TALKS

Luis Apolo (University of Amsterdam)

TTbar in string theory [blackboard talk]

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Luis Apolo (University of Amsterdam)

Holographic entanglement entropy for TTbar-deformed CFTs [blackboard talk]

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TTbar-deformed CFTs with a negative deformation parameter have been proposed to be dual to Einstein gravity on AdS3 with a finite cutoff. In this talk, I will present an extension of cutoff AdS3 holography that is valid for positive values of the deformation parameter. In this case, turning on the deformation parameter brings the asymptotic boundary of AdS3 to a finite cutoff in an auxiliary AdS3 spacetime. I will show that the theory living at the cutoff surface reproduces many features of TTbar-deformed CFTs with a positive deformation parameter. Furthermore, I will give a geometric prescription for computing the holographic entanglement entropy of an interval at the cutoff surface. The holographic entanglement entropy is finite and features a minimum length scale, as expected for TTbar-deformed CFTs.

Minkyoo Kim (Sogang University)

Comments on Heavy Operators in N=4 SYM

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For N=4 SYM in the planar limit, its spectrum is completely solved at finite coupling through integrability.  The dynamics of single-trace operators and worldsheet formulation of holographic dual strings are such cases.  On the other hand, the spectrum of heavy operators, which is a holographic dual to giant graviton D-branes, cannot be restricted to the planar limit and therefore is not generally integrable.  In this talk, I would like to introduce techniques that have been developed to achieve analytic results in these situations.  In particular, I shall review some established ideas based on group representation theory and comment on some new results.

John Z. Imbrie (University of Virginia)

Integrability from Disorder in Quantum Many-Body Systems [slide]

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A quantum system is said to be many-body localized (MBL) if one can find a complete set of local integrals of motion (LIOMs). This leads to a breakdown of thermalization. I will summarize a proof that certain one-dimensional spin chains have an MBL phase (the proof depends on a certain assumption on level statistics). In recent work with Morningstar and Huse, we develop specific RG flow equations to study the critical phenomenon at the edge of the MBL phase.  This turns out to be similar to the Kosterlitz-Thouless (KT) flow, but there are important differences that place the MBL transition in a new universality class.

Max A. Metlitski (MIT)

Boundary and plane defect criticality in the 3d O(N) model

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It is known that the classical O(N) model in dimension d > 3 at its bulk critical point admits three boundary universality classes: the ordinary, the extraordinary and the special. The extraordinary fixed point corresponds to the bulk transition occurring in the presence of an ordered boundary, while the special fixed point corresponds to a boundary phase transition between the ordinary and the extra-ordinary classes. While the ordinary fixed point survives in d = 3, it is less clear what happens to the extraordinary and special fixed points when d = 3 and N is greater or equal to 2. I'll show that formally treating N as a continuous parameter, there exists a critical value Nc > 2 separating two distinct regimes. For N < Nc the extra-ordinary fixed point survives in d = 3, albeit in a modified form: the long-range boundary order is lost, instead, the order parameter correlation function decays as a power of log r.  For N > Nc there is no fixed point with order parameter correlations decaying slower than power law. I'll discuss how these findings compare to recent Monte-Carlo studies of classical and quantum spin models with SO(3) symmetry.

Christian Ferko (UC Davis)

Universal Stress Tensor Deformations

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Christian Ferko (UC Davis)

Root-TT Deformations [slide]

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In this talk I will discuss a new proposal for a classically marginal deformation of any two-dimensional quantum field theory which is constructed from the stress-energy tensor. Although this deformation is non-analytic in the stress tensor because it involves a square root, the resulting flow equation for the classical Lagrangian has some surprisingly nice properties, and can be solved in closed form for a large class of seed theories. I will also explain some work in progress concerning a holographic interpretation of this deformation in terms of modified boundary conditions for the bulk metric in the AdS3 gravity dual.

