# “Strings, CFT & Integrability”

## Fall Semester 2020

Tuesday, 16:00, zoom: 940 4378 1498

Organised by: Niklas Beisert, Matthias Gaberdiel, Marc-Antoine Fiset, Pietro Longhi

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Tue

29.09.

16:00

zoom: 940 4378 1498

Jakub Vosmera (ETH Zurich)

“Exploring novel open string boundary conditions with string field theory”

abstract (click to view)
We will first give a brief overview of open (super)string field theory and discuss how its classical solutions can be used to extract information about open string backgrounds (D-branes) for a given closed string worldsheet CFT. We will then analyze two particular settings where this approach sheds new light on non-trivial conformal boundary conditions: new exotic D-branes wrapping a 2-torus in the bosonic string and D(-1)/D3 bound states wrapping a 4-torus in type II superstring. (click to hide)

Tue

06.10.

16:00

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Francesco Galvagno (ETH Zurich)

“Emitted radiation and geometry”

abstract (click to view)
We discuss the computation of the radiated energy by an accelerated heavy particle. This quantity is captured by the one-point function of the stress energy tensor in presence of a Wilson line. In N=2 superconformal theories we prove that this observable is exactly related to a small deformation of the background geometry, just by a consistent implementation of the symmetries. In a four dimensional case, SUSY localization allows to express the emitted energy in terms of a matrix model on a squashed sphere. Finally we analyze the consequences of our results for a weak coupling perturbative expansion. (click to hide)

Tue

13.10.

16:00

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Alexander Maloney (McGill)

“Gravity from Averaging CFTs”

abstract (click to view)
I will explore the idea that certain theories of gravity in Anti-de Sitter space are dual to an average over an ensemble of quantum theories, rather than to a specific quantum theory. I will describe an average over Narain’s family of two-dimensional conformal field theories which describe free bosons on a torus, and compute the partition function using the Siegel-Weil formula. The result takes the form of a sum over geometries as one would expect in a theory of gravity. But the gravitational theory looks more like a Chern-Simons theory than like Einstein gravity. (click to hide)

Tue

20.10.

16:00

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Luigi Tizzano (Stony Brook & SCGP)

“Delayed Deconfinement and the Hawking-Page Transition”

abstract (click to view)
We revisit the confinement/deconfinement transition in N=4 super Yang-Mills (SYM) theory and its relation to the Hawking-Page transition in gravity. Recently there has been substantial progress on counting the microstates of 1/16-BPS extremal black holes. In this context, we discuss a phenomenon in complex matrix models which we call "delayed deconfinement". It turns out that when the action is complex, due to destructive interference, tachyonic modes do not necessarily condense. We demonstrate this phenomenon in a simple unitary matrix model and finally in the context of N=4 SYM. Delayed deconfinement implies a first-order transition, in contrast to the more familiar cases of higher-order transitions in unitary matrix models. We determine the deconfinement line and find remarkable agreement with the prediction of gravity. (click to hide)

Tue

03.11.

16:00

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Shlomo Razamat (Technion)

“Where do supersymmetric dualities come from?”

abstract (click to view)
We will overview the program of realizing supersymmetric quantum field theories in 4d by compactifying 6d SCFTs on Riemann surfaces. In particular we will discuss how strong coupling IR phenomena in 4d, such as dualities and emergence of global symmetries, can be understood geometrically through such compactification constructions. As a concrete example we will show that the 4d SU(N+2) SQCD in the middle of the conformal window can be engineered by compactifying certain 6d SCFTs on three punctured spheres. We will discuss here the interplay between simple geometric and group theoretic considerations and complicated field theoretic strong coupling phenomena. In particular we will show how many known and novel dualities and symmetry enhancement phenomena of supersymmetric gauge theories with SU(M) type gauge groups are related to the Weyl group of SO(12+4N). (click to hide)

Tue

10.11.

16:00

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Marcos Marino (Geneva)

“Resurgence, BPS states, and quantum knot invariants”

abstract (click to view)
Resurgent analysis associates, to a perturbative series, a rich structure of exponentially small corrections, or trans-series. Recent developments indicate that, in some cases, this structure encodes the counting of BPS states in an appropriate quantum theory. After reviewing some background material, I will

focus on the resurgent structure of the perturbative Chern-Simons invariants of hyperbolic knots. In this example, singularities in the Borel plane appear in infinite towers, and the corresponding Stokes constants are integer numbers. This gives a new mechanism for obtaining integer invariants in these theories. I present various conjectures/computations of these integers, which appear as coefficients of q-series, and I relate them to BPS invariants of hyperbolic knots. State integrals and their holomorphic blocks play an important role in the story. (click to hide)

focus on the resurgent structure of the perturbative Chern-Simons invariants of hyperbolic knots. In this example, singularities in the Borel plane appear in infinite towers, and the corresponding Stokes constants are integer numbers. This gives a new mechanism for obtaining integer invariants in these theories. I present various conjectures/computations of these integers, which appear as coefficients of q-series, and I relate them to BPS invariants of hyperbolic knots. State integrals and their holomorphic blocks play an important role in the story. (click to hide)

Tue

17.11.

