# Wednesday Seminars

The majority of seminars take place remotely these days as a consequence of the ongoing Covid-19 pandemic. There is also a nordic seminar series.

If you would like to attend via Zoom but have not subscribed to the mailing list yet, you can contact the organizers in order to obtain the passcode.

**Organizers:** Luca Cassia (luca.cassia@physics.uu.se), Nicolò Piazzalunga (nicolo.piazzalunga@physics.uu.se)

## Spring 2022

### Niccolò Cribiori (Munich U., Max Planck Inst.)

16 February 2022, 13.45

Location: Zoom

Title: TBA

Abstract: TBA

### Miroslav Rapčák (UC, Berkeley)

09 February 2022, **16.30**

Location: Zoom

Title: TBA

Abstract: TBA

### Ingmar Saberi (Munich U.)

26 January 2022, **14.30**

Location: Zoom

Title: TBA

Abstract: TBA

### Francesco Aprile (Sao Paulo, IFT)

19 January 2022, 13.45

Location: Zoom

Title: TBA

Abstract: TBA

## Fall 2021

### Jakub Vošmera (ETH)

Tuesday 14 December 2021, 14.15 (Nordic HET)

Location: Zoom https://uu-se.zoom.us/j/67470448765

Title: D-branes in AdS_3 x