Wednesday Seminars

2019/2020

Matteo Parisi (Oxford)

18 December 2019

Title: TBA

Abstract: TBA

Anthony Charles (Leuven)

11 December 2019

Title: TBA

Abstract: TBA

Paul McFadden (Southampton)

4 December 2019

Title: TBA

Abstract: TBA

Matthias Wilhelm (NBI)

27 November 2019

Title: TBA

Abstract: TBA

Adrien Bouhon (Uppsala University)

20 November 2019

Title: Non-Abelian Reciprocal Braiding of Weyl Nodes

Abstract:
Weyl points trapped within a C_2T-symmetric plane (C2 = 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.   

2018/2019

Song He (Chinese Academy of Sciences, Beijing)

26 June 2019

Title: Generalized Particles and Strings from Combinatorial Geometries

Abstract:
The search for a “theory at infinity” for the S-Matrix has revealed surprising geometric structures underlying scattering amplitudes. I will present a novel picture where certain scattering amplitudes of both particles and strings directly emerge from polytopes known as generalized associahedra. I will show how tree- and one-loop amplitudes for bi-adjoint scalars are given by “volume forms” of such associahedra living in kinematic space, and how (generalized) associahedra for string worldsheet and (generalized) string integrals defined on them naturally arise in this construction. The field-theory limit of such string-like integral is given by the pushforward using scattering equations that map worldsheet associahedra to kinematic ones, and I will show that in fact this last result applies to any polytopes.

Andy O'Bannon (Southampton)

12 June 2019

Title: Wilson Surface Central Charges from Holography

Abstract:
M-theory is currently our best candidate for a theory of everything, but remains mysterious. We know M-theory has M2- and M5-branes. The low-energy theory on a stack of coincident M2-branes is well-understood: it is maximally supersymmetric Chern-Simons matter theory. However, the low-energy theory on a stack of coincident M5-branes remains poorly understood: it is a maximally supersymmetric theory of self-dual strings with zero tension. In this talk I will discuss one type of probe of the M5-brane theory, namely self-dual strings with infinite tension. These play a role analogous to Wilson lines in gauge theories, but are two-dimensional surfaces rather than lines, and hence are called Wilson surfaces. I will describe holographic calculations of entanglement entropy associated with Wilson surfaces, from which we extract a key parameter characterizing them, namely their central charge. This provides a count of the number of massless degrees of freedom living on them, and thus may shed light on some of the fundamental degrees of freedom of M-theory and the M5-brane theory. Along the way I will summarize recent developments in studying two-dimensional defects in conformal field theories more generally.

Lance Dixon (SLAC)

10 June 2019

Title: The Energy-Energy Correlator at Small Angles

Abstract:
How energy is distributed in quantum field theories is of broad interest, from conformal field theories to jets at the Large Hadron Collider. There has been a lot of recent theoretical progress in analyzing the correlation of energy deposited in two detectors separated by an angle, in electron-positron annihilation, in Higgs boson decays to light hadrons, and in conformal field theories. I will present a factorization formula for the small angle limit, which allows large logarithms to be resummed based on timelike evolution. Next-to-next-to-leading logarithmic accurate predictions will be presented for QCD, N=1 and N=4 supersymmetric Yang-Mills theories. In the latter, conformally invariant theory, a reciprocity relation between time-like and space-like evolution can be used to establish an equivalence to other recent approaches based on light-ray operators and Mellin representations. These results may lead to a more quantitative understanding of jet substructure at the LHC.

Matin Mojaza (AEI Potsdam)

5 June 2019

Title: Multiloop Soft Theorem for Gravitons and Dilatons in the Bosonic String

Abstract:
I will describe how one can write down closed-form expressions of closed-string amplitudes at any loop order in the bosonic string theory. I will then show that this makes it possible to extract the soft behavior of closed massless strings, and in particular establish a multiloop soft theorem through subsubleading order for the symmetrically polarized states, namely the graviton and dilaton, exposing nontrivial cancellations in the soft expansion at any loop order. Finally, if time permits, I will comment on a curious application at the tree level, which reveals a hidden symmetry of graviton amplitudes in ordinary Einstein gravity.

Erik Panzer (Oxford)

3 June 2019

Title: Single-valued multiple elliptic polylogarithms and modular graph functions

Abstract:
Superstring scattering amplitudes in genus one have a low-energy expansion in terms of real analytic modular forms, called modular graph functions (after D'Hoker, Green, Gürdogan and Vanhove). They are examples of single-valued iterated integrals of modular forms (generalizing Eichler integrals) as studied by Brown, which explains many of their properties. I will sketch the underlying integration theory on configuration spaces of points on an elliptic curve. The key ingredient is the construction of single-valued variants of Brown and Levin's elliptic multiple polylogarithms, and an idea of Schnetz.

Roberto Auzzi (UCSC Brescia)

22 May 2019

Title: A Galilean non-renormalization theorem

Abstract:
I will consider a Galilean N=2 supersymmetric theory in 2+1 dimensions with F-term couplings, obtained by null reduction of a relativistic Wess-Zumino model. I will compute quantum corrections and check that, as for the relativistic parent theory, the F-term does not receive quantum corrections. Even more, I'll give evidence that the causal structure of the non-relativistic dynamics together with particle number conservation constrain the theory to be one-loop exact.

Amit Sever (CERN)

15 May 2019

Title: The Holographic Fishchain

Abstract:
I'll talk about the paper 903.10508, where we derive from first-principles a weak-strong duality between the fishnet theory in four dimensions and a discretized string-like model living in five dimensions. At strong coupling, the dual description becomes classical and we demonstrate explicitly the classical integrability of the model. We test our results by reproducing the strong coupling limit of the 4-point correlator computed before non-perturbatively from the conformal partial wave expansion. Due to the extreme simplicity of our model, it could provide an ideal playground for holography with no super-symmetry. Furthermore, since the fishnet model and N=4 SYM theory are continuously linked our consideration could shed light on the derivation of AdS/CFT for the latter.

Silviu Pufu (Princeton)

8 May 2019

Title: CFT correlators, string theory scattering amplitudes, and supersymmetric localization

Abstract:
In this talk, I will discuss how a combination of supersymmetric localization, Mellin space, and string theory scattering amplitudes can be used to extract new information about the 4-point functions of single trace superconformal primaries in N=4 SYM theory. These computations also provide new precision tests of AdS/CFT beyond the supergravity approximation.

Alexandre Belin (CERN)

29 April 2019

Title: Einstein Gravity from ANEC correlators

Abstract:
In this talk, I will discuss properties of the averaged null energy (ANEC) operator in conformal field theories. I will describe how to compute correlation functions involving ANEC operators and local operators, by deriving an OPE between the ANEC operator and local operators. This technology is particularly useful for large N CFTs with a large gap to higher spin operators. In such a scenario, I will show that the correlation function with two local operators and an arbitrary number of ANECs is simply given by a sequence of differential operators acting on the local two-point function. I will use this technology to show that in any large N CFT with a large gap, the bulk dual must be Einstein gravity, in the sense that the OPE coefficients must match those of Einstein gravity with minimally coupled matter.

Stefan Förste (Bonn)

24 April 2019

Title: NAdS2 Holography and Supersymmetric Black Holes

Abstract:
I will review some aspects of the NAdS2/SYK duality and discuss the extension to supersymmetry. Then I will review how 4 point functions of a 3d CFT dual to a 4d black hole can be matched to SYK 4 point functions. I will present work in progress applying this to a supersymmetric black hole.

Matthew Buican (Queen Mary)

17 April 2019

Title: Rationalizing CFTs

Abstract:
2D rational conformal field theories (RCFTs) are typically thought of as being the “nicest” interesting CFTs we can study: They have large enough symmetry so that there are only a finite number of primary fields, but they also have applications to the real world (e.g., to various condensed matter systems). In this talk, I will describe a program that aims to understand connections between exotic 4D theories with N=2 superconformal symmetry and more down-to-earth 2D RCFTs, thereby enriching our understanding of both.

Giuseppe Dibitetto (Uppsala)

10 April 2019

Title: Plenty of reasons to worry about 'Nothing'

Abstract:
I will start by reviewing the original construction of Witten's “bubble of nothing” destroying the 5D KK vacuum. Subsequently I'll present a fully analytic treatment for bubbles of nothing in higher dimensions by using the Hamilton-Jacobi formalism. The actual geometry will be related to non-extremal black branes by double analytic continuation. I will then continue by discussing the relevance of this analysis in the context of the conjectured instability of non-supersymmetric AdS vacua in string theory. Finally, I will conclude by presenting an explicit example in a non-supersymmetric AdS7 vacuum of M-theory.

Alessandro Torielli (Surrey)

3 April 2019

Title: Massless scattering in lower-dimensional AdS/CFT

Abstract:
In recent years the framework of AdS/CFT integrability has seen a significant paradigm change due to the appearance of massless modes in the associated scattering problem. We shall briefly introduce the framework of the exact S-matrix theory and adapt it, following ideas of Zamolodchikov, to massless scattering, where we will make connections with integrable flows between conformal fixed points. We shall then translate this scenario to the case of AdS3/CFT2, and demonstrate the appearance of a 2D critical theory in the so-called Berenstein-Maldacena-Nastase limit of the integrable scattering problem. This talk is primarily based on work with R. Borsato, A. Fontanella, O. Ohlsson Sax, A. Sfondrini, B. Stefanski and J. Stroemwall.

Claire Zukowski (Amsterdam)

27 March 2019

Title: Kinematic Space and the Orbit Method

Abstract:
Coadjoint orbits are symplectic manifolds that are the classical analogues of a Lie group’s unitary irreducible representations. In this talk I will argue that the space of Ryu-Takayanagi surfaces in anti de Sitter spacetime, known as kinematic space, is a particular coadjoint orbit of the conformal group. In addition, I will show that the Crofton form on kinematic space, that was shown to compute the lengths of bulk curves, is equal to the standard Kirillov-Kostant symplectic form on the coadjoint orbit. Since kinematic space is Kähler in addition to symplectic, it can be quantized. The orbit method then translates geometrical properties of holographic auxiliary spaces like kinematic space into statements about the representation theory of the conformal group. This is a new application of the orbit method to holography that extends the kinematic space dictionary and suggests generalizations as well as obstructions for kinematic space.

