# Wednesday Seminars

### 2019/2020

Due to the ongoing COVID-19 situation all local spring seminars are cancelled. There are exceptions for speakers using online tools, such as Zoom. In the meantime there are joint Nordic online seminars. Details can be found here.

### Andrea Puhm (CPHT)

23 June 2020, 14:15 PM

Location: Zoom

Title: Asymptotic symmetries and celestial CFT

Abstract: The realization that soft theorems in gauge theory and gravity are manifestations of Ward identities for asymptotic symmetries has reinvigorated recent attempts at flat space holography. One of the key milestones from applying this paradigm is that it solidifies a conjectured extension of the BMS group to include superrotations, based on a newly discovered subleading soft graviton theorem. This precipitated two parallel initiatives within the field. On the one hand, the subleading soft graviton mode is a natural stress tensor candidate for a putative dual celestial CFT with a Virasoro symmetry, which provoked reexamining scattering amplitudes in a basis that makes conformal covariance manifest. On the other hand, the correspondence between the subleading soft graviton theorem and the proposed asymptotic Virasoro superrotation symmetry does not appear to be bijective and an extension to arbitrary diffeomorphisms on the celestial sphere was suggested. In this talk I will provide a unified treatment of conformally soft Goldstone modes of spontaneously broken asymptotic symmetries which will land us at the crossroads of the two ongoing debates about what the appropriate conformal basis for celestial CFT is and what the asymptotic symmetry group of Einstein gravity at null infinity should be.

### Alessandra Gnecchi (CERN)

09 June 2020, 14:15 PM

Location: Zoom

Title: On AdS7 Stability – a perturbative and non-perturbative analysis

Abstract: The stability of non-supersymmetric vacua obtained from string theory is receiving a lot of attention due to its implications for the recent generalizations of the Weak Gravity Conjecture. Starting from the known classification of supersymmetric AdS7 vacua in massive type IIA supergravity, I will present the study of their non-supersymmetric counterparts. For a large subset of them, perturbative instabilities can be found both from a 7D Supergravity analysis as well as from the study of polarization of the D6 brane action in 10D. For the remaining, perturbative stable ones, I will show the existence of a non-perturbative decay channel mediated by an NS5 bubble nucleation.

### Alberto Zaffaroni (Milano-Bicocca)

12 May 2020, 14:15

Location: Zoom

Title: Gluing gravitational blocks for AdS black holes

Abstract: I describe an entropy functional and an attractor mechanism that apply to all known supersymmetric black hole in AdS4 x S7 and AdS5 x S5 with arbitrary rotation and arbitrary electric and magnetic charge. The construction is inspired by recent results about the microscopic counting of degrees of freedom for AdS black holes.

### Gregory Korchemsky (IPT Saclay)

05 May 2020, 14:15

Location: Zoom

Title: Exact correlation functions of heavy half-BPS operators in planar N=4 SYM

Abstract: I will report on a recent progress in computing four-point correlation functions of infinitely heavy half-BPS operators in planar N = 4 SYM. Taking advantage of integrability of the theory, it has recently been realized that these correlation functions can be constructed in terms of fundamental building blocks - the octagon form factors. We demonstrate that these functions satisfy a system of nonlinear integro-differential equations which are powerful enough to fully determine their dependence on the ’t Hooft coupling and two cross ratios. At weak coupling, solution to these equations yields a known series representation of the octagon in terms of ladder integrals. At strong coupling, we develop a systematic expansion of the octagon in the inverse powers of the coupling constant and calculate accompanying expansion coefficients analytically.

### Iñaki García-Etxebarria (Durham University)

21 April 2020, 14:15

Location: Zoom

Title: IIB flux non-commutativity and the global structure of field theories

Abstract: I will discuss the origin of the choice of global structure (or equivalently, the choice for which higher p-form symmetries are present in the theory) for various Lagrangian and non-Lagrangian field theories in terms of their realization in IIB and M-theory. I will explain how this choice on the field theory side can be traced back to the fact that fluxes in string/M-theory do not commute in the presence of torsion. I will illustrate how these ideas provide a stringy explanation for the fact that six-dimensional (2,0) and (1,0) theories generically have a partition vector (as opposed to a partition function) and explain how this reproduces the classification of N=4 theories provided by Aharony, Seiberg and Tachikawa. Time permitting, I will describe how to use these ideas to determine the global structure for the Argyres-Douglas theories, and how this can be used to test recently proposed N=1 Lagrangians for some of these theories.