Vladimir Kazakov (LPENS, Paris)

The Loom for Generalized Fishnet CFTs [slide]

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I will review  basic facts about the Fishnet CFT proposed in 2014 by O.Gurdogan and myself. I will expose its origin as a special scaling limit of gamma-twisted N=4 SYM theory. Then I will explain some generalizations (any dimension, anisotropic deformations etc).  I will show some results of analytic  computations of anomalous dimensions based on the integrability of this model in the 't Hooft limit. Then I will present the results of our forthcoming paper (with Enrico Olivucci and Michail Alfimov) on the most general construction of Fishnet CFTs stemming from A.Zamolodchikov 1980 work on  integrable Fishnet graphs, based on Baxter lattice and star-triangle relations.

Andre LeClair (Cornell University)

On the classification of UV completions of  integrable  TT deformations  of CFT [slide] 

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It is well understood that 2d conformal field theory (CFT) perturbed by an irrelevant TT perturbation  of dimension $4$ has universal properties.

In particular, for the most interesting cases, the theory develops a singularity in the ultra-violet(UV), signifying a shortest possible distance, with a Hagedorn transition in applications to string theory. We show that by adding an infinite number of higher TT_{s>1} irrelevant operators of positive integer scaling dimension 2(s+1) with tuned couplings, this singularity can be resolved and the theory becomes UV complete with a Virasoro central charge c_{UV} > c_{IR} consistent with the c-theorem. We propose an approach to classifying the possible UV completions of a given CFT perturbed by TT_s that are integrable. The main tool utilized is the thermodynamic Bethe ansatz. We study this classification for  theories with scalar (diagonal) factorizable S-matrices. For the Ising model with  c_{IR} = \1/2 we find 3 UV completions based on a single massless Majorana fermion description with c_{UV} = 7/10 and 3/2,  which both have N=1 SUSY and were previously known, and we argue that these are the only solutions to our classification problem based on this spectrum of particles. We find 3 additional ones with a spectrum of 8 massless particles related to the Lie group E_8 appropriate to a magnetic perturbation with c_UV = 21/22, 15/2 and 31/2. We argue that it is likely there are more cases for this $E_8$ spectrum.   We also study simpler cases based on $\suthree$ and $\sufour$ where we can propose complete classifications. For $\suthree$ the infra-red (IR) theory is the 3-state Potts model with c_IR=4/5 and we find 3 completions with  4/5 < c_{UV} < 16/5. For the $\sufour$ case, which has 3 particles and c_{IR}=1,  and we find 11 UV completions with 1 < c_{UV} < 5, most of which were previously unknown. (Based on work with Changrim Ann).  

Shinji Hirano (Huzou University)

The holography of duality in N=4 Super-Yang-Mills-theory

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Via the AdS/CFT correspondence, I discuss the gravity dual description of a class of N = 4 Super-Yang-Mills theories with su, so and sp gauge algebras, differing by the global 1-form symmetries and its associated line operator spectrum. These fine prints are encoded in the boundary conditions on the NSNS and RR 2-forms in type IIB string theory on AdS_5 x S^5 and AdS_5 x RP^5 and described by simple 5d topological field theories. The holographic description provides an elementary and intuitive understanding of the rich properties of these theories such as an intricate web of the SL(2,Z) duality, mixed ’t Hooft anomalies, interfaces as well as the existence of more N = 4 Super-Yang-Mills theories. Time permitting, I might make a brief comment on non-invertible (higher category) symmetries and add a speculative discussion on an ensemble of N = 4 SYM theories.

Balazs Pozsgay (Eötvös Loránd University)

Medium range spin chains -- Motivations: The folded XXZ model

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Most spin chains that are studied have nearest neighbour interactions. However, recently there appeared some models in the literature, where the fundamental interaction spans more than two sites. We consider such models, their integrability, and the algebraic constructions ensuring the existence of commuting transfer matrices. Our main example will be the so-called folded XXZ model, but we will also consider other examples. If time allows, extensions to systems with discrete time will be presented too.