Pavel Putrov (ICTP Trieste)

20 March 2019

Title: 6d (1,0) theories on 4-manifolds

Abstract:
In my talk I will discuss topologically twisted compactification of 6d (1,0) theories on 4-manifolds with background flavor symmetry bundles. The effective 2d theory generically has (0,1) supersymmetry and a residual flavor symmetry. Evaluation of its elliptic genus thus produces an invariant of the 4-manifold equipped with a principle bundle valued in the ring of (equivariant) modular forms. By further including torsion valued invariants of (0,1) 2d theories, one obtains an invariant of 4-manifolds valued in (equivariant) topological modular forms (TMF). I will describe basic properties of this map and present a few simple examples. I will also mention some new results on 't Hooft anomalies of 6d (1,0) theories. The talk is based on a joint work with Gukov, Pei and Vafa.

Paul Ryan (Trinity College Dublin)

14 March 2019

Title: Separated Variables & Wave Functions for rational gl(N) spin chains

Abstract:
We review recent advancements in the Separation of Variables (SoV) program for rational gl(N) spin chains. Building on these, we propose a basis for such spin chains with the physical space in an arbitrary rectangular representation of gl(N) that factorises the Bethe vectors into products of Slater determinants in Baxter Q-functions. This basis is constructed by repeated action of fused transfer matrices on a suitable reference state. We prove that it diagonalises the so-called B-operator, hence the operatorial roots of the latter are the separated variables. The spectrum of the separated variables is also explicitly computed and it turns out to be labelled by Gelfand-Tsetlin patterns, providing a direct link between SoV and Yangian representation theory.

Congkao Wen (Queen Mary)

6 March 2019

Title: 6D tree-level S matrices from rational maps

Abstract:
In this talk, we will discuss some recent progress on the study of six-dimensional S matrices. The theories we are interested in include the world-volume theories of single probe M5-brane and D5-brane, as well as 6D super Yang-Mills and supergravity. We will present twistor-string-like formulas for all these theories, analogue to that of Witten’s twistor string formulation for 4D N=4 SYM. These formulas allow us to study properties of these theories, such as moduli structures of supergravity theories. We also deduce new formulas for scattering amplitudes of theories in lower dimensions, such as SYM and supergravity in five dimensions, and 4D N=4 SYM on the Columbo branch.

Irene Valenzuela (Cornell)

27 February 2019

Title: The String Swampland and Emergence of Global Symmetries

Abstract:
Consistency with quantum gravity can have significant consequences on low energy physics. Interestingly, it seems that not every effective field theory can be consistently coupled to quantum gravity unless it satisfies some additional consistency constraints dubbed Swampland constraints. In this talk, I will revisit such constraints and their emergence as a quantum gravity obstruction to restore global symmetries. I will then focus on the Swampland Distance Conjecture for which infinite distances in field space imply an infinite tower of states becoming exponentially light. We present new string theory evidence for this conjecture in Calabi-Yau manifolds, by studying the monodromy discrete symmetries associated to the infinite distance singularities in the moduli space. This monodromy generates an infinite orbit within the spectrum of BPS states and allow us to classify the different types of infinite distance loci. We apply the results to the complex structure and Kähler moduli spaces of Type II and M/F-theory compactifications.

Michelangelo Preti (Angers)

21 February 2019

Title: Generalized Fishnets and Exact Four-Point Correlators in Chiral CFT4 

Abstract:
We study the Feynman graph structure and compute certain exact four-point correlation functions in the chiral CFT4  obtained as a double scaling limit of γ-deformed N = 4 SYM theory. We give full description of bulk behavior of large Feynman graphs: it shows a generalized “dynamical fishnet” structure, with a dynamical exchange of bosonic and Yukawa couplings. We compute certain four-point correlators in the full chiral CFT4, generalizing recent results for a particular one-coupling version of this theory – the bi-scalar “fishnet” CFT. We sum up exactly the corresponding Feynman diagrams, including both bosonic and fermionic loops, by the Bethe-Salpeter method. This provides explicit OPE data for various twist-2 operators with spin, showing a rich analytic structure, both in coordinate and coupling spaces.

Souvik Banerjee (Uppsala)

13 February 2019

Title: Relativistic correlators at large space-like momenta, thermality and evanescence

Abstract:
In this talk I shall present results from several ongoing projects which involve study of generic features of thermal correlators in large space-like momenta. First I shall discuss Wightman functions in a relativistic quantum field theory in this limit and show that this correlator must die off at least as fast as Exp[Rβ] where R is the radius of the smallest sphere that contains the polygon formed by the momenta and β, the inverse temperature. I shall give examples both in the weakly coupled field theories and in holographic theories and argue that these exponentially decaying modes are essential features of thermalisation. If time permits, I shall discuss quench in a coupled system of vector models resulting in an “approximate“ thermal state.

Andras Laszlo (Wigner Institute)

6 February 2019

Title: A possible alternative mechanism to SUSY: conservative extensions of the Poincaré group

Abstract:
In this talk a mathematical argument is outlined, showing that even without the field theoretical context of the Coleman-Mandula theorem, the Lie group theoretical possibilities for unification of a compact internal symmetry group with the Poincaré symmetry group of the spacetime are rather limited. From the Lie group theoretical arguments, however, it shall be seen that there are some backdoors to these no-go arguments. SUSY, for example, is possible because of these backdoors. In the talk we will show a non-SUSY unified compact + Poincaré group, using these backdoors. The price to pay in case of the pertinent mechanism is to allow also a nilpotent internal symmetry group in addition. Its analogies to SUSY as well as some possible field theoretical implications will be also addressed.

Andrei Marshakov (Lebedev Institute)

5 February 2019

Title: SUSY gauge theories and cluster integrable systems

Abstract:
I am going to present recent point of view on the well-known relation between N=2 SUSY gauge theories and integrable systems. It turns to be possible to make a strict statement that its (dual) partition function as a function of the coupling constant satisfies a non-autonomous differential equation. This statement is actually proven or verified for some class of uplifted to 5d SUSY gauge theories in (a special) Omega-background, where now a difference equation comes out of deautonomization of a cluster integrable system. I will present a few explicit examples, including the families of q-Painlevé equations and topological strings for YN,k geometry. Based on joint works with M. Bershtein, P. Gavrylenko and M. Semenyaki.

Andrea Fontanella (University of Padova)

17 December 2018

Title: Beyond the horizon conjecture and bulk extension

Abstract:
The horizon conjecture, proved in a case by case basis, states that every supersymmetric horizon in any supergravity theory admits a sl(2, R) symmetry algebra. In the first part of this talk, I will discuss the horizon conjecture in heterotic theory beyond the supergravity approximation. Global theorems valid in the supergravity approximation, such as the Lichnerowicz theorem, are spoiled by the string corrections. However, a sufficient condition to establish the horizon conjecture beyond the supergravity approximation is identified. In the second part, I will discuss the extension of a prescribed horizon into the bulk. The horizon fields will be deformed at first order in the radial direction (the moduli), and the dimension of the moduli space will be shown to be finite. This talk is based on work done in collaboration with Jan Gutowski and George Papadopoulos, arXiv:1605.05635 and arXiv:1610.09949.

NO SEMINAR ()

12 December 2018

Everyone is welcome to go to the “QCD meets gravity” workshop at Nordita.
http://agenda.albanova.se/conferenceDisplay.py?confId=6238

Semeon Klevtsov (Köln)

5 December 2018

Title: Geometry and physics of Quantum Hall states on Riemann surfaces

Abstract:
Quantum Hall states are N-particle wave functions, successfully describing the fractional quantum Hall effect (QHE). It was understood early on, that much can be learned about QHE when QH states are considered on a Riemann surface. We define QH states on a Riemann surface and study their topological degeneracy, adiabatic transport of QH states on moduli spaces and geometric responses to the metric, magnetic field, etc. – for the large number of particles N. We will discuss the Hall viscosity, Hall central charge and topological phases from this point of view.

Yihao Yin (Universidad Andres Bello)

29 November 2018

Title: Vasiliev's equations, (A)dS backgrounds and perturbations

Abstract:
In my talk I will introduce a method of solving Vasiliev’s equations of higher-spin gravity (in 4D with only integer spins). I will elaborate on the method of finding suitable initial data, such that the resulting solutions at the linearized level describe perturbative fields with desirable symmetry properties on an (A)dS background. Two examples in dS and AdS respectively will be discussed.

Marco Meineri (EPFL)

28 November 2018

Title: Reflection and transmission coefficients for conformal interfaces

Abstract:
We define a set of coefficients which measure reflection and transmission of conformal matter scattered off an interface in two dimensions. We elucidate the physical meaning of a previous definition, which we recover as a special case and for which we find unitarity bounds. We discuss an example and future directions.

Georgios Papathanasiou (DESY)

21 November 2018

Title: Cluster algebras, integrability and scattering amplitudes

Abstract:
I present recent progress towards determining the planar S-matrix of maximally supersymmetric Yang-Mills theory, thanks to the rich interplay between its perturbative analytic properties in general kinematics, and its integrable structure in special kinematics. The former are related to cluster algebras, and allow for the computation of amplitudes with six/seven gluons up to seven/four loops, whereas the latter yields all amplitudes in the multi-Regge limit at any loop order.

Sacha Zhiboedov (CERN)

14 November 2018

Title: Light ray operators, event shapes and Einstein gravity

Abstract:
I will describe recent developments and new results in the understanding of light ray operators in CFTs. One application of these techniques is computation of event shapes that are used to describe collider experiments for generic CFTs. Another application is new constraints on low-energy gravitational theories that admit a UV completion.

Olof Ohlsson Sax (Nordita)

14 November 2018

Title: Closed Strings and Moduli in AdS3 x S3

Abstract:
String theory on AdS3 x S3 x T4 has 20 moduli. I will discuss how the perturbative closed string spectrum changes as we move around this moduli space in both the RR and NSNS flux backgrounds. At weak string coupling, only four of the moduli affect the energies. In the RR background the only effect of these moduli is to change the radius of curvature of the background. On the other hand, in the NSNS background, the moduli introduce worldsheet interactions which enable the use of integrability methods to solve the spectral problem. These results show that the worldsheet theory is integrable across the 20 dimensional moduli space.

Valentin Reys (Milano-Bicocca)

7 November 2018

Title: Localization in supergravity and precision tests for AdS/CFT

Abstract:
Localization techniques have proven to be a powerful tool for obtaining exact results in rigidly supersymmetric theories. In this talk, we will discuss how this framework can be used in the study of locally supersymmetric theories, i.e. in supergravity. We will discuss the BRST quantization of supergravity theories on spaces with an asymptotic boundary via a suitable background field formalism. When the background is restricted to have a residual isometry group, an equivariant BRST algebra arises as a deformation of the standard nilpotent BRST algebra. This equivariant algebra can then be used to localize supersymmetric partition functions or expectation values. As an illustration of this general formalism, we will present recent results for the exact entropy of asymptotically AdS BPS black holes and compare with exact results previously obtained in the dual CFT. Such exact results on both sides will pave the way for precision tests of the AdS/CFT correspondence beyond the large N limit.

