford U.)

25 May 2022, 13.45

Location: Å80121

Title: The twistor origin of hidden w-infinity symmetries in celestial gravity

Abstract: Recently in their celestial holography programme, Strominger and coworkers attempt to provide a holographic description of conventional 4d gravity. In their investigations, they uncovered a hidden w-infinity symmetry in their ‘celestial soft OPEs’ for graviton scattering. This talk will explain the origin of this symmetry in terms of old ideas of Newman and Penrose based on light-cone cuts of null infinity and their description in terms of asymptotic twistors and certain sigma models in asymptotic twistor space. Wn symmetries were introduced by Zamolodchikov as higher spin symmetries in 2d conformal field theories. These were given a geometric interpretation for n=infinity as area-preserving diffeomorphisms of the plane. I will explain how the corresponding loop algebra becomes a hidden symmetry of self-dual gravity via Penrose's nonlinear graviton construction. The action of this symmetry on the tree-level S-matrix of full gravity beyond the self-dual sector will then be obtained from its action on a sigma model in the asymptotic twistor space of a general space-time. This talk is based on 2110.06066 and 2103.16984.

Emil Bjerrum-Bohr (NBI)

18 May 2022, 15.15

Location: Å80121

Title: Binary Merger Dynamics from Scattering Amplitudes

Abstract: Gravity is a fundamental force but so weak that we have limited understanding of its extreme interactions. A recent exciting development is the detection of collisions of black holes in the Universe. These phenomena witnessed by the Laser Interferometer Gravitational-Wave Observatory (LIGO) have refreshed enthusiasm for exploration of gravitational interactions and stimulated critical advancement of precision analysis of merger events. We will in this talk outline how to derive dynamics in general relativity by application of modern on-shell amplitude techniques and discuss how such methods can complement existing technology and facilitate computational progress.

Davide Passaro (Amsterdam U.)

18 May 2022, 13.45

Location: Å80121

Title: 3-manifolds and VOAs through Ẑ invariants

Abstract: Q-series are a concise and elegant way of encoding physical and mathematical information into a single function. Examples of such q-series are the Ẑ 3-manifold invariants also known as homological blocks. Surprisingly Ẑ invariants appear in widely different areas throughout mathematics and physics, such as low dimensional topology and vertex operator algebras (VOA). The appearance of the Ẑ invariants in these two contexts originates from the unexpected number theoretical properties that Ẑ invariants have. In this talk I will discuss how the connection between 3-manifolds and  VOAs was established and how it has led to an increased understanding of the number theoretical aspects of Ẑ invariants.

Michelangelo Preti (King's College)

11 May 2022, 13.45

Location: Å80101

Title: Bootstrability in defect CFT

Abstract: We study how the exact non-perturbative integrability methods in 4D N = 4 Super-Yang-Mills can work efficiently together with the numerical conformal bootstrap techniques to go beyond the spectral observables and access previously unreachable quantities such as correlation functions at finite coupling. We consider the 1D defect CFT living on a 1/2-BPS Wilson line, whose non-perturbative spectrum is governed by the Quantum Spectral Curve (QSC). In addition, we use that the deformed setup of a cusped Wilson line is also controlled by the QSC. In terms of the defect CFT, this translates into two nontrivial relations connecting integrated 4-point correlators to cusp spectral data, such as the Bremsstrahlung and Curvature functions – known analytically from the QSC. Combining these new constraints and the spectrum of the 10 lowest-lying states with the Numerical Conformal Bootstrap, we obtain very sharp rigorous numerical bounds for the structure constants of the first non-protected states. Furthermore, we also develop analytic functional bootstrability obtaining weak coupling results for several structure constants.