Balazs Pozsgay (Eötvös Loránd University)

Medium range spin chains -- Algebraic theory

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Most spin chains that are studied have nearest neighbour interactions. However, recently there appeared some models in the literature, where the fundamental interaction spans more than two sites. We consider such models, their integrability, and the algebraic constructions ensuring the existence of commuting transfer matrices. Our main example will be the so-called folded XXZ model, but we will also consider other examples. If time allows, extensions to systems with discrete time will be presented too.

Deliang Zhong (Tel Aviv University)

Line Operators in Chern-Simons-Matter Theories and Bosonization in Three Dimensions [slide]

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We study Chern-Simons theories at large N with either bosonic or fermionic matter in the fundamental representation. The most fundamental operators in these theories are mesonic line operators, the simplest example being Wilson lines ending on fundamentals. We classify the conformal line operators along an arbitrary smooth path as well as the spectrum of conformal dimensions and transverse spins of their boundary operators at finite 't Hooft coupling. These line operators are shown to satisfy first-order chiral evolution equations, in which a smooth variation of the path is given by a factorized product of two line operators. We argue that this equation, together with the spectrum of boundary operators, are sufficient to uniquely determine the expectation values of these operators. We demonstrate this by bootstrapping the two-point function of the displacement operator on a straight line. We show that the line operators in the theory of bosons and the theory of fermions satisfy the same evolution equation and have the same spectrum of boundary operators.

Alessandro Sfondrini (Padova University)

Integrability for Root-TTbar deformations

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After briefly reviewing the constructions of Root-TTbar deformations (which Chris Ferko will present in more detail in his talk earlier in the week), I will discuss how to construct the Lax connection for a rather large class of theories which include semi-symmetric space sigma models with the addition of a Kalb-Ramond interaction.

Matthew Heydeman (IAS)

Near BPS black holes and microstates in supergravity [slide]

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It is an open problem to understand the ultimate fate of the path integral in gravity and quantum black hole thermodynamics. Recent progress has focused on supersymmetric black holes in asymptotically AdS spacetimes in which the sum over classical BPS solutions reproduces the superconformal index of the dual CFT. In a parallel line of development, it has been understood that certain models of 2d gravity describe the near horizon region of a near-extremal black hole. At strong coupling the integral over metrics in these models may be done exactly, and the solution reveals a coarse-grained chaotic spectrum with no sharply defined microstates.  In this talk, we will show that these two approaches are compatible and compute the low-temperature partition function of near-BPS black holes via a gravitational path integral. Our approach uses an effective field theory of the near horizon geometry which reduces to an exactly solvable supersymmetric extension of the Schwarzian theory found in toy models of 2d gravity. The quantization of this theory gives a quantum gravitational derivation of the extremal entropy and the superconformal index, showing that supergravity at strong coupling may exhibit both exact integrable microstates as well as a chaotic spectrum of near-BPS states. This approach gives universal predictions which apply to both AdS and flat space black holes as well as dual superconformal field theories such as N=4 SYM.

Roberto Tateo (Turin University)

Fun with a nonlinear quantum oscillator [slide]

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The purpose of this informal talk is to discuss the quantization of a
Born-Infeld type oscillator and its interpretation as a possible
regularization of the Berry-Keating xp model.

Roberto Tateo (Turin University)

TTbar-like deformations in arbitrary dimensions [slide]

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We discuss a one-parameter family of composite fields which generalize the
TTbar operator to arbitrary spacetime dimensions.
We show that they induce a deformation of the classical action, which is
dynamically equivalent to a field-dependent modification of the metric
according to a specific flow equation.

Zoltan Bajnok (Wigner Research Centre for Physics)

CFT 3pt-functions from integrability [slide]

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I will explain how the diagonal 3pt-functions in 1+1 dimensional conformal field theories might be expressed in terms of the integrable structure of the underlying CFT. The idea is to introduce a massive integrable perturbation, calculate the spectrum and expectation values, and then investigate their CFT limit. In doing so, we expand the ground-state energy at small volume, a problem which resisted solutions for decades. As a result, we can express 3-point functions and mass-coupling relations purely in terms of the CFT kink functions.