Washington Taylor (MIT)

6 November 2018

Title: Lessons from six dimensions for geometry, the string landscape, and the nature of matter

Abstract:
The geometrical formulation of string theory through “F-theory” provides a nonperturbative framework that connects geometry with the physics of gauge theories and matter fields coupled to supergravity. In six dimensions, this has led to new insights into the global structure of the “landscape” of supergravity theories and string theory vacua, has provided new mathematical results on Calabi-Yau geometries and mirror symmetry, and shows that certain gauge groups and matter content are “generic” in physical theories, and that while some exotic matter structures can be fine tuned in string theory, most gauge groups and matter representations cannot arise in string vacua. This talk describes many of these developments and some of their implications for four dimensional physics, the standard model and dark matter.

Francesca Ferrari (UvA, Amsterdam)

31 October 2018

Title: A look into 3d Modularity

Abstract:
The aim of the talk is to give an overview of my recent work in collaboration with Cheng, Chun, Gukov and Harrison. Starting from the so-called half-index (also known as homological block) of certain 3d N=2 theories, I will present how to predict specific topological data of 3-manifolds through the modular properties of the associated half-index. I will then explain the importance of quantum, false and mock modular forms in the analysis of 3-manifold invariants. To conclude, I will describe our conjecture on homological blocks of 3-manifolds with positive definite orientation and propose the form of superconformal indices for the associated 3d N=2 theories.

Natalia Pinzani Fokeeva (Leuven)

24 October 2018

Title: Schwinger-Keldysh effective field theories

Abstract:
​I will discuss novel effective field theories for hydrodynamics. After identifying the relevant low-energy degrees of freedom, I will show how the symmetries of the problem can be efficiently implemented using supersymmetry. Interestingly, a conserved entropy current naturally arises as a Noether current in superspace. If time permits, I will also show how to derive such effective actions from holography.

Kostas Skenderis (Southampton)

17 October 2018

Title: Renormalized 3-point functions in CFT

Abstract:
I will present an overview of the computation and renormalization of CFT 3-point function of the stress energy tensor, conserved currents and scalar operators in momentum space and discuss how (old and new) conformal anomalies arise.

Sasha Anokhina (ITEP)

10 October 2018

Title: Is there a state model for the Khovanov – Rozansky invariants?

Abstract:
Based on joint work with A. Yu. Morozov.

A breakthrough in the knot theory happened when a number of knot invariants (such as Alexander, Jones, HOMFY, and Kauffman polynomials, including the colored ones) found the universal “state model” description. As a notable consequence, all these invariants show specific exponential “evolution” along the (discrete) “trajectories” in the “space of knots”, which correspond to inserting the replicated pieces of a tangles.

Somewhat surprisingly, very similar evolution formulas hold for Khovanov – Rozansky (KhR) invariants. The latter ones are originally defined in terms of the knot homology. This means that the Hilbert space of the corresponding “state model” (if one exists) is a  homology space of a complex, or, in other words, the states are implicitly defined modulo an involved system of “gauge transformations” (this might resemble the description of supersymmetric models in terms of complexes).

The essential property of the evolution formulas for the KhR polynomials is a violation of their analytic properties at certain points on the trajectory, or, more generally, on the boundary of the certain domains in the knot space. This makes the KhR polynomials strictly different from the “true state sum” polynomials, as well as from the superpolynomials, which can be obtained as a certain limit of KhR polynomials.

Yet, even such a “piecewise” state sum formula for the KhR polynomials would be a good achievement and might give rise to an alternative constructive definition for these invariants, which by now look rather mysterious and remain hardly approachable.

During the talk, I will review the basic concepts of this subject and illustrate the problems outlined above with a number of toy examples.

Pierre Vanhove (Saclay)

4 October 2018

Title: On shell method for classical and quantum gravity

Abstract:
We outline the program to apply modern quantum field theory methods to calculate observables in classical general relativity through a truncation to classical terms of the multi-graviton two-body on-shell scattering amplitudes between massive fields. We will explain how classical post-Minkowskian contributions are contained in quantum scattering amplitudes. We will illustrate on some derivation of the two-body gravitational potential and the bending angle.

Dmytro Volin (Uppsala)

26 September 2018

Title: Non-compact Young diagrams

Abstract:
We will present the classification of all unitary representations of su(p,q|m) non-compact Lie super-algebras. Furthermore, for the case of integer weights, we will introduce the notion of an extended Young diagram. Similarly to the case of ordinary Young diagrams, the extended diagram is non-sensible to the rank of the algebra and hence it serves to define representations of su(p,q|m) with various different p,q,m. Thus one can deduce some interesting relations e.g. between characters of su(p,q|m) and ordinary Schur polynomials.
Based on 1712.01811, 1701.03704

Alessandro Sfondrini (ETH Zurich)

19 September 2018

Title: Integrability, correlation functions, and stringy WZW models

Abstract:
Integrability techniques have been crucial to compute non-protected observables in AdS/CFT, starting from the planar spectrum (two-point functions) of the theory. Recently it was understood that planar three- and higher-point functions, as well as non-planar observables, can be computed by integrability techniques. I will discuss these advances and focus on one recently-discovered integrability setup: the AdS3xS3xT4 stringy Wess-Zumino-Witten model.

2017/2018

Andreas Gustafsson (Uppsala)

5 June 2018

Å73121 – 13.45

Title: The non-Abelian tensor multiplet

Abstract: I will present off-shell supersymmetry variations for the 5d SYM vector and hypermultiplets in a 6d covariantly way where I introduce a background vector field along which all the fields have vanishing Lie derivatives and then 5d SYM is shown to have the full 6d superconformal symmetry. This formulation also allows for couplings to supergravity background fields.

Julius Engelsöy (Stockholm)

30 May 2018

Å73121 – 13.45

Title: Considerations regarding AdS2 backreaction and holography

Abstract: In this talk I will discuss the 2d dilaton gravity Jackiw–Teitelboim model in a holographic context. I will show how we can derive a boundary time reparametrization equation and compute the holographic stress tensor and how these can be obtained from an effective boundary model. Large time separation commutators of matter fields in the boundary model are shown to agree with gravitational shockwave interaction. I will also show how one-loop quantum effects in the bulk allow for modeling black hole evaporation in AdS2.

Elli Pomoni (DESY)

24 May 2018

Å73121 – 11.00

Title: Exact results for class Sk

Abstract: We will introduce a large class of N=1 superconformal theories, called Sk, which is obtained from Gaiotto’s N=2 class S via orbifolding. We will study the Coulomb branch of the theories in the class by constructing and analyzing their spectral curves. Using our experience from the N=2 AGT correspondence we will search for a 2D/4D relations (AGTk) for the N=1 theories of class Sk. From the curves we will identify the 2D CFT symmetry algebra and its representations, namely the conformal blocks of the Virasoro/W-algebra, that underlie the 2D theory and reproduce the Seiberg-Witten curves of the N = 1 gauge theories. We find that the blocks corresponding to the SU(N) Sk gauge theories involve fields in certain non-unitary representations of the WkN algebra. These conformal blocks give a prediction for the instanton partition functions of the 4D N = 1 SCFTs of class Sk.

EXTRA – Alessandro Tomasiello (Milano B.)

17 May 2018

Å73121 – 13.45

Title: Holographic duals of S-folds

Abstract: S-folds are analogues of manifolds where transition functions are not just coordinate changes, as in any theory of gravity, but also involve elements of the S-duality group. Peculiar solutions like this have been the subject of intense speculation in the past few years, but it is usually hard to establish their existence, because the string coupling necessarily becomes strong in some region. I will present some classes of S-folds with an AdS4 spacetime. They have supersymmetric CFT duals, and their sphere partition function can be computed, showing agreement with the supergravity result when the fields are varying slowly. This gives nontrivial evidence of their existence.

Sibasish Banarjee (Bonn)

16 May 2018

Å73121 – 13.45

Title: 5D gauge theories and strings

Abstract: Five-dimensional gauge theories preserving 8 supercharges possess a rich spectrum of BPS states. Compactified on a torus, these theories reduce to sigma-models whose target space gets instanton corrections from these BPS states and carries an isometric action of the torus modular group. I will present a joint work with Alexandrov and Longhi where we showed that this sigma model can be captured in a rigid limit of type II string theory compactified on a Calabi-Yau manifold. The rigid limit is induced by a certain local limit on the Calabi-Yau. I will also illustrate how the different instanton corrections on the gauge theory side, which thus far remained elusive, can be understood in the light of the recent progress made in studying the hypermultiplet moduli space of string theory.

William Cottrell (Stanford)

9 May 2018

Å73121 – 13.45

Title: Towards Proving Weak Gravity

Abstract: In this talk I will discuss various paths towards proving the Weak Gravity Conjecture, particularly focusing on complexity and entropy bounds. I will also describe the implications of these conjectures for cosmology.

Daniele Dorigoni (Durham)

2 May 2018

Å73121 – 13.45

Title: Cheshire Cat Resurgence in QM and QFT

Abstract: We start by reviewing some background material on Borel transform and resurgent analysis emphasising their importance in standard perturbation theory for physical observables. We then specialise our discussion to some simple supersymmetric quantum mechanical model showing how resurgence can be applied also in the case of a truncating, or even vanishing perturbative expansion in a nice example of what is called Cheshire cat resurgence. We continue with a two-dimensional quantum field theory example studying the prototypical CPN-1 model with N=(2,2) supersymmetry. After having computed the two-sphere partition function of this model via susy-localisation we show how Cheshire cat resurgence appears also in this quantum field theory example, allowing us to reconstruct non-perturbative physics even in the case of a truncating perturbative expansion.

Riccardo Borsato (Stockholm)

25 April 2018

Å73121 – 13.45

Title: Deformations of integrable models and AdS/CFT

Abstract: Integrable models have played a crucial role in our understanding of various areas of Physics, including the celebrated AdS/CFT correspondence. In this context unprecedented results, such as the exact spectrum in the planar limit, were obtained thanks to the integrability of the string sigma-model on AdS5 x S5 and of the dual N=4 super Yang-Mills. I will discuss methods to generate deformations of these maximally-symmetric theories, where the original symmetries are typically broken. The important advantage of these methods is that the underlying integrable structure is preserved under the deformation, thus opening the possibility of applying the powerful tools of integrability also to the less-understood deformed models. I will also present the main challenges to be solved in order to achieve such goal.