Fabrizio Del Monte (Montreal U., CRM)

04 May 2022, 13.45

Location: Å4004

Title: Supersymmetric partition functions and isomonodromic tau functions

Abstract: Class S theories are four-dimensional N=2 QFTs that are obtained by wrapping M5-branes on Riemann surfaces with singular points. In recent years a correspondence between class S theories and isomonodromic deformations equations (whose simplest examples are the six Painlevé equations) is emerging, where the so-called isomonodromic tau function is identified with the dual partition function of the QFT. The tau functions can be typically written as Fredholm determinants or Pfaffians, providing a fully nonperturbative approach to study partition functions of class S theories, relying only on the geometric data entering in the string theory construction, and not on a classical action. After reviewing the Fredholm determinant formulation of tau functions for spheres with regular singularities, corresponding to linear quiver gauge theories, I will show how the description can be extended to tau functions with orthogonal gauge groups, that are written as Fredholm Pfaffians, and to tau functions for tori with regular singularities, corresponding to circular quiver gauge theories. I will conclude by showing how the tau function description allows to study the S-duality transformation of the resulting gauge theory dual partition function.

Anton Nedelin (Geneva U.)

27 April 2022, 13.45

Location: Å90101

Title: Integrable Models from N=1 Superconformal Indices

Abstract: In this talk I will present a method of derivation of previously known and new integrable finite difference operators using N=1 quantum field theories. I will start with brief review of the 4d compactifications of 6d SCFTs leading to the wide class of 4d SCFTs. Then I will show how to derive various finite difference operators using superconformal indices of this 4d theories as well as some intuition coming from the geometry of 6d compactifications. The talk is based on the preprint 2106.08335 as well as ongoing work in collaboration with Shlomo Razamat and Belal Nazzal.

Pietro Longhi (Uppsala U.)

20 April 2022, 13.45

Location: Å80115 or Zoom

Title: Instanton partition functions beyond weak coupling

Abstract: Localization provides a way to compute instanton partition functions of 4d N=2 QFTs by means of a Lagrangian description. The resulting expressions take the form of power series in the instanton counting parameters, and accordingly their range of validity is restricted to a neighbourhood of the weak-coupling regime where the series converges. I will review a proposal formulated in joint work with Coman and Teschner for the analytic continuation of 4d N=2 instanton partition functions beyond weak coupling, based on a relation to isomonodromic deformations of quantum Seiberg-Witten curves.

Raffaele Savelli (Rome U.)

06 April 2022, 13.45

Location: Å101136.B

Title: Computing OPE coefficients for Argyres-Douglas theories

Abstract: Argyres-Douglas theories are a special class of N=2 superconformal field theories in four dimensions, featuring an intrinsically strong dynamics. Due to the lack of a Lagrangian description, determining their properties quantitatively is a challenge. In this talk, I will present a general formula, which, inspired by the fact that such theories appear as saddle points of the sphere partition function of asymptotically-free gauge theories, computes, through localization techniques, the OPE coefficients between their Coulomb-Branch operators. The formula only depends on intrinsic data of the conformal field theory, that can be extracted from its Seiberg-Witten geometry. Although it neglects higher gravitational corrections, this formula gives results in surprising agreement with the bootstrap bounds. The talk is based on the work ArXiv:2112.11899, done in collaboration with A. Bissi, F. Fucito, A. Manenti, and F. Morales.

Alex Ochirov (Oxford U.)

30 March 2022, 13.45

Location: Å90101

Title: Classical scattering of spinning black holes from quantum amplitudes

Abstract: In view of the observations of gravitational-wave signals from black-hole mergers, classical black-hole scattering has received considerable interest due to its relation to the classical bound-state problem of two black holes inspiraling onto each other. In this talk I will discuss the link between classical scattering of spinning black holes and quantum scattering amplitudes for massive spin-s particles. I will derive the spin-exponentiated structure of the relevant tree-level amplitude from (in some sense) “minimal” coupling to Einstein's gravity, which in the s → ∞ limit generates the black holes' complete series of spin-induced multipoles. I will phrase this limit rigorously in the language of coherent-spin quantum states. Considering the first post-Minkowskian (PM) order, I will show how the resulting scattering function encodes in a simple way the classical net changes in the black-hole momenta and spins at 1PM order and to all orders in spins. I will then comment on the results and challenges at 2PM order and beyond.