### Itamar Yaakov (Milano-Bicocca)

25 March 2020

Title: Supersymmetric Rényi entropy and charged hyperbolic black holes

Location: Zoom

Abstract: I will present an AdS/CFT computation matching the entropy of supersymmetric electrically charged hyperbolic black holes in four and six dimensions. Hyperbolic black holes are a class of black holes with non-compact horizons which are especially simple to deal with. The CFT calculation is a localization calculation on the squashed/branched sphere with vortex-like defect insertions. It is related in an interesting way to the Supersymmetric Rényi entropy of the dual gauge theory.

### Michele Levi (NBI)

13 March 2020

Title: QFT in the service of gravity

Abstract: In this talk I will present state of the art in PN gravity, and its significant advancement via the EFT of gravitating spinning objects. First, I will introduce the tower of EFTs for the binary inspiral problem. I will then present the intricate formulation of the EFT of spinning objects. Finally, I will present advanced results accomplished uniquely within this framework.

### Michael Reiterer (Hebrew University)

04 March 2020

Title: A homotopy BV algebra for Yang-Mills and color-kinematics

Abstract: Standard Feynman rules for Yang-Mills tree amplitudes fail to comply with the Bern-Carrasco-Johansson/color-kinematics duality. I will make the point that this failure is due to homotopies. These homotopies are local multilinear operations that jointly define a homotopy Batalin-Vilkovisky algebra structure for Yang-Mills on Minkowski spacetime, an algebraic upgrade of standard Yang-Mills. When this homotopy BV structure is rectified, it yields Feynman rules that do comply with BCJ/color-kinematics duality. All relevant algebraic terminology will be introduced from scratch. Based on arXiv:1912.03110.

### Thorsten Schimannek (University of Vienna)

27 February 2020

Title: The quantum geometry of genus one fibered Calabi-Yau threefolds

Abstract: The enumerative geometry of Calabi-Yau manifolds and its interplay with string dualities, most notably mirror symmetry, is by now a classical topic. Nonetheless it continues to lead to new insights in mathematics as well as string theory. In particular, there has been a renewed interest in the properties of genus one fibered Calabi-Yaus, mainly due to their importance for F-theory and geometrical engineering of gauge theories. I will review the basics of topological strings and topological branes and then describe recent progress in understanding and leveraging the quantum geometry of Calabi-Yau manifolds that exhibit a genus one fibration. The results are, via dualities, related to the supersymmetry protected quantities of a variety of different physical theories. The talk is based on 1902.08215, 1910.01988, 1911.09697 and 1912.09493.

### Anton Nedelin (Technion)

19 February 2020

Title: Black holes, Bethe Ansatz and toric geometry

Abstract: Recently there has been a considerable progress in understanding microstate counting of asymptotically-AdS black holes using index computations in the dual CFTs. One of the methods that was crucial for this progress is so called Bethe Ansatz method which was used to evaluate large-N behaviour of the superconformal index of N=4 SYM. In this talk I will briefly review this method and discuss its generalization to the large class of toric theories. I will give examples of calculations for T11 theory and infinite class of Ypq theories. Finally I will argue how our results relate large-N behaviour of the index with the toric geometry of the underlying theory.

### Karapet Mkrtchyan (Scuola Normale Superiore)

12 February 2020

Title: Some remarks related to on-shell methods for amplitudes

Abstract: I will discuss the group-theoretical approach to the amplitudes, employing spinor-helicity variables concentrating on the example of the description of massive spinning particles in four dimensions to illustrate the main concept. The generalizations to higher dimensions will be discussed briefly. This talk is based on earlier work [JHEP 1608 (2016) 040] with E. Conde and E. Joung and an ongoing work with E. Joung and M. Mojaza.