Stefan Hohenegger (Lyon)

18 April 2018

Å73121 – 13.45

Title: Triality in Little String Theories

Abstract: I discuss a class of little string theories (LSTs) with eight supercharges on the world-volume of $N$ M5-branes probing a transverse $\mathbb{Z}_M$ orbifold. These M-brane configurations compactified on a circle are dual to $M$ D5-branes intersecting $N$ NS5-branes on $T^2\times \mathbb{R}^{7,1}$ as well as to F-theory compactified on a toric Calabi-Yau threefold $X_{N,M}$. I argue that the Kähler cone of $X_{N,M}$ admits three regions associated with weakly coupled quiver gauge theories of gauge groups $[U(N)]^M$, $[U(M)]^N$ and $[U(NM/k)]^k$ where $k=\text{gcd}(N,M)$. These provide low-energy descriptions of different LSTs. The duality between the first two gauge theories is well known and is a consequence of the S-duality between D5- and NS5-branes or the T-duality of the LSTs. The triality involving the third gauge theory is new and I illustrate it using several examples.

Yegor Zenkevich (Milano B.)

4 April 2018

Å73121 – 13.45

Title: Why is Ding-Iohara-Miki algebra relevant?

Abstract: In the first part of the lecture we will introduce several subjects in which similar structures seem to appear “mysteriously” and tie them together:

1. The study of gauge theories using localization. Here Nekrasov partition function (which is the Omega-deformation of the Seiberg-Witten prepotential) is given by the instanton series, where each term is labelled by a tuple of Young diagrams. The series satisfies some interesting recurrence relations, which are known as qq-characters and form a W-algebra. Also, Nekrasov functions of certain theories, which are naively quite different, turn out to actually coincide (this is known as the spectral duality).

2. 2d CFT, where conformal blocks, which are holomorphic parts of the correlators, turn out to be equal to Nekrasov partitions functions (the fact known as the AGT duality).

3. Refined topological strings are natural deformations of ordinary topological strings on toric Calabi-Yau manifolds and reproduce 5d Nekrasov partition functions. The partition functions are computed by a certain glueing algorithm, which depends on the choice of a preferred direction on the toric diagram. The final answer, however, turns out to be independent of this choice.

Having gathered the motivation from different fields, in the second part of the lecture we proceed to define the DIM algebra and describe its representations. We show how all the mysterious correspondences between various theories are explained by the properties of this remarkable mathematical object.

EXTRA – Saso Grozdanov (MIT)

29 March 2018

Å73121 – 13.45

Title: Generalized global symmetries in field theory and holography

Abstract: After an extended introduction on hydrodynamics, which will include its recently uncovered connection with many-body chaos, I will discuss the concept of generalised global symmetries and their applications. From the point of view of effective field theory, I will first present a recent, comprehensive reformulation of magnetohydrodynamics, which is a theory of low-energy excitations in plasmas. I will then discuss our construction of holographic duals to theories with generalised global symmetries. Of particular focus will be a five-dimensional bulk theory with a dynamical two-form gauge field, which is dual to a field theory in which magnetohydrodynamics captures the dynamics of its infra-red limit. In the final part of this talk, I argue that generalised global symmetries are a powerful tool for constructing theories with dynamical defects even in the absence of known microscopic origins of such symmetries. As an example, I will use the theory of viscoelastic states and formulate it both in effective field theory and holography.

Andre Lukas (Oxford)

28 March 2018

Å73121 – 13.45

Title: Calabi-Yau Manifolds and SU(3) Structure

Abstract: String compactifications on Calabi-Yau manifolds and on manifolds with SU(3) structure are usually seen as complementary. In this talk I will explore the possibility of combining these two approaches by studying non-trivial SU(3) structures on Calabi-Yau manifolds. It turns out that large classes of Calabi-Yau manifolds do indeed support such SU(3) structures. Moreover, for certain choices these structures are potentially relevant for string theory. In particular, it turns out that complete intersection Calabi-Yau manifolds – and likely even more general classes – support a Strominger-Hull system. These results may lead to a new avenue for string compactification and I will discuss the prospects and current problems.

Alejandra Castro (Amsterdam)

21 March 2018

Å73121 – 13.45

Title: Black Holes & Number Theory: how to bootstrap a black hole via modular forms.

Abstract: In the language of statistical physics, an extremal black hole is a zero temperature system with a huge amount of residual entropy. Understanding which class of counting formulas can account for a large degeneracy will undoubtedly unveil interesting properties of quantum gravity. In this talk I will discuss the application of Siegel modular forms to black hole entropy counting. The role of the Igusa cusp form in the D1D5P system is well-known in string theory, and its transformation properties are what allows precision microstate counting in this case. We implement this counting for other Siegel modular and paramodular forms, and we show that they could serve as candidates for other types of black holes.

Diederik Roest (Groningen)

14 March 2018

Å73121 – 13.45

Title: Can the vector be a Goldstone boson?

Abstract: Non-linear symmetries for scalars play an important role in many fields of physics, including cosmology. We will provide a pedagogical review of the scalar case, and then turn towards the fermion and the vector: can one realize similar non-linear symmetries on these fields? In the vector case, we will investigate this question from a number of perspectives, including coset constructions and generalized geometry.

Nikolay Bobev (Leuven)

7 March 2018

Å3419 – 13.45

Title: An Etude on 3d N=2 Conformal Manifolds.

Abstract: I will describe the one-dimensional complex conformal manifold that controls the infrared dynamics of a three-dimensional N=2 supersymmetric theory of three chiral superfields with a cubic superpotential. Two special points on this conformal manifold are the XYZ model and three decoupled copies of the critical Wess-Zumino model. The conformal manifold enjoys a discrete duality group and can be thought of as an orbifold of CP1. I will discuss how to compute the spectrum of low lying operators and their OPE coefficients as a function of the coordinate on the conformal manifold using the epsilon-expansion and the numerical conformal bootstrap.

EXTRA – Olaf Hohm (Simons)

1 March 2018

Å73121 – 13.45

Title: Aspects of duality covariant theories.

Abstract: I give an overview of a research program that aims to formulate the target space description of string/M-theory in a way that is manifestly covariant under the duality symmetries that conventionally are only visible for special compactifications. Such theories, referred to as double and exceptional field theory, are defined on an extended generalized spacetime. I outline three major research projects where these frameworks seem very promising: 1) Determining or constraining the higher-derivative α' corrections of string theory; 2) formulating extended versions of “stringy” gravity that include higher massive string modes; 3) investigating possible applications in the cosmology of the early universe.

Madalena Lemos (DESY)

28 February 2018

Å3419 – 13.30

Title: Bootstrapping an Argyres-Douglas theory.

Abstract: We approach the “simplest” Argyres-Douglas theory by employing numerical and analytical bootstrap techniques. We “zoom in” to the theory by specifying Coulomb branch data, and obtain constraints on its operator spectrum and OPE coefficients. We conclude with an outlook for the prospects of approaching other Argyres-Douglas theories.

Gabriele Honecker (Mainz)

21 February 2018

Å73121 – 13.45

Title: D-Brane Model Building Tools Beyond Factorisable Tori.

Abstract: Geometric constructions of D-brane models provide powerful tools to obtain Type II string vacua with particle physics spectra. Deriving the corresponding low-energy effective action, such as gauge couplings at one-loop and Yukawa interactions already at tree-level, usually require to go well beyond supergravity, in particular using CFT techniques. This kind of D-brane model building has in the past mainly focused on toroidal orbifolds, where the underlying six-torus factorises into three two-tori. However, other factorisations are interesting due to (i) their reduced number of twisted geometric moduli and (ii) their potential to accommodate higher-form background fluxes. In this talk, I will first discuss generic model building requirements and then proceed with exemplary so-called non-factorisable Zorientifolds, first in terms of the topology of three-cycles and secondly by means of a factorised description with an additional shift symmetry acting on various two-tori simultaneously. This latter description allows for adjustments of standard CFT techniques, by which the non-chiral spectrum and the low-energy effective action can be derived.

Arthur Lipstein (Durham)

14 February 2018

Å73121 – 13.45

Title: M-theory Beyond The Supergravity Approximation.

Abstract: I will consider the 4-point function of stress tensor multiplets in the 6d (2,0) theory and describe solutions to the crossing equations whose superconformal partial wave expansions are truncated in spin. Using holographic arguments, I will show that the spin-0 solution corresponds to the leading correction to the tree-level supergravity prediction for the 4-point function in the large-N limit, and higher-spin solutions are subleading compared to the 1-loop supergravity prediction, which has yet to be calculated.

Paolo Benincasa (NBI)

7 February 2018

Å73121 – 13.45

Title: Cosmological Polytopes and the Wavefunction of the Universe.

Abstract: In this talk I will present a novel approach to understanding cosmological observables, such as the wavefunction of the universe and the late-time correlators. The general questions we ask are: Which features should these observables have to come from a consistent unitary evolution in cosmological space-time? If someone hands them to us, how do we check that they are correct? Concretely, I will focus on the wavefunction of the universe for a class toy models of scalars with time-dependent coupling constants, including conformally coupled scalars (with non-conformal interactions) in FRW cosmologies as a special case. Each Feynman diagram turns out to be associated with an universal rational integrand, which in turn can be identified as a canonical form of a new polytope, the cosmological polytope. Strikingly, these objects have a first principle definition, which does not refer to physics. Beautifully, the cosmological polytope geometrizes the singularity structure of the wavefunction, which is a universal feature of all the theories. Different representations for the wavefunction of the universe can be obtained as different triangulations of the cosmological polytope: This feature allow to find new representations, some of them with no current physical interpretation. Finally, I will comment on the relation between the wavefunction of the universe, the cosmological polytope and the flat-space S-matrix, and how these two observables are related to one another.

Andreas Braun (Oxford)

31 January 2018

Å73121 – 13.45

Title: G2 Manifolds and String Dualities.

Abstract: G2 manifolds prominently feature in dualities of string
compactifications preserving four supercharges. In this talk I will
discuss the geometric origin of such dualities and present several novel
constructions of explicit examples for G2 manifolds realized as twisted
connected sums. In particular, I will review recent progress for mirror
symmetry of type II strings on G2 manifolds, as well as the 4D N=1
duality between heterotic string theory and M-theory on G2 manifolds.

Antoine Bourget (Oviedo)

24 January 2018

Å73121 – 13.45

Title: Quivers and Deconstruction of (2,0) theories

Abstract: In this talk we will review the deconstruction of six-dimensional N=(2,0) theories using quiver theories in four dimensions. We check the deconstruction proposal for using exact methods based on Higgs branch Hilbert series computation in four dimensions and the half-BPS index in six dimensions. We introduce a technical tool needed for the computation, namely the Weyl integration formula over a class of non-connected Lie groups, which is used in this context to integrate over orthogonal groups.