16:00

zoom: 940 4378 1498

Manus Visser (Geneva)

“Gravitational Thermodynamics of Causal Diamonds”

abstract (click to view)
Black hole (more generally, horizon) thermodynamics is a window into quantum gravity. In this talk I discuss whether horizon thermodynamics can be quasi-localized. A special case is the static patch of de Sitter space, known since the work of Gibbons and Hawking to admit a thermodynamic equilibrium interpretation. I will review this example, and present its generalization to finite causal diamonds in maximally symmetric spacetimes. This story includes a Smarr formula and first law of causal diamonds, analogous to those of black hole mechanics. I will also give a holographic interpretation of the first law of causal diamonds in AdS3/CFT2, in terms of differential entropy and holographic complexity. (click to hide)

Tue

24.11.

16:00

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Matthias Volk (Niels Bohr Institute)

“One-point functions in defect CFTs with holographic duals”

abstract (click to view)
In conformal field theories with defects (dCFTs) one-point functions of scalar operators constitute interesting observables. In this talk we discuss dCFTs that have a dual holographic description in terms of probe brane systems, in particular the D3-D5 and D3-D7 probe brane intersections. We describe how the non-trivial vacuum expectation values of the scalars in the dCFT lead to a mixing problem in the quadratic terms of the Lagrangian that can be solved by coupling group representations, thereby reviewing the work of arXiv:1611.04603 for the D3-D5 system. For the D3-D7 system we explain how similar methods allow us to prepare the dCFT for perturbative calculations by deriving the propagators for the theory. As an application we compute the one-loop one-point function of the scalar operator tr(Z^L) finding agreement with a holographic computation in a certain double-scaling limit. We comment on the connection to recent progress in the integrability of one-point functions in which the latter are expressed in terms of overlaps of Bethe eigenstates with matrix product states.

Based on arXiv:1810.11463 and arXiv:1912.02468 in collaboration with A. Gimenez Grau, C. Kristjansen, M. Wilhelm. (click to hide)

Based on arXiv:1810.11463 and arXiv:1912.02468 in collaboration with A. Gimenez Grau, C. Kristjansen, M. Wilhelm. (click to hide)

Tue

01.12.

16:00

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Xinan Zhou (Princeton)

“All holographic four-point correlators in maximally supersymmetric CFTs”

abstract (click to view)
Holographic correlators are natural objects to test and exploit the AdS/CFT correspondence. However, brute force calculations are notoriously difficult and have led to only a handful results in the past two decades. In this talk, I will introduce a powerful new method to constructively derive all tree-level four-point functions in holographic CFTs with maximal supersymmetry. I will introduce the notion of maximally R-symmetry violating (MRV) limit, which is key to our construction. In this limit, the correlators simplify drastically and become easy to compute. I will then show that the full correlators can be reconstructed from the MRV limit by using symmetries. A remarkable feature is that the four-point functions do not have intrinsic contact interactions, and can be expressed as sums of exchanges alone. Using this method we reproduce the known results for AdS5xS5 dual to 4d N=4 SYM, and also compute all four-point functions from AdS4xS7 and AdS7xS4, dual respectively to 3d N=8 ABJM and the 6d (2,0) theory. The amplitudes for these different theories exhibit a surprising universality. (click to hide)

Tue

08.12.

16:00

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Fabrizio Nieri (DESY)

“ε-deformed W algebras: 4d N=2 gauge theories, 2d CFT and (bi)spectral duality”

abstract (click to view)
I will present a constructive definition of ε-deformed or Yangian version of q-W algebras which feature in the brane engineering of 5d N=1 quiver gauge theories. This new type of algebras fill in the missing intermediate level between q-deformed and un-deformed W algebras, and I will explain in which sense the ε-deformed algebras are (bi)spectral duals to the ordinary ones. In particular, I will discuss how the ε-deformed conformal blocks manifestly reproduce instanton partition functions of 4d N = 2 quiver gauge theories and give dual (unconventional) integral representations of Toda conformal blocks via AGT duality. Finally, I will also consider an application to integrable many-body systems. (click to hide)

Tue

15.12.

16:00

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Owen Gwilliam (Massachusetts, Amherst)

t.b.a.

last modified: Tue, 17 Nov 2020, 17:15 CET