Naveen Prabhakar

16 March 2022, 10.30

Location: Zoom

Title: A Hilbert space for large N Chern-Simons matter theories

Abstract: Chern-Simons theories in three dimensions coupled to vector-like matter are exactly solvable in the large N limit. This led to the discovery of ‘bosonisation’ in three dimensions wherein a Chern-Simons gauged fermion is dual to a Chern-Simons gauged boson with levels and ranks interchanged. The coupling of Chern-Simons gauge theory to bosons and fermions modifies the spin and the statistics of the matter excitations. Recently, the occupation number distribution of such Chern-Simons coupled matter was computed in the large N limit and was found to be a one-parameter interpolation between Fermi-Dirac and Bose-Einstein statistics. Subsequently, this modification of the statistics was understood in terms of an exclusion principle for bosons that arises due to the coupling to Chern-Simons gauge theory. In the talk, we will discuss these matters briefly and allude to possible future developments in this rich and fertile subject.

Johannes Walcher (Heidelberg U.)

09 March 2022, 13.45

Location: Zoom

Title: Irrationality of MUMs and extended mirror symmetry

Abstract: It was pointed out a while ago that the generic predictions of extended mirror symmetry depend on arithmetic data such as the field of definition of the algebraic cycle representing the D-brane in the B-model. Recently, we are exploring closed string analogues of this phenomenon in the guise of Calabi-Yau differential equations with points of maximal unipotent monodromy that are equally irrational. In my talk, I will explain the mathematical meaning and interest of these statements through examples, though we do not understand their physical interpretation.

Miroslav Rapčák (UC, Berkeley)

23 February 2022, 16.30

Location: Zoom

Title: Branes, BPS Algebras and COHA

Abstract: According to the famous AGT correspondence, there exists an action of the WN algebra on the cohomology of the moduli space of instantons. This module admits an interpretation in terms of the space of vacua of a quantum mechanics describing the low energy dynamics of D0-branes bound to a stack of N D4-branes. The WN algebra then captures processes of bounding and separating D0-branes from the stack. It is natural to ask if the story generalizes to D0-branes bound to intersecting branes and branes of different dimensions. In this talk, I will 1. Review the construction of the module structure using correspondences in algebraic geometry. 2. Illustrate the construction on the simplest example of D0-branes bound to a single D2-brane, recovering the vector representation of the gl1 affine Yangian 3. Comment on some recent developments in the cohomological Hall algebra approach to the correspondence between branes and representation theory.

Rodolfo Panerai (Uppsala U.)

Tuesday 22 February 2022, 14.15 (Nordic HET)

Location: Zoom

Title: Exact TTbar-deformation of two-dimensional Yang-Mills theory

Abstract: In this talk, I will discuss the TTbar deformation of two-dimensional Yang-Mills theory on the sphere. Focusing on unitary gauge groups, I will show how the deformed partition function can be obtained by solving the relevant flow equation at the level of individual monopole sectors. For positive values of the deformation parameter, the quantum spectrum of the theory experiences a truncation, the partition function reducing to a sum over a finite set of energy states. For negative values, instead, the appearance of non-perturbative contributions drastically modifies the structure of the partition function, regularising its naive divergences and seemingly breaking locality. In the large-N expansion, the theory exhibits a rich phase diagram. The transitions between different phases are driven by instantons both in the 't Hooft coupling (leading to a deformed Douglas-Kazakov phase transition) and in the deformation parameter.

Niccolò Cribiori (Munich U., Max Planck Inst.)

16 February 2022, 13.45

Location: Zoom 658 2223 8569

Title: Open-Closed correspondence of K-theory and Cobordism

Abstract: Non-trivial K-theory groups and non-trivial cobordism groups can lead to global symmetries which are conjectured to be absent in quantum gravity. In this talk, a correspondence between the two groups is proposed. It is inspired by open-closed string duality and relies on a mathematical result known as Conner-Floyd isomorphism. As an application, it is shown how to recover known tadpole cancellation conditions of string theory from a purely bottom-up perspective. In the spirit of the swampland program, this can be used to give further support to the string lamppost principle.

Ingmar Saberi (Munich U.)

