# General info

Journal clubs are on Tuesdays from 13 to 14 in Seminar Room 3 (building 1).

Feel free to suggest papers in the comments section. Note that you can also add attachments and comment on other people's suggestions.

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# Winter semester 2019-2020

## Journal club 24-03-2020

- Saskia Demulder, (TBA)

## Journal club 03-03-2020

- Lorenzo Quintavalle, (TBA)

## Journal club 25-02-2020 (TBA)

## Journal club 18-02-2020

- Madalena Lemos (CERN), Surface defects in 4d superconformal theories and chiral algebras
Abstract:

We study symmetry constraints on BPS surface defects in four-dimensional superconformal field theories, showing how supersymmetry imposes relations on anomaly coefficients. Turning to dynamics, we analyze a protected subsector of N=(2,2) surface defects that is captured by a two-dimensional chiral algebra. We discuss how to compute observables of interacting defects from the chiral algebra, including the aforementioned anomaly coefficients.

## Journal club 11-02-2020 (Integrability school)

## Journal club 04-02-2020

- Taro Kimura (Bourgogne U.), Yet another affinization of geometric Langlands correspondence
Abstract:

One of the implications of the geometric Langlands correspondence is the isomorphism between conformal blocks of W-algebra and affine Lie algebra. This correspondence has a natural q-deformation, providing a relation between q-deformation of W-algebra and quantum affine algebra. In this talk, I'll discuss yet another affinization of this correspondence between doubly affine W-algebra and quantum toroidal algebra, based on the formalism of quiver W-algebra. I'll also mention its possible physical interpretation in gauge theory with the surface defects.

## Journal club 28-01-2020

- Federico Carta (DESY), Supersymmetry Enhancement

## Journal club 21-01-2020

- Lorenzo Quintavalle (DESY), Celestial sphere amplitudes

## Journal club 10-12-2019

- Fabrizio del Monte (SISSA), Class S theories and isomonodromic deformations on the torus.

Abstract:In the last few years there have been many new results connecting (linear quiver) N=2 class S theories, and the topological strings that engineeer them, to the theory of isomonodromic deformations on the sphere and their q-deformations. These gauge theories are constructed by compactifying the 6d N=(0,2) SCFT on a punctured Riemann Sphere, whose moduli, which are the marginal deformations of the gauge theory, are the times of the isomonodromic flows. The aim of this talk is to show how this connection can be extended beyond the case of genus zero, for more general (asymptotically superconformal) class S theories. We will discuss in detail the case of circular quiver gauge theories, that are obtained from the N=(0,2) SCFT on punctured tori, and see how the genus one case displays new qualitative features that are absent on the sphere, due to the possibility of various inequivalent vector bundles, and how this actually provides new interesting relations satisfied by the gauge theory partition function.

## Journal club 03-12-2019

Jack Foster (Southampton University): Cluster Adjacency, Tropical Geometry, and Scattering Amplitudes

Abstract: I will discuss two new areas of interest in scattering amplitudes: cluster adjacency and tropical geometry. The former describes how the analytic structure of planar amplitudes in N=4 Super Yang-Mills is controlled by mathematical objects called cluster algebras. The latter has been used to calculate amplitudes in the biadjoint phi^3 theory, which I will discuss briefly, but it also has implications for cluster adjacency.

## Journal club 26-11-2019

- Francesco Galvano (Torino): Emitted radiation and geometry (This seminar will take place in
**Building 3 Seminar room BAH 2**)

Abstract: 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 a N=2 superconformal theory we prove that this observable is exactly related to a small geometric deformation of the background geometry. In a four dimensional case, supersymmetric localization allows to express the emitted energy in terms of a matrix model on a squashed sphere.

## Journal club 19-11-2019

- No seminar scheduled.

## Journal club 12-11-2019

- Zhengwen Liu: Scattering Equations and Multi-Regge Kinematics

Abstract**:**The scattering equations, a system of algebraic equations connecting the space of kinematic invariants and the moduli space of punctured Riemann spheres, provide a novel way to construct scattering amplitudes. In this framework, the tree-level S-matrix in many quantum field theories can be reformulated as a multiple integral that is entirely localized on the zeroes of the scattering equations. After presenting a very minimal introduction to the scattering equations, I will discuss the asymptotic behavior of the scattering equations in the so-called Multi-Regge kinematic regime and the corresponding factorizations of amplitudes in gauge theory and gravity.

References**:**Multi-Regge kinematics and the scattering equations

Gravitational Scattering in the High-Energy Limit

Journal club 05-11-2019

- Yuta Sekiguchi (U. Bern, AEC), O(d,d) transformations preserve classical integrability.
Abstract: We studied the classical integrability of O(d,d) transformations including not only O(d,d;Z) duality but also global O(d,d;R) deformation. The latter is known in the traditional literature to generate so-called current-current (JJbar) deformations of 2D CFTs. In this talk, I first plan to give brief reviews Yang-Baxter deformations of string backgrounds as well as the doubled sigma model. Then I will present how to construct the Lax pairs in the O(d,d) deformed WZNW models via O(d,d) map through very easy examples. The resulting Lax connections are in general non-local because they depend on the winding modes. Finally I will briefly comment on open questions under consideration in relation to the recent irrelevant integrable deformation. This talk is based on the recent work [1907.03759] with Domenico Orlando, Susanne Reffert and Kentaroh Yoshida.

## Journal club 29-10-2019

Till Bargheer: Exact Four-Point Functions: Genus Expansion and Strong Coupling (This seminar will take place in

**Building 3 Seminar room BAH 2**)

Using integrability, correlation functions of local operators in planar N=4 super Yang-Mills theory can be computed by reconstructing the dual worldsheet from a set of "hexagon" patches via "gluing" (performing complete state sums over mirror Bethe states). After reviewing this general procedure, I will explain how it can be used to compute a certain class of four-point correlators as an exact function of the coupling at any order in the 1/Nc genus expansion, and how the resulting series can be re-summed (via a suitable matrix model) to recover the full Nc dependence. If time permits, I will also comment on the strong-coupling limit of these correlators.

## Journal club 22-10-2019

Gleb Kotousov: "Reflection operators in integrable QFT".

## Journal club 15-10-2019

Sylvain Lacroix, Gauge Theory And Integrability, III (focusing on the second part about "disorder defects").

# Summer 2019

## Journal club 08-10-2019

Alessandro Pini, Extremal Correlators and Random Matrix Theory.