### Oscar Henriksson (Helsinki)

05 February 2020

Title: Stringy instabilities in holographic gauge theories at finite density

Abstract: Holographic duality offers a tool for studying gauge theories at finite density. It is amusing (but perhaps to be expected) that the gauge theory phase that is best understood with conventional perturbative methods has proven the most subtle to understand in holography: the color superconductor. In this talk, I will discuss a stringy instability of charged black brane solutions termed brane nucleation, wherein the system can lower its free energy by placing some of the N D-branes at a finite radius in the bulk, thus realizing a phase analogous to a color superconductor. We will see this occurring in the dual of both N=4 SYM at finite R-charge density and in the Klebanov-Witten gauge theory at finite baryon density.

### Extra: Cumrun Vafa (Harvard)

28 January 2020

Title: A String Theory Perspective on Cosmology

Abstract:

I review some of the lessons we have learned from the identification of what can be constructed in string theory (string landscape) and what cannot (string swampland) and apply it to questions of interest in cosmology. These ideas lead to strong constraints both on early cosmology as well as the present and the future of our universe.

### Kiril Hristov (INRNE Sofia)

22 January 2020

Title: Gluing gravitational blocks

Abstract:

The main purpose of the talk (based on 1909.10550) is to introduce the concept of gravitational blocks, basic building blocks used for supergravity observables. They are directly inspired by the holomorphic blocks appearing in the factorization of supersymmetric partition functions in three and four dimensions. In particular I will focus on the use of the gravitational blocks for determining the entropy function of various black holes and black strings in AdS space. The resulting gluing rules give a prediction about the dual field theoretic superconformal and twisted indices.

### Lorenzo Tancredi

17 January 2020

Title: Analytic treatment of some classes of elliptic Feynman integrals

Abstract:

In this seminar I will present a set of techniques and algorithms that can be used to compute analytically complicated Feynman integrals of elliptic type. In particular, I will show how to write the results in compact form, how to analytically continue them to the whole phase-space relevant for physical applications and how to evaluate them numerically efficiently.

### Nikita Nekrasov

8 January 2020

Title: Blow-up methods in gauge theory, BPS/CFT, and quantum/classical relations

Abstract: TBA

### Matteo Parisi (Oxford)

18 December 2019

Title: Amplituhedra: from Geometry to Scattering Amplitudes

Abstract:

The Amplituhedra A(n,k,m) are generalisations of polytopes introduced as a geometric construction encoding scattering amplitudes in N=4 supersymmetric Yang-Mills theory. These are extracted from a differential form, the canonical form of the Amplituhedron, which emerges from a purely geometric definition.

I will give a gentle introduction to the (tree) amplituhedra, describing them in various special cases, and explaining their relation with scattering amplitudes.

Following my recent works, I will explain how the Jeffrey-Kirwan residue, a powerful concept in symplectic and algebraic geometry, computes the canonical form for whole families of objects, namely for Amplituhedra of type A(n,1,m), which are cyclic polytopes and for their conjugates A(n,n-m-1,m) for even m, which are not polytopes.

This method connects to the rich combinatorial structure of triangulations of Amplituhedra, captured by what we refer to as ‘Secondary Geometry’. For polygons, this is the ‘Associahedron’, explored by Stasheff in the sixties; for polytopes, it is the ‘secondary polytope’ constructed by the Gelfand's school in the nineties. Whereas, for Amplituhedra, we are the first to initiate the studies of what we called the ‘Secondary Amplituhedra’. The latter encodes representations of scattering amplitudes, many not obtainable with any physical method, together with their algebraic relations produced by global residue theorems.

Finally, I will briefly illustrate some of new geometric directions in my works on the Amplituhedron in momentum space and on the geometric origin of the cluster phenomena in scattering amplitudes.