26 January 2022, 14.30

Location: Zoom 664 3233 5042

Title: Twisted eleven-dimensional supergravity and exceptional Lie superalgebras

Abstract: Twists of supersymmetric field theories have been the subject of intense and fruitful study for more than thirty years. Recently, there has been a great deal of progress on extending the technique of twisting to supergravity theories, building on the definition of twisted supergravity given by Costello and Li. Much of what is explicitly known about these theories comes from the topological B-model, whose string field theory, as studied by Bershadsky-Ceccotti-Ooguri-Vafa, conjecturally produces the holomorphic twist of type IIB supergravity. Progress on other supergravity theories, including eleven-dimensional supergravity, has been hindered by the lack of such a worldsheet approach. I will discuss a rigorous computation of the twist of the free eleven-dimensional supergravity multiplet, as well as an interacting BV theory with this field content that passes a large number of consistency checks. Surprisingly, the resulting holomorphic theory on flat space is closely related to the infinite-dimensional exceptional simple Lie superalgebra E(5,10). This is joint work with Surya Raghavendran and Brian Williams.

Francesco Aprile (Sao Paulo, IFT)

19 January 2022, 13.45

Location: Zoom

Title: Conformal blocks in diverse dimensions and the superconformal Cauchy identity

Abstract: I will describe 4pt conformal blocks for scalar operators in diverse dimensions by using a single unified formalism. I will discuss a property of the conformal blocks known as stability. This implies that when writing conformal blocks as certain multivariate series, the coefficients of this expansion only depend on Young diagrams. In particular, we can bosonise the conformal computation and simply focus on theories with compact groups. In this framework the blocks are polynomials of the BC root system after we apply complementation. I will then explain the connection with mathematics and show how to formulate a superconformal Cauchy identity which yields the CPW of any free theory diagram in any dimension. For discussion, I will finally mention q-deformations results in our framework.

Fall 2021

Jakub Vošmera (ETH)

Tuesday 14 December 2021, 14.15 (Nordic HET)

Location: Zoom

Title: D-branes in AdS3 x S3 x T4 at k = 1 and their holographic duals

Abstract: Following the recent work of Eberhardt, Gaberdiel and Gopakumar, exact comparison between various quantities living on the two sides of the AdS/CFT duality has become a possibility. The goal of this talk will be to extend the existing holographic dictionary to include some non-perturbative vacua on both sides. I will start by reviewing the original, purely closed-string setup, giving arguments that string theory on AdS3 x S3 x T4 with minimal k=1 NS-NS flux is exactly dual to the symmetric-product orbifold CFT with the T4 as the seed. I will then construct various D-branes of this string theory and calculate their associated cylinder amplitudes. We will observe that these amplitudes match with the cylinder correlators of certain boundary states of the dual CFT, thus suggesting a direct correspondence between these boundary conditions. I will also show that the disk amplitudes of these D-branes localise to those points in the worldsheet moduli space where the worldsheet disk holomorphically covers the spacetime disk. This talk is based on 2110.05509.

António Antunes

08 December 2021, 13.45

Location: Zoom

Title: Lightcone Bootstrap at higher-points

Abstract: Higher-point functions of scalar operators are a rich observable in CFTs, as they contain OPE data involving multiple spinning operators. In this talk, I will derive the lightcone blocks for five- and six-point functions in the snowflake channel and use them to bootstrap these correlators in the lightcone limit. As a result, I will determine the large spin expansion of OPE coefficients involving two or three spinning operators, which can then be checked in simple theories. Finally, I will point out many open directions of research involving CFT higher-point functions.

Joshua Kames-King (Bonn U. and Santa Barbara, KITP)

Tuesday 30 November 2021, 15.45 (Nordic HET)

Location: Zoom

Title: No Page curves for de Sitter

Abstract: The nature of de Sitter horizon and a possible microscopic interpretation are still a mystery. In this talk I will explain how we investigated the question of unitary evaporation for the de Sitter horizon in [2108.09318]. More concretely, in the out-of-equilibrium Unruh de Sitter state, analogous to black hole evaporation for cosmological horizons we study the time-dependent radiation entropy for both the meta-observer and the static patch observer. I will conclude by commenting on work in progress relating to recent approaches to static patch holography and quantum information in cosmological spacetimes.

Mathew Bullimore (Durham U.)