Arash Arabi Ardehali (Uppsala)

17 January 2018

Å73121 – 13.45

Title: Holography of the subleading central charges in 2d and 4d SCFTs.

Abstract: Superconformal field theories (SCFTs) in two, four, and six dimensions have “Weyl anomalies”---i.e. anomalies under Weyl scalings---that are characterized by their so-called “central charges”. I explain how these central charges can be produced holographically through computations in the dual AdS supergravity theories. In particular, I present simple formulas---discovered in collaboration with Larsen, Liu, and Szepietowski---relating the subleading central charges of 2d and 4d SCFTs to the superconformal index of their dual supergravity theories. The formulas confirm many well-understood dualities, and lead to novel predictions for several still not-well-understood holographic SCFTs. Finally, I describe the ongoing work on the analogous problem for 6d holographic SCFTs.

Dimitrii Galakhov (Berkeley)

10 January 2018

Å73121 – 13.45

Title: On ADE link homology.

Abstract: I will describe rules for computing a homology theory of knots and links colored by minuscule representations of simply laced Lie algebras and embedded in 3d Euclidean space. It is derived from the theory of framed BPS states bound to domain walls separating two-dimensional Landau-Ginzburg models with (2,2) supersymmetry. I will illustrate the rules with some sample computations and argue that the results are compatible with purely geometrical constructions of such homology theories due to Khovanov and Rozansky.

Marcus Sperling (Vienna)

20 December 2017

Å73121 – 13.45

Title: Algebraic properties of the monopole formula.

Abstract: The monopole formula provides the Hilbert series of the Coulomb branch for a 3-dimensional N = 4 gauge theory.
In this talk, I will discuss how the two geometric notions “fan” and “monoid” can be very fruitful for the understanding of the monopole formula.
After a brief reminder of the monopole formula, I will introduce the matter fan and reorganise the monopole formula accordingly. I then discuss the resulting benefits such as:
(1) Explicit expressions for the Hilbert series for any gauge group.
(2) Proof that the order of the pole at t=1 and t → ∞ equals the complex or quaternionic dimension of the Coulomb branch.
(3) Identification of a sufficient set of chiral ring generators.

Vyacheslav Rychkov (IHES and CERN)

13 December 2017

Å73121 – 14.30

Title: Long-range Ising model as a physical example of non-local CFT.

Abstract: TBA.

Murad Alim (Hamburg)

12 December 2017

Å73121 – 13.45

Title: Algebraic structure of tt* equations, modularity and topological strings.

Abstract: The tt* equations put forward by Cecotti and Vafa are attached to moduli spaces of N=2 theories in 2 dimensions. When these theories are realized as sigma models into a target manifold X, the tt* equations give the moduli space of X the structure of a special Kähler manifold and generalizations thereof. In this talk, I will discuss an algebraic reformulation of the tt* equations, which is intimately connected to modular forms and their generalizations. For Calabi-Yau threefolds, this algebraic structure of tt* equations provides an algebraic formulation of topological string theory.

EXTRA – Fabio Apruzzi (Pennsylvania)

12 December 2017

Å73121 – 11.00

Title: Green-Schwarz Automorphisms and 6D SCFTs.

Abstract: All known interacting 6D superconformal field theories (SCFTs) have a tensor branch which includes anti-chiral two-forms and a corresponding lattice of string charges. Automorphisms of this lattice preserve the Dirac pairing and specify discrete global and gauge symmetries of the 6D theory. In this talk I will present the general features of a classification of this automorphism group for 6D SCFTs, and some significative explicit examples. This discrete data determines the geometric structure of the moduli space of vacua. Upon compactification, these automorphisms generate Seiberg-like dualities, as well as additional theories in discrete quotients by the 6D global symmetries. When a perturbative realization is available, these discrete quotients correspond to including additional orientifold planes in the string construction.

Shamil Shakirov (Harvard)

7 December 2017

Å73121 – 10.00

Title: Mapping class groups and difference operators.

Abstract: We review the representation of SL(2,Z) – the mapping class group of the torus – by automorphisms of a simple algebra of difference operators. The algebra, known as spherical double affine Hecke algebra (DAHA) plays an important role in many developments in modern representation theory and mathematical physics. We will define a new algebra which is a direct analogue of spherical DAHA for a genus two surface, and sketch the proof of the corresponding mapping class group action. Time permitting, we will explain the connection to the Reshetikhin-Turaev construction, and possible generalizations to higher genus.

Carlos Shahbazi (Hannover)

29 November 2017

Å73121 – 13.45

Title: On the spin geometry of M theory.

Abstract: I will present a classification of bundles of weakly faithful Clifford modules over a pseudo-Riemannian manifold (M,g) in terms of Lipschitz structures, obtaining the topological obstruction for their existence. For bundles of irreducible real Clifford modules, I will classify all Lipschitz structures, finding a spinorial structure which to the best of my knowledge is new in the literature. As an application of the previous classification, I will show that in signature (10,1) and under a mild assumption on the existence of a Dirac operator, M theory can be defined on (M,g) if and only if (M,g) is Spin, ruling out the possibility of defining M theory on non-orientable manifolds.

EXTRA – Sourav Sarkar (Humboldt)

28 November 2017

Å3419 – 11.00

Title: Mellin Amplitudes for Fermionic Correlators.

Abstract: We define Mellin amplitudes for the fermion-scalar four point function and the fermion four point function. The Mellin amplitude thus defined has multiple components each associated with a tensor structure. In the case of three spacetime dimensions, we explicitly show that each component factorizes on dynamical poles onto components of the Mellin amplitudes for the corresponding three point functions. The novelty here is that for a given exchanged primary, each component of the Mellin amplitude may in general have more than one series of poles. We present a few examples of Mellin amplitudes for tree-level Witten diagrams and tree-level conformal Feynman integrals with fermionic legs, which illustrate the general properties.

Benjamin Assel (CERN)

22 November 2017

Å73121 – 13.45

Title: RG flows in 3d N=4 gauge theories.

Abstract: I will present a new approach to study the RG flow in 3d N=4 gauge theories, based on an analysis of the Coulomb branch of vacua. The Coulomb branch is described as a complex algebraic variety and important information about the strongly coupled fixed points of the theory can be extracted from the study of its singularities. I will use this framework to study the fixed points of USp(2N) gauge theories with fundamental matter, revealing some surprising scenarios at low amount of matter.

Oscar Henriksson (Helsinki)

8 November 2017

Å73121 – 13.45

Title: Holographic phases of matter from the top down.

Abstract: Gauge/gravity duality can provide a theoretical handle on exotic phases of matter lacking conventional descriptions. However, there are still open questions regarding the exact interpretation of such holographic phases. I will argue that careful “top-down” studies of known dualities, where the full machinery of supergravity comes into play, are important in making progress on such questions. As an example, I will discuss the computation of correlation functions of both fermionic and bosonic operators in states of finite density ABJM theory; these states are described by black holes in N=8 supergravity. Strict adherence to the top-down framework means that meaningful quantitative comparisons between those computations can be made, and allows one to search for previously unseen correlations.

EXTRA – Jacob Bourjaily (NBIA)

7 November 2017

Å73121 – 13.45

Title: The Surprising Simplicity of Scattering Amplitudes.

Abstract: The way that we understand, teach, and use quantum field theory to make predictions for experiments is rapidly changing in order to make the simplicity of our predictions less surprising (and hence also easier to compute). Indeed, whether for theoretical exploration or practical applications, the methods described in textbooks are no longer those being used by experts today. In this talk, I will describe some of our recent advances that have been made to explain this simplicity and how they have been used to dramatically extend predictive reach. Much of this progress has so far been made for especially simple quantum field theories, but many of the lessons learned have much wider applicability. I will describe the status of these generalizations today, and the concrete roads ahead.

Sebastian Franco (CUNY)

1 November 2017

Å73121 – 13.45

Title: Going Beyond Duality Using Branes.

Abstract: Engineering quantum field theories in String Theory in terms of branes is a powerful approach for understanding their dynamics. I will first present recent progress in the realization of 2d N=(0,2) gauge theories in terms of branes. I will discuss Brane Brick Models, a novel class of Type IIA brane configurations, which are T-dual to D1-branes over singular toric Calabi-Yau 4-folds. They fully encode the infinite class of 2d (0,2) quiver gauge theories on the worldvolume of the D1-branes and substantially streamline their connection to the probed geometries. In a second part, I will explain how mirror symmetry provides a geometric unification of order n gauge theory dualities in different dimensions.

EXTRA – Andreas Gustafsson (KIAS)

26 October 2017

Å73121 – 15.00

Title: Five-dimensional fermionic Chern-Simons theory.

Abstract: We study 5d fermionic CS theory with a fermionic 2-form gauge potential. This theory can be obtained from 5d MSYM theory by performing the maximal topological twist. We put the theory on a five-manifold and compute the partition function. We find that it is a topological quantity, which involves the Ray-Singer torsion of the five-manifold. For abelian gauge group we consider the uplift to the 6d theory and find a mismatch between the 5d partition function and the 6d index, due to the nontrivial dimensional reduction of a selfdual two-form gauge field on a circle. We also discuss an application of the 5d theory to generalized knots made of 2d sheets embedded in 5d.

Anton Nedelin (Milano Bicocca)

25 October 2017

Å73121 – 13.45

Title: T[U(N)] duality web: mirror symmetry and gauge/CFT correspondence.

Abstract: In this talk I will review recently obtained correspondence between partition functions of 2d GLSM and certain correlators of Toda CFT. To illustrate this correspondence I will consider a particular example: the partition function of 3d T[U(N)] theory. I will start by reviewing mirror self-duality of this class of theory and will briefly derive the correspondence between its holomorphic blocks and Dotsenko-Fateev representation of the correlators in q-deformed Toda CFT. Finally I will show how upon the circle reduction 3d mirror symmetry reduces to correspondence between the partition function of certain 2d N=(2,2) quiver theory and Toda CFT correlators.

Yunfeng Jiang (ETH)

18 October 2017

Å73121 – 13.45

Title: Algebraic geometry and Bethe ansatz.

Abstract: In this talk, I will discuss how to apply methods of modern computational algebraic geometry to Bethe ansatz. I will show that algebraic geometry provides natural mathematical languages and powerful tools to understand the structure of solutions of Bethe ansatz equations (BAE). In particular, I will present new methods to count the number of physical solutions with fixed quantum numbers based on Gröbner basis and quotient ring. This method can be applied to study the completeness of Bethe ansatz. I will also discuss an analytical method to compute the sum of on-shell physical quantities over all physical solutions without explicitly solving BAE. This method has important applications in calculating the sum rules of OPE coefficients in N=4 super-Yang-Mills theory.