### Anthony Charles (Leuven)

11 December 2019

Title: Euclidean Black Saddles and AdS_{4} Black Holes

Abstract:

The entropy of a class of asymptotically-AdS_{4} black holes can be reproduced by the partition function of the dual ABJM theory via localization. However, establishing this match requires a particular extremization over field theory parameters. This begs the question: what are the bulk dual geometries when we do not extremize in the field theory? In this talk, I will show that these bulk duals are smooth Euclidean geometries with finitely-capped throats. These geometries generically have no clear interpretation in Lorentzian signature, but when their throat becomes infinitely long they become black holes with an AdS_{2} near-horizon geometry. For any set of field theory parameters whose extremization is compatible with a black hole, we find a large family of Euclidean geometries whose on-shell action reproduces the ABJM partition function exactly, without the need to extremize, thus establishing a more complete understanding of AdS_{4}/CFT_{3} holography.

### Paul McFadden (Newcastle)

4 December 2019

Title: Double copy structure in conformal field theory

Abstract:

Conformal symmetry places strong constraints on the form of correlation functions. In this talk, we review recent developments in conformal field theory (CFT) from the perspective of momentum space, which is natural for many applications including inflationary cosmology as well as for making connections with scattering amplitudes. We show how scattering amplitudes can be extracted from CFT correlators by taking a certain flat space limit, and how the double copy structure of amplitudes is then naturally inherited by CFT correlators. In particular, stress tensor correlators have poles whose residues correspond to gravitational scattering amplitudes, and these residues are a double copy of the analogous residues appearing in current correlators, which correspond to gauge theory amplitudes. The same also holds for mixed correlators featuring marginal scalars. In odd spacetime dimensions these residues are easy to extract, but in even dimensions correlators have branch cuts meaning a specific analytic continuation is required.

### Matthias Wilhelm (NBI)

27 November 2019

Title: Thermodynamics of AdS5/CFT4: From Hagedorn to Lee-Yang

Abstract:

The AdS/CFT correspondence provides a rich setup to study the properties of gauge theories and the dual theories of gravity, in particular their thermodynamic properties. On RxS^3, the maximally supersymmetric Yang-Mills theory with gauge group U(N) exhibits a phase transition that resembles the confinement-deconfinement transition of QCD. For infinite N, this transition is characterized by Hagedorn behavior. We show how the corresponding Hagedorn temperature can be calculated at any value of the 't Hooft coupling via integrability. For large but finite N, we show how the Hagedorn behavior is replaced by Lee-Yang behavior.

### Adrien Bouhon (Uppsala University)

20 November 2019

Title: Non-Abelian Reciprocal Braiding of Weyl Nodes

Abstract:

Weyl points trapped within a C_{2}T-symmetric plane (C_{2} = 180 degrees rotation, and T = time reversal) possess non-Abelian topological charges on top of their chirality. E.g. three-level systems realize the quaternion group. This picture requires to go beyond the modeling of a band structure as a Grassmannian (where a single spectral gap is specified). The non-Abelian nature of Weyl points implies new types of obstruction, where, for instance, two Weyl points with opposite chiralities may not annihilate. Also, the non-Abelian charges can be converted through the braiding of Weyl points in momentum space. I will review three different yet equivalent ways of computing the non-Abelian charges: (i) as a non-cyclic phase defined from the parallel transport of the Hamiltonian along a base loop, (ii) as the Euler class of a two-band subspace over a patch bounded by the base loop, and (iii) as the winding number of the Pfaffian of the Wilsonnian Hamiltonian – of the two-band subspace – as the base loop flows over the patch.

### Franz Ciceri (AEI Potsdam)

13 November 2019

Title: Higher-derivative invariants from maximal conformal supergravity

Abstract:

Supersymmetric extensions of conformal gravity have been known for a long time. In four dimensions, N=4 conformal supergravity is the maximally supersymmetric theory of this type. While the underlying multiplet of fields and their full non-linear transformations rules were already derived almost 40 years ago, no complete invariant action had been constructed so far. I will present the most general class of actions which turns out to be characterized by a holomorphic function of the scalar fields. This deviates from the non-maximally supersymmetric cases where the action is unique. These results also provide the basis for a formalism in which certain higher-derivative invariants of N=4 Poincare supergravity can be studied off-shell. In particular, I will argue that one of the superconformal actions, after gauge fixing the conformal symmetries and carefully integrating out the auxiliary fields, should give the complete supersymmetric expression of the counterterm that cancels the duality symmetry anomaly of the N=4 Poincare theory.