17 November 2021, 13.45

Location: Zoom

Title: 3d Supersymmetric Gauge Theories on an Elliptic Curve

Abstract: TBA

Matteo Bertolini (SISSA)

Tuesday 09 November 2021, 14.15 (Nordic HET)

Location: Zoom

Title: On fixed-points and phase transitions in five dimensions

Abstract: Supersymmetric gauge theories in five dimensions, although power counting non-renormalizable, are known to be in some cases UV completed by a superconformal field theory (SCFT). Similar results are not available without supersymmetry and it is in fact an open question whether non-supersymmetric CFT exist at all in five dimensions. A concrete way to try and answer this question is to deform known superconformal fixed points by non-supersymmetric relevant deformations and study the end-point of the RG-flow. Recently, a supersymmetry-breaking deformation of one such SCFT, the so-called E1 theory, was proposed which, at weak gauge coupling, leads to pure SU(2) Yang-Mills and which was conjectured to lead to an interacting CFT at strong coupling. Using both field theory and brane-web techniques, I will show that for large enough gauge coupling a global symmetry is spontaneously broken and the theory enters a new phase which, at infinite coupling, displays an instability. The Yang-Mills and the symmetry broken phases are separated by a phase transition, which can be first or second order. Quantum corrections to this analysis are discussed, as well as possible outlooks.

Adam Chalabi

03 November 2021, 13.45

Location: Zoom

Title: Defect Central Charges

Abstract: Conformal defects can be characterised by their contributions to the Weyl anomaly. The coefficients of these terms, often called defect central charges, depend on the particular defect insertion in a given conformal field theory. I will review what is currently known about them across dimensions, and present novel results. I will discuss many examples where they can be computed exactly without requiring any approximations or limits. These include defects in free theories, and recently developed tools for defects in superconformal field theories.

Hans Jockers (Hamburg U.)

27 October 2021, 13.45

Location: Zoom

Title: Quantum K-Theory and Wilson Loop Algebras

Abstract: In this talk we review certain aspects of Wilson line operators in 3d N=2 supersymmetric gauge theories with a Higgs branch that is geometrically described by complex projective varieties. We discuss the relationship between Wilson loop algebras of the gauge theory and the quantum K-theoretic ring of the associated complex projective variety.

Eric Sharpe (Virginia Tech.)

06 October 2021, 16.00

Location: Zoom

Title: Decomposition of quantum field theories

Abstract: In this talk, I will outline higher-form symmetries in quantum field theories, focusing on two-dimensional theories with one-form symmetries. A fundamental QFT result (first worked out in 2006, but recently becoming more widely understood) is that such theories are equivalent to disjoint unions of other quantum field theories, an equivalence known as ‘decomposition’. We will outline some basic examples, as well as applications to Gromov-Witten theory, gauged linear sigma models, elliptic genera, and anomaly resolution in orbifolds.

Dmitry Galakhov

29 September 2021, 13.45

Location: Zoom

Title: Quiver BPS Algebras

Abstract: In this talk we will discuss BPS algebras associated to BPS states in systems of D-branes wrapping toric Calabi-Yau threefolds. These algebras are labeled by framed quivers and are usually referred to as shifted quiver Yangians generalizing a notion of affine Yangians introduced in integrable models. Accordingly, quiver BPS algebras appear nested naturally in rational-trigonometric-elliptic hierarchies. I will derive these algebras from an effective gauge field description of D-brane systems as well as their representations, usually referred to as MacMahon modules, or molten crystal models. The analysis of these theories suggests that there exist even richer classes of algebras associated with generalized cohomology theories going beyond the traditional rational-trigonometric-elliptic hierarchy. This talk is based on arXiv:2108.10286 with Wei Li and Masahito Yamazaki.

Markus Dierigl (Munich U., ASC)

15 September 2021, 13.45

Location: Zoom

Title: The anomaly that was not meant IIB

Abstract: In this talk I will discuss possible quantum anomalies in the duality group of type IIB string theory. Quite surprisingly, these do not vanish, which would signal an inconsistency of F-theory. A subtle modification of the type IIB Chern-Simons term cancels the anomalies in a highly non-trivial way. However, in principle there are alternative ways to cancel the anomaly which might point towards the existence of a discrete string theory landscape.

Slides: pdf

Senast uppdaterad: 2022-06-22