Ingmar Saberi (Heidelberg)

11 October 2017

Å73121 – 13.45

Title: Brane quantization and the double affine Hecke algebra.

Abstract: Double affine Hecke algebras, first introduced by Ivan Cherednik, are a family of algebras labeled by a choice of root system. Cherednik used their representation theory to give a uniform proof of the Macdonald constant-term conjectures; thereafter, they have been a source of interest in mathematics and also physics, especially after they were used to construct homological invariants of torus knots by Gorsky, Oblomkov, Rasmussen, and Shende. I'll discuss some new calculations that recover aspects of DAHA representation theory from the study of a particular Hitchin moduli space, in the framework of the brane quantization program of Gukov and Witten. In the associated physical picture, the so-called “spherical” DAHA appears as an algebra of line operators in four-dimensional N = 2* theory. This is ongoing work, to appear soon with Sergei Gukov, Peter Koroteev, and Satoshi Nawata.

EXTRA – Timothy Adamo (Imperial C.)

10 October 2017

Å73121 – 13.45

Title: Worldsheet methods for interactions on curved backgrounds.

Abstract: The study of string theory on curved background fields is notoriously difficult: quantum consistency of the worldsheet CFT imposes an infinite tower of constraints on the background fields, and calculations which were straightforward for flat backgrounds (e.g., writing vertex operators, computing 3-point functions) are practically impossible. I will discuss how certain chiral, constrained worldsheet theories (known as ‘ambitwistor strings’) describe QFT degrees of freedom directly and avoid many of the difficulties associated with ordinary strings on curved backgrounds. This provides a new tool for studying perturbation theory on curved backgrounds from a worldsheet perspective. As an example, we will consider certain plane wave backgrounds, and see that the worldsheet theory can be quantized, vertex operators written down, and 3-point functions computed explicitly.

Pedro Liendo (DESY)

4 October 2017

Å73121 – 13.45

Title: Bootstrapping N=4 SYM with defects.

Abstract: We study the constraints of superconformal symmetry on 4d N=4 theories in the presence of a defect from the point of view of the bootstrap. The superconformal algebra implies that the bootstrap equations have an exact truncation which is tractable analytically. Due to a surprising connection between superconformal blocks, these results apply both to conformal interfaces and to line defects. Moreover, the truncated equations are polynomial and in general solvable, which allows to extract analytic non-perturbative results for the CFT data.

Matt von Hippel (Copenhagen)

27 September 2017

Å73121 – 13.45

Title: Up to Six Loops, with help from Steinmann.

Abstract: As observed by Steinmann in the early 60's, functions appearing in scattering amplitudes not only have physical branch cuts, but also obey conditions on their double discontinuities, known as Steinmann relations. We have found these Steinmann relations to be particularly powerful in the context of the six-particle amplitude in planar N=4 super Yang-Mills. There, my collaborators and I build ansatze of “Steinmann Hexagon Functions” and then constrain them using a small list of physical conditions. We also obtain additional constraints based on Cosmic Galois Theory, a mathematical conjecture that constrains the constants which appear in scattering amplitudes. Using these methods we compute the complete amplitude through six loops with no need to draw Feynman diagrams or perform Feynman integrals.

Sibasish Banarjee (Saclay)

20 September 2017

Å73121 – 13.45

Title: TBA.

Abstract: TBA.

Johan Henriksson (Oxford)

13 September 2017

Å73121 – 13.45

Title: High Spin Bootstrap of Conformal Gauge Theories.

Abstract: The analytic conformal bootstrap offers new possibilities of studying conformal field theories using the crossing equation and consistency conditions. We present the recent method of using twist conformal blocks to study the high spin asymptotics of CFT data. We apply the method to weakly coupled gauge theories and derive the most general form of the one-loop four-point correlator without referring to the usual Feynman approach. It enables us to recover known correlators from the N = 4 super Yang-Mills theory.

-------------------------------------------------------------

2016/2017

Dongwook Ghim (Seoul Nat'l University)

21 June 2017

Å73121 – 13.45

Title: Elliptic Genera of 2d (0, 2) gauge theories from Brane Brick Models.

Abstract: I will present recent developments in understanding the dynamics of 2d (0,2) supersymmetric gauge theories, in particular obtained by a novel brane construction, called brane brick. Those theories are world-volume theories of D1 brane which transverse geometry is given by non-compact toric Calabi-Yau 4-cone. Briefly reviewing their construction and nice interplay between physics and geometry around them, I will introduce the recent progress in quantitative study of them with elliptic genus, an elliptic analog of Witten index. Recent findings suggest that non-linear sigma model (NLSM), which target space is a toric CY4 we probe, arises at the IR fixed point of RG flow triggered at 2d (0,2) gauge theories from brane brick. Several subtle but important issues, like gauge anomaly, in defining and computing the elliptic genus would be also discussed.

Sergio Benvenuti (SISSA)

16 June 2017

Å73121 – 13.45

Title: Abelianization and Sequential Confinement in 2 + 1 dimensions.

Abstract: We consider the lagrangian description of Argyres-Douglas theories of type A2N−1, which is a SU(N) gauge theory with an adjoint and one fundamental flavor. An appropriate reformulation allows us to map the moduli space of vacua across the duality, and to dimensionally reduce. Going down to three dimensions, we find that the adjoint SQCD “abelianizes”: in the infrared it is equivalent to a N = 4 linear quiver theory. Moreover, we study the mirror dual: using a monopole duality to “sequentially confine” quivers tails with balanced nodes, we show that the mirror RG flow lands on N = 4 SQED with N flavors. These results provide a physical derivation of previous proposals for the three dimensional mirror of AD theories. A crucial ingredient in our analysis is a concept of chiral ring stability, that dynamically removes some superpotential terms.

Marcus Berg (Karlstad)

15 June 2017

Å73121 – 11.15

Title: Hybrid String Amplitudes and RNS.

Abstract: The RNS/pure spinor hybrid formalism for computing string amplitudes can keep less-than-maximal supersymmetry manifest. It also provides a natural description of backgrounds sourced by RR fields, like Anti-de Sitter space. I will give a brief introduction and sketch a few tree-level and one-loop string amplitude calculations, some old and some work in progress, e.g. recreating flat-space results from arXiv:1603.05262.

Alexios Polychronakos (CUNY)

13 June 2017

Å73121 – 13.45

Title: Duality, Solitons and Hydrodynamics in Generalized Calogero Models.

Abstract: TBA.

Peter Koroteev (UC Davis)

07 June 2017

Å73121 – 13.45

Title: BPS/CFT and Large-N Gauge Theories.

Abstract: We shall revisit the BPS/CFT correspondence (AGT in particular) in the context of recent progress in mathematical literature. It was found that representations of some algebras (i.e. double affine Hecke algebra) or rank N have interesting stable limits when N becomes large. These observations have both physical and mathematical significance, in particular one may formulate the BPS/CFT correspondence in this way. I shall review recent results and work in progress on these matters.

Francesco Sala (IPMU)

02 June 2017

Å73121 – 13.45

Title: From instanton counting to Cohomological Hall algebras.

Abstract: Cohomological Hall algebras associated with preprojective algebras of quivers play a preeminent role in geometric representation theory and mathematical physics. For example, if the preprojective algebra is the one of the Jordan quiver (i.e., the quiver with one vertex and one loop), the corresponding CoHA contains the Maulik-Okounkov Yangian associated with the Jordan quiver. It acts on the equivariant cohomology of Hilbert schemes of points on the complex affine plane (“extending” the previous results of Nakajima, Grojnowski, Vasserot, etc. for actions of Heisenberg algebras) and of moduli spaces of framed sheaves on the complex projective plane. The latter action yields an action of W-algebras and hence provides a proof of the Alday-Gaiotto-Tachikawa conjecture for pure supersymmetric gauge theories on the real four-dimensional space. In the present talk, I will introduce and describe CoHAs associated with the stack of Higgs sheaves on a smooth projective curve. Moreover, I will address the connections with representation theory and gauge theory.

Amihay Hanany (Imperial College)

31 May 2017

Å73121 – 13.45

Title: Five Dimensional Gauge Theories and Higgs branch at Infinite Coupling.

Abstract: A long standing problem is to figure out the Higgs branch of a 5d supersymmetric gauge theory when all parameters are set to zero. Such theories are generically constructed with branes by a web of five branes in Type IIB superstring theory. The problem is solved in an elegant way by presenting a 3d N=4 quiver theory, invoking an interesting 5d-3d correspondence. The Coulomb branch of the 3d gauge theory is the Higgs branch at infinite coupling of the 5d gauge theory, both being Hyperkähler cones. Many features of the 5d theory including mesons, baryons and relatively new objects called instanton operators are captured by monopole operators, Casimir invariants, and dressed monopole operators of the 3d theory.

Gabriele Ferretti (Chalmers)

24 May 2017

Å73121 – 13.45

Title: Gauge theories of partial compositeness.

Abstract: We present a class of gauge theories in which the Higgs boson arises as a pseudo-Nambu-Goldstone boson (pNGB) and top-partners arise as bound states of three hyperfermions. A common feature they all share is the presence of specific additional scalar resonances, namely two neutral singlets and a colored octet, described by a simple effective Lagrangian. We study the phenomenology of these scalars and develop a generic framework which can be used to constrain them.

Piotr Surowka (Harvard)

17 May 2017

Å73121 – 13.45

Title: Topology, supersymmetry and hydrodynamics.

Abstract: Fluid mechanics can be formulated in terms of effective actions. However, different quantum field theories can lead to different fluids. Microscopic details are encapsulated in the hydrodynamic gradient expansion and a set of transport coefficients that include, for example, viscosity or coefficients due to quantum anomalies. First I will explain how effective actions can be constructed for non-dissipative fluids. Next I will show how to include dissipation which leads to supersymmetric degrees of freedom. I will explicitly analyze the Langevin dynamics, the simplest model that illustrates a construction of the effective action for a dissipative system.

Johanna Knapp (Vienna)

15 May 2017

Å73121 – 13.45

Title: Gauged Linear Sigma Model, Calabi-Yaus and Hemisphere Partition Function.