### Humberto Gomez (NBI)

6 November 2019

Title: L_{\infty}-algebras in the Perturbiner expansion

Abstract:

First of all, we are going to introduce L_{\infty}-algebras formalism. After that, we will argue that the minimal model (the cohomology) for the L_{\infty}-algebra that governs a classical field theory has enough information to determine its Perturbiner expansion. We will present some examples.

### Shlomo Razamat (Technion)

30 October 2019

Title: N=1 conformal dualities

Abstract:

We consider on one hand the possibility that a supersymmetric N = 1 conformal gauge theory has a strongly coupled locus on the conformal manifold at which a different, dual, conformal gauge theory becomes a good weakly coupled description. On the other hand we discuss the possibility that strongly coupled theories, e.g. SCFTs in class S, having exactly marginal N = 1 deformations admit a weakly coupled gauge theory description on some locus of the conformal manifold. We present a simple algorithm to search for such dualities and discuss several concrete examples. In particular we find conformal duals for N = 1 SQCD models with G2 gauge group and a model with SU(4) gauge group in terms of simple quiver gauge theories. We also find conformal weakly coupled quiver theory duals for a variety of class S theories: T4, R0,4, R2,5, and rank 2n Minahan-Nemeschansky E6 theories. Finally we derive conformal Lagrangians for four dimensional theories obtained by compactifying the E-string on genus g > 1 surface with zero flux. The pairs of dual Lagrangians at the weakly coupled loci have different symmetries which are broken on a general point of the conformal manifold. We match the dimensions of the conformal manifolds, symmetries on the generic locus of the conformal manifold, anomalies, and supersymmetric indices. The simplicity of the procedure suggests that such dualities are ubiquitous.

### Edoardo Vescovi (Imperial College)

23 October 2019

Title: Exact structure constants of determinant operators

Abstract:

In this talk, based on [1906.07733] and [1907.11242] with Y. Jiang and S. Komatsu, we derive the first non-perturbative result for the structure constants of two determinant operators and a non-BPS single-trace operator of finite length in planar N=4 SYM. First, we introduce a new method based on large-N collective fields, which efficiently computes correlators of such non-single-trace operators in free theory and also realizes an example of Gopakumar's “open-closed-open” string triality. The form of the result supports the interpretation of the three-point function as an overlap between an integrable boundary state, which we determine using symmetry and integrability, and the state describing the single-trace operator. Second, we use thermodynamic Bethe ansatz to derive a non-perturbative expression for such overlap with an excited state in the SL(2) sector. Finally, we briefly discuss some interesting applications that could be addressed with these methods.

### Donald Youmans (Geneva)

21 October 2019

Title: Two-dimensional BF theory as a CFT

Abstract:

Two-dimensional abelian BF theory is an example of a topological gauge theory. Imposing the Lorenz gauge-fixing condition introduces an auxiliary geometric data in form of a metric. We will show that the theory becomes topological conformal, i.e. it depends only on the conformal structure of the introduced metric. Moreover, the stress-energy tensor is Q-exact (hence vanishes in Q-cohomology and therefore on physical states). Going beyond Q-cohomology, i.e. studying correlation functions and OPEs of non Q-closed objects, allows one to define interesting structures such as topological correlation functions, a BV algebra structure on the Q-cohomology and an analog of Gromov-Witten invariants on the moduli space of punctured Riemann surfaces. The Q-primitive of the stress-energy tensor can be used to deform the model. In particular, the non-abelian theory can be seen as a deformation of the abelian one in the space of TCFTs. The former shares many features of a logarithmic CFT, such as the appearance of logarithmic singularities in OPEs. Notably, the presence of infinite Jordan cells of the Hamiltonian lead to vertex operators. This is a joint work with Andrey Losev (University Higher School of Economics Moscow) and Pavel Mnev (University of Notre Dame).