Subir Sachdev (Harvard University)

Quantum statistical mechanics of strange metals and black holes [Youtube]

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Complex many-particle quantum entanglement is a central theme in two distinct major topics in physics: the strange metal state found in numerous correlated electron compounds, and the quantum theory of black holes in Einstein gravity. The Sachdev-Ye-Kitaev model provides a solvable theory of entangled many-particle quantum states without quasiparticle excitations. I will describe how this toy model has led to realistic universal models of strange metals, and to new insights on the quantum states of black holes.

Hugo Camargo (GIST)

TBA

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Kyungsun Lee (GIST)

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Zoltan Bajnok (Wigner Research Centre for Physics)

From perturbative to non-perturbative in 2d O(N) sigma models [slide]

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In my talk I will investigate the free energy of the 2d O(N) sigma models in a magnetic field. I will review how large number of perturbative coefficients can be derived and used to reveal non-perturbative corrections. I will also use the integrability of the model to extract the non-perturbative corrections directly.  I will compare the O(3) model to the O(4) model in detail.

Stefano Negro (New York University)

Topological gauging and non-relevant deformations of Quantum Field Theories [slide]

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In the last few years, much attention has been devoted to the study of a peculiar class of irrelevant deformations of 2-dimensional Quantum Field Theories, known as “Solvable Irrelevant Deformations”. The poster child of these is the celebrated “TTbar deformation”. They display unusual properties in the UV, which can be described exactly, their irrelevant nature notwithstanding. For this reason they represent a sensible extension of Quantum Field Theory beyond the Wilsonian paradigm and have attracted a considerable attention from the high energy theory community. The property of being solvable is shared with a wider class of deformations, constructed out of pairs of conserved currents. In general these are marginal deformations, thus presenting very different UV properties. Nonetheless their structures are similar to the TTbar ones, hinting at the existence of a universal description.

In this talk I will present a very general framework that accommodates both solvable irrelevant and solvable marginal deformations, which amounts to a “topological gauging” of the symmetries of the system. Through simple path integral computations, I will recover the main features of these theories and show their equivalence to TST and Yang-Baxter deformations. For the case of TTbar, the topological gauging perspective explains the previously not understood relation to field theory in non-commutative Minkowski space-time and to the centrally extended Poincaré algebra.

Tarek Anous (University of Amsterdam)

Sailing Past the Edge and Discovering the Island

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Quantum extremal surfaces and islands have led us to rethink the very meaning of microscopic entropy in the presence of dynamical gravity. I will present some work elucidating how the island/QES prescription emerges in a simple doubly holographic setup where we have analytic control. We will consider two holographic 2d CFTs joined at an interface  such that the bulk dual is a geometry that interpolates between two locally 3d AdS geometries with different AdS lengths. In particular we will consider systems where the transition region is a thin brane with tension T. In the limit of large tension, the brane gets pushed outward towards the boundary, meaning the effective theory on the brane is conformal and gravity almost decouples. We show precisely how the canonical CFT entropy and the remaining gravity dynamics leads to an island formula in this setup. The location of the island is identified with where the RT surface crosses the brane in the full holographic 3d picture.

James Bonifacio (University of Cambridge)

Bootstrap bounds for hyperbolic manifolds [slide]

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I will discuss how bootstrap methods can be used to derive bounds on the eigenvalues of the Laplace-Beltrami operator on closed hyperbolic manifolds. As an example, I will discuss an upper bound on the spectral gap of the Laplace-Beltrami operator in two dimensions and show that this is very close to being saturated by a particular genus-2 surface called the Bolza surface. I will also mention some results for 3-manifolds.

Minjae Cho (Princeton University)

Bootstrapping the Ising model on the lattice

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We discuss a bootstrap approach to the statistical Ising model of spins on the infinite lattice, where spin-flip equations and positivity conditions such as reflection positivity and Griffiths inequalities lead to rigorous two-sided bounds on spin correlators.