Abstract: The gauged linear sigma model (GLSM) is a supersymmetric gauge
theory in two dimensions which captures information about Calabi-Yaus and
their moduli spaces. Recent result in supersymmetric localization provide
new tools for computing quantum corrections in string compactifications.
This talk will focus in particular on the hemisphere partition function in
the GLSM which computes the quantum corrected central charge of B-type
D-branes. Several concrete examples of GLSMs and the application of the
hemisphere partition function in the context of transporting D-branes in
the Kähler moduli space will be given.

Sergey Alexandrov (Montpellier)

10 May 2017

Å73121 – 13.45

Title: D-instantons, mock modular forms and BPS partition functions.

Abstract: I'll discuss the modular properties of D3-brane instantons
appearing in Calabi-Yau string compactifications. I'll show that
the D3-instanton contribution to a certain geometric potential on
the hypermultiplet moduli space can be related to the elliptic
genus of (0,4) SCFT. The modular properties of the potential imply
that the elliptic genus associated with non-primitive divisors
of Calabi-Yau is only mock modular. I'll show how to construct
its modular completion and to make manifest the modular invariance 
of the twistorial construction of D-instanton corrected hypermultiplet
moduli space.

Juan Pedraza (Amsterdam)

3 May 2017

Å73121 – 13.45

Title: Linear response of entanglement entropy from holography.

Abstract: In this talk I will present recent results of the propagation of entanglement entropy after a global quench in the context of AdS/CFT. I will start by reviewing the heuristic picture for the entanglement propagation after instantaneous quenches dubbed as ‘entanglement tsunami’, and explain why this interpretation fails for general quenches and entangling regions. In the second part of the talk I will present an analytic perturbative calculation for small subregions and show that in this regime the spread of entanglement exhibit some distinct features, most notably, that the instantaneous rate of growth is not constrained by causality, but rather its time average. Finally, I will argue that for small subregions, the growth of entanglement after general global quenches can be physically understood in terms of a linear response theory.

Vishnu Jejjala (Johannesburg)

26 April 2017

Å3419 – 13.45

Title: Hot Attractors.

Abstract: Non-extremal black holes, which emit thermal Hawking radiation, have two horizons: the event horizon or outer horizon and the Cauchy horizon or inner horizon. Surprisingly, for a broad class of solutions to the Einstein equations, the product of the areas of the inner and outer horizons is the square of the area of the horizon of the zero temperature black hole obtained from taking the smooth extremal limit. We use the attractor mechanism in supergravity to motivate this result. Non-extremal geometries at the Reissner-Nordström point, where the scalar moduli are held fixed, can be lifted to solutions in supergravity with a near-horizon AdS3×S2. These solutions have the same entropy and temperature as the original black hole and therefore allow an interpretation of the underlying gravitational degrees of freedom in terms of CFT2. Symmetries of the moduli space enable us to explicate the origin of entropy in the extremal limit.

Nadav Drukker (King's)

19 April 2017

Å73121 – 13.45

Title: Matrix models for the gauge-gravity correspondence.

Abstract: Localization reduces the calculation of certain observables in supersymmetric field theories to matrix models. I will review three examples of matrix models that arise in N=4 SYM and ABJM theory. In particular I will focus on how the solution of the matrix model, either the exact solution, or the large N one, leads to exact predictions for the AdS duals of these theories. In the case of the Schur index, there is still no satisfactory holographic counterpart to the gauge theory result.

Ilmar Gaharamanov (MPI)

12 April 2017

Å73121 – 13.45

Title: Progress in integrable statistical models inspired by supersymmetric gauge theories.

Abstract: Recently, there has been observed several connections of integrable models to supersymmetric gauge theories and special functions of hypergeometric type. One of such connections is a correspondence between supersymmetric quiver gauge theories and integrable lattice models such that the integrability emerges as a manifestation of supersymmetric dualities. Particularly, partition functions of supersymmetric quiver gauge theories with four supercharge on different manifolds can be identified with partition functions of two-dimensional exactly solvable statistical models. This relationship has led to the construction of new exactly solvable models of statistical mechanics, namely the Yang-Baxter equation was solved in terms of new special functions. In the talk, I will present some new solutions of the Yang-Baxter equation and related basic and hyperbolic hypergeometric integral identities.

Fabio Riccioni (La Sapienza University)

05 April 2017

Å73121 – 13.45

Title: On orientifold truncations and spacefilling branes.

Abstract: We consider in any dimension the supersymmetric Z2 truncations of the maximal supergravity theories. These include all orientifold truncations together with additional truncations that can be formally interpreted as being generated by non-perturbative duals of the orientifold planes. In each dimension and for each truncation we determine all the sets of space-filling branes that preserve the supersymmetry of the truncated theory. We show that in any dimension below eight these sets always contain exotic branes, that are objects that do not have a ten-dimensional origin. We finally repeat the same analysis for half-maximal theories and for the N=2 theory in four dimensions.

David Marsh (Cambridge)

29 March 2017

Å73121 – 13.45

Title: Many-field inflation.

Abstract: Inflation provides a remarkably simple explanation of the origin of structure in the universe. However, the micro-physics governing inflation is poorly known, and even some of the most basic questions like “What degrees of freedom were dynamically important in the early universe?” remains unanswered. Present cosmological observations are consistent with single-field models of inflation, but, as I will review, multi-field models are both theoretically well-motivated and phenomenologically rich. Unfortunately, they are also hard to study. This has confined the vast majority of previous studies to the two-field case, while the general case of N ≫1 interacting fields has remained intractable. In this talk, I will present a new method for studying models of many-field inflation using non-equilibrium random matrix theory, and I will discuss the generation of observables during inflation in these models. Strikingly, as the number of interacting fields (and hence the complexity of the model) increases, the power spectra of curvature perturbations simplify, and become more predictive. These simplifications can be attributed to eigenvalue repulsion in the Hessian matrix, which can be expected to extend to a much broader class of models than those studied here. I will present explicit results for N=100 fields, and briefly discuss observational targets for the next generation of cosmological experiments.

Lorenzo Bianchi (DESY)

22 March 2017

Å73121 – 13.45

Title: Conformal defects: from entanglement to emitted radiation.

Abstract: In this talk, after introducing conformal defects and describing some of their interesting features, I will apply this general framework to two apparently unrelated fields. First I will discuss Rényi entropies in conformal field theories and then the Bremsstrahlung function for integrable superconformal field theories (in particular N=4 SYM and ABJM). In conclusion, I will point out some interesting relations and connections between these two examples.

Can Kozcaz (Harvard)

15 March 2017

Å73121 – 13.45

Title: Cheshire Cat Resurgence and Self-Resurgence.

Abstract: We explore a one parameter ζ-deformation of the quantum-mechanical Sine-Gordon and Double-Well potentials which we call the Double Sine-Gordon (DSG) and the Tilted Double Well (TDW), respectively. In these systems, for positive integer values of ζ, the lowest ζ states turn out to be exactly solvable for DSG – a feature known as Quasi-Exact-Solvability (QES) – and solvable to all orders in perturbation theory for TDW. For DSG such states do not show any instanton-like dependence on the coupling constant, although the action has real saddles. On the other hand, although it has no real saddles, the TDW admits all-orders perturbative states that are not normalizable, and hence, requires a non-perturbative energy shift. Both of these puzzles are solved by including complex saddles. We also show that the perturbative series is self-resurgent, a one-to-one relation between the early terms of the perturbative expansion and the late terms of the same expansion.

Alexandros Anastasiou (Nordita)

08 March 2017

Å73121 – 13.45

Title: Yang-Mills origin of gravitational symmetries.

Abstract: A recurring theme in the attempts of understanding the quantum theory of gravity is the idea of “gravity as the square of Yang-Mills”. This involves the tensoring of the multiplet content of two super-Yang-Mills theories to obtain the multiplet content of a supergravity theory. A complete understanding of this correspondence requires studying how the gravitational symmetries originate from the corresponding Yang-Mills factors. In this talk I will show how both the global and local symmetries of linear supergravity arise from the corresponding super-Yang-Mills ones through a dictionary between both fields and transformation parameters. Finally, I will comment on how the dictionary seems to restrict the study of dynamics to particular gauge choices.

Yang-Hui He (Oxford)

01 March 2017

Å73121 – 13.45

Title: Gauge Theories: Quivers, Dessins and Calabi-Yau.

Abstract: We discuss how bipartite graphs on Riemann surfaces capture a wealth of information about the physics and the mathematics of large classes of gauge theories, especially those arising from string theory in the context of AdS/CFT.

The dialogue between the physics, the underlying algebraic geometry of Calabi-Yau varieties, the combinatorics of dimers and toric varieties, as well as the number theory of dessin d'enfants becomes particularly intricate and fruitful under this light.

Daisuke Yokoyama (King's)

22 February 2017

Å73121 – 13.45

Title: Localization of supersymmetric field theories on non-compact hyperbolic three-manifolds.

Abstract: I am going to talk about how to derive an exact partition function of supersymmetric field theories on a supersymmetry-preserving quotient of Euclidean AdS3 by localization. We utilized three-methods to compute 1-loop contributions because the situation in non-compact space is quite different from the one in compact space, and we wanted to make ourselves be confident about the result. I will explain at least one method out of three depending on time, and discuss the features of the partition function we derived.

Nicolò Petri (Milano Statale)

15 February 2017

Å73121 – 13.45

Title: TBA

Abstract: TBA

Diego Rodriguez-Gomez (Oviedo)

08 February 2017

Å73121 – 13.45

Title: Correlation functions in 4d N=2 theories.

Abstract: Correlation functions are central objects in QFT. Owing to very recent results using localization, correlators for Chiral Primary Operators (CPO’s) in 4d N=2 CFT’s can be exactly computed. In this talk we will be interested on correlators for CPO’s in large N theories, focusing in particular on N=4 SYM as well as on necklace quivers arising from its orbifolding (which, in a corner of parameter space, reduce to superconformal QCD). In the N=4 SYM case we will recover the well-known non-renormalization theorem. In the case of necklace quivers, at the perturbative orbifold point, we find that untwisted sector operators follow the same behavior as N=4, while for twisted sector operators we find striking cancellations. We also compute correlation functions between CPO’s and Wilson lines, finding, in a completely different manner, striking agreement with the conjectured matrix model of Giombi & Pestun. As a by-product of our computations, we find a factorized structure of the final result for the large-N partition function.

Lotte Hollands (Heriot-Watt)

01 February 2017

Å73121 – 13.45

Title: BPS states, spectral networks and non-Lagrangian QFT's.

Abstract: BPS states in four-dimensional quantum field theories have a geometric interpretation in terms of so-called spectral networks on two-dimensional (punctured) Riemann surfaces. In this talk I will explain that the relevant spectral networks for non-Lagrangian theories can be generated from generalized Strebel differentials. I will describe how one can count the BPS states as special Lagrangian discs ending on the spectral network, and make this very concrete in one particularly interesting example, the E6 Minahan-Nemeschansky theory. Some rather beautiful pictures come up!

Shamil Shakirov (Harvard)

25 January 2017

Å73121 – 13.45

Title: Refined Chern-Simons theory at genus 0,1,2.

Abstract: TBA

Antonio Pittelli (Surrey)

18 January 2017

Å73121 – 13.45

Title: Dualities and Integrability in Low Dimensional AdS/CFT.

Abstract: Dual superconformal symmetry (DSC) underlies the duality between scattering amplitudes and Wilson loops in N=4 super Yang-Mills theory. Via AdS/CFT, this corresponds to the invariance of the AdS5 x S5 superstring action under specific T-dualities. Furthermore, DSC and T-self-duality are deeply connected with the integrability and the Yangian symmetry of the model. We show that this symmetry holds also for AdS2 x M8 and AdS3 x M7 superstrings with exceptional isometry supergroups, which imply the DSC of their still mysterious CFT duals. We then focus on AdS2 x S2 x T6 superstrings and discuss their peculiar worldsheet S-matrix, as well as the infinite dimensional symmetry behind the model: the Y[gl_c(1|1)] Yangian.

Alessandro Pini (Oviedo)

14 December 2016

Å73121 – 13.45

Title: Hilbert Series and aspects of the moduli space of instantons on CP2.

Abstract: In the first part of the talk I give an introduction to the computational tool called “Hilbert Series” (HS). I analyze how it can be employed for the characterization of the moduli space of vacua of a QFT and of the moduli space of instantons. Then, in the second part of the talk, I discuss the moduli space of (framed) self-dual instantons on CP2. These are described by an ADHM-like construction which allows to compute the Hilbert Series of the moduli space. The latter has been found to be blind to certain compact directions. I probe these directions, finding them to correspond to a Grassmannian, upon considering appropriate ungaugings. Moreover I discuss the ADHM-like construction of instantons on CP2/Zn as well as compute its Hilbert series. As in the unorbifolded case, these turn out to coincide with those for instantons on C2/Zn.

David Vegh (Utrecht)

7 December 2016

Å73121 – 13.45

Title: Segmented strings.

Abstract: Segmented strings in flat space are piecewise linear string solutions. Kinks between the segments move with the speed of light and their worldlines form a lattice on the worldsheet. This description can be generalized to anti-de Sitter spacetime where the string motion is non-linear. In this talk, I will review the properties of segmented strings and relate them to discrete-time Toda-type lattices.

Masanori Hanada (Stanford and Kyoto)

30 November 2016

Å73121 – 13.45

Title: Black Holes and Random Matrices.

Abstract: Maldacena suggested that the late-time behavior of the correlation functions can diagnose the discrete energy spectrum of the black hole microstates. In order to determine such long-time property in the quantum field theories dual to black holes, we consider the Sachdev-Ye-Kitaev (SYK) model as an ideal toy model. We study the correlation functions and the spectral form factor, which is essentially the analytic continuation of the thermal partition function to the Minkowski time, and numerically demonstrate that the long-time behavior of the spectral form factor of the SYK model is reproduced by the Random Matrix Theory (RMT). We argue that super Yang-Mills theory would exhibit the same property, and comment on possible implications to quantum gravity.

Costis Papageorgakis (Queen Mary)

23 November 2016

Å73121 – 13.45

Title: The Nekrasov-Shatashvili Limit of the 5D Superconformal Index.

Abstract: I will motivate and introduce the Nekrasov-Shatashvili (NS) limit of the five-dimensional N=1 superconformal index. I will then summarise recent results of Cordova and Shao, who related the so-called Schur limit of the four-dimensional N=2 index to the BPS spectrum of 4D theories on the Coulomb branch. Finally, I will describe how the technology of Cordova-Shao can also reproduce the NS index in 5D for simple examples of U(1) theories. In this picture, the 5D instanton solitons are interpreted as additional flavour nodes to an associated 5D BPS quiver.

Cyril Closset (CERN)

16 November 2016

Å73121 – 13.45

Title: The three-dimensional A-twist and supersymmetric partition functions.

Abstract: Three-dimensional N=2 supersymmetric theories can be defined on curved spaces, such as the three-sphere, that preserve (at least) half of the supersymmetry of flat space. To a large extend, this can be understood in terms of the topological A-twist of a two-dimensional N=(2,2) supersymmetric theory obtained by compactification on a circle. In this talk, I will show how this 2d point of view allows us to compute supersymmetric partition functions of 3d N=2 gauge theories (and other correlation functions of loop operators) on any U(1) principal bundle over a Riemann surface Σg. This includes in particular the round three-sphere partition function, and the so-called twisted indices on the product spaces Σg*S1. I will also discuss some applications of those results.

Sara Pasquetti (Milano Bicocca)

09 November 2016

Å73121 – 13.45

Title: 3d N=2 dualities with monopole superpotentials.

Abstract: D3 branes stretching between webs of (p,q) 5branes provide an interesting class of 3d N=2 theories. IIB S-duality is realized as 3d mirror symmetry. In the Abelian case we use the Kapustin-Strassler piece-wise dualization to obtain the low energy description of D3 branes ending on pq-webs made of many coincident NS5's intersecting one D5. Interestingly, these theories involve monopole operators in the superpotential. We also consider generalizations to non Abelian gauge theories, leading to Aharony-Seiberg dualities with monopole superpotentials.

Yang Zhang (ETH)

02 November 2016

Å73121 – 13.45

Title: A ZURIch-based InTEgral-determination method.

Abstract: Multiloop amplitude is very important for the study of LHC physics. Given large number of Feynman diagrams, we need to consider the reduction to a small number of master integrals. Usually this is achieved by Integration-by-parts (IBP) relations, however, IBP computation is often very heavy. In this talk, we show the recent progress of IBP reduction, based on generalized unitarity and new mathematical method: computational algebraic geometry. Our package, AZURITE, which is designed to find master integrals will be presented.

Mathew Bullimore (Oxford)

26 October 2016

Å73121 – 13.45

Title: Vortices and Vermas.

Abstract: In 3d gauge theories, monopole operators create and destroy vortices. I will explore this idea in the context of 3d supersymmetric gauge theories in the presence of an omega background, and explain how it leads to a finite version of the AGT correspondence.

Johan Blåbäck (Saclay)

19 October 2016

Å73121 – 13.45

Title: There and back again: A T-brane's tale.

Abstract: In some particular backgrounds the non-Abelian dynamics of a
stack of Dp-branes can be described by an Abelian polarised D(p+2)-brane
with worldvolume flux. By investigating the defining equations for
T-branes, we find a new “polarised” state for a stack of non-Abelian
Dp-branes where the “polarised” frame is a single Abelian Dp-brane where the
original T-branes data is encoded as worldvolume curvature. We will join an
explicit T-brane construction's journey through T-dualities and brane
polarisation to reach this new Abelian T-brane description.

Seyed Morteza Hosseini (Milano Bicocca)

12 October 2016

Å73121 – 13.45

Title: The origin of AdS black hole/string microstates from topologically twisted index.

Abstract: We provide general formulae for the topologically twisted index of general three/four-dimensional N = 2 / N = 1 quiver gauge theories in the limits for which holography becomes applicable. After reviewing the results for the ABJM theory, we move to compute the index of N = 4 SYM and N = 1 Klebanov-Witten theory, and reproduce the “entropy” of AdS5 black strings. An extremization of the index with respect to a set of chemical potentials is required which is equivalent to the c-extremization principle. As a final bonus, our computations reveal an intriguing universal RG flow from 3d/4d to 1d/2d.

Alessandro Sfondrini (ETH)

5 October 2016

Å73121 – 13.45

Title: Integrable bootstrap in AdS/CFT.

Abstract: I will review how symmetries and integrability techniques can be used to efficiently compute certain AdS/CFT observables exactly. For most of my talk, I will be focusing on the AdS3/CFT2 backgrounds, and more specifically on the spectrum of AdS3xS3xT4 and AdS3xS3xS3xS1 super-strings in the planar (or 't Hooft) limit. I will introduce the rich physics of these backgrounds, that are related to the D1-D5 system, to CFT2s with N=(4,4) super-conformal symmetry, and in a certain limit higher-spin theories. Interestingly, and despite considerable efforts, many features of these AdS/CFT setups remain to be understood. Leaving aside most technical details, I will introduce the “integrable bootstrap” approach on the string world-sheet, highlight some key physical features that it has revealed, and discuss how it can be used to address open questions. I will conclude by sketching how this approach can be adapted to compute more general observables, such as form factors, three-point functions and one-point functions in the presence of defects.

Marco Chiodaroli (Uppsala)

28 September 2016

Å73121 – 13.45

Title: N=2 supergravities as double copies of gauge theories.

Abstract: This talk reports on recent progress in amplitude computations for broad
classes of N=2 Maxwell-Einstein and Yang-Mills-Einstein supergravities,
using the framework provided by color/kinematics duality and the
double-copy construction. After a review of the main theoretical tools, we discuss the extension of the double-copy construction to a particular infinite family of N=2
Maxwell-Einstein theories in four and five dimensions, the generic
Jordan family of supergravities. We show that the global symmetries of these theories can be easily gauged, giving the amplitudes of the corresponding Yang-Mills-Einstein
supergravities. We also discuss how the construction can be modified to
describe spontaneous symmetry breaking. Finally, we present an extension
of the construction that captures an even larger class of N=2
supergravities with homogeneous target spaces.

Ali Mollabashi (IPM)

21 September 2016

Å73121 – 13.45

Title: Entanglement in the Field Space: Scalar Field.

Abstract: I will start with a brief review of how the notion of quantum entanglement is uplifted from QM to QFT. Introducing various types of entanglement in the framework of QFT, the rest of the talk will be focused on a specific type which I call “entanglement in the field space”. I will introduce a family of solvable scalar models and report the result of various entanglement measures in these models. I will discuss about some features of their entanglement structure and also about their possible holographic dual.

Gustav Mogull (Edinburgh)

7 September 2016

Å73121 – 13.45

Title: Local integrands for pure Yang-Mills theory.

Abstract: Finding compact integrand representations of amplitudes in pure Yang-Mills theory requires convenient integral bases. In this seminar I will demonstrate how the infrared divergences of some D-dimensional Feynman integrals can be regulated using special choices of numerators. These choices are inspired by the existing four-dimensional local integrand structures due to Arkani-Hamed et al, valid only in the planar limit of maximally supersymmetric N=4.