### Yuta Sekiguchi (Bern)

16 October 2019

Title: O(d,d) transformations preserve classical integrability

Abstract:

In our recent work [1907.03759], we studied the interplay between the classical integrability of WZNW models and the action of O(d,d) transformations in the doubled formalism. Along this bottom-up approach, by identifying O(d,d)-deformed Lax pairs, we concluded that any O(d,d;R) deformation preserves classical integrability. In this talk, I will start from motivations of studying O(d,d) transformations, or current-current deformations in relation to Yang-Baxter deformations. Then I will review O(d,d;R) deformations as well as the classical integrability of WZNW models. After introducing the doubled sigma model as a useful tool for these deformations, explicit constructions of deformed Lax pairs will be presented using the O(d,d)(-duality) map.

### Diego Hofman (Amsterdam)

9 October 2019

Title: Higher form symmetries and superfluids

Abstract:

I will describe superfluid hydrodynamics as the hydrodynamic theory of a system with an emergent anomalous higher-form symmetry. The higher-form charge counts the winding planes of the superfluid -- its constitutive relation replaces the Josephson relation of conventional superfluid hydrodynamics. This formulation puts all hydrodynamic equations on equal footing. The anomalous Ward identity can be used as an alternative starting point to prove the existence of a Goldstone boson, without reference to spontaneous symmetry breaking. This provides an alternative characterization of Landau phase transitions in terms of higher-form symmetries and their anomalies instead of how the symmetries are realized. This treatment is more general and, in particular, includes the case of BKT transitions.

### Thales Azevedo (Rio de Janeiro Federal University)

8 October 2019

Title: (DF)^{2} gauge theories and strings

Abstract:

Recently, a gauge theory built out of dimension-six operators such as (DF)^{2} appeared in the double-copy construction of conformal supergravity amplitudes. In this talk, I will show how theories of that kind are related to conventional, sectorized and ambitwistor string theories.

### Biswajit Sahoo (Harish-Chandra Research Institute)

2 October 2019

Title: Status of Soft Theorem in D=4 (Its classical limit and understanding as Ward identity)

Abstract:

In recent years we explored the understanding of the soft factorization property of the S-matrix for a theory containing massless particles(photon/graviton) when the energy of external massless particles are small (soft particles). Though the leading factorization is discovered long ago (1965) by Weinberg, it's understanding in the subleading order (for all loop order in S-matrix) was not much explored prior to our (with Sen) work due to the infrared divergence of S-matrix in D=4. For loop corrected S-matrix, we found that the subleading soft factorization contains terms logarithmic in soft energy. The classical limit of this logarithmic terms in soft graviton theorem provides a new classical tail memory with the known permanent shift between the mirrors of gravitational wave detector. Currently, we are trying to understand whether this soft expansion can be understood as the Ward identity of any asymptotic symmetry.

### Andrew Strominger (Harvard)

30 September 2019

Title: Operator Products of Gluons and Gravitons on the Celestial Sphere

### No seminar (Nordita workshop)

25 September 2019

### Michele Del Zotto (Durham)

19 September 2019

Title: The Spectral Problem of Quantum Fields – Lessons from String Theory

Abstract:

Determining the whole spectrum of stable excitations of a quantum field theory (QFT) is a well-known open problem. To tackle this question a good theoretical laboratory is provided by supersymmetric field theories (SQFTs) with enough conserved supercharges to constrain the QFT dynamics towards exact results. In this context, string theory techniques can be exploited to compute the spectrum of excitations of infinitely many classes of SQFTs in various dimensions. After a brief overview of these methods, we will discuss some concrete applications. Here the string theory formalism can be viewed as a tool that on one hand provides several surprising insights about the physics of this problem, and on the other unveils novel insightful connections among the theory of quantum fields and mathematics.

### Ivano Basile (Scuola Normale Superiore)

18 September 2019

Title: Vacuum stability in non-supersymmetric strings

Abstract:

We propose a holographic correspondence between gravitational vacuum bubbles and renormalization group flows studying entanglement entropy. In order to obtain a concrete top-down realization, we study instabilities of anti-de Sitter (AdS) flux compactifications in effective field theories that arise from the non-supersymmetric USp(32) and U(32) orientifold models and the SO(16) x SO(16) heterotic string. We frame the vacua in terms of near-horizon brane stacks, then we describe vacuum bubbles generated by branes leaving the stack, computing the associated decay rate. We conclude briefly discussing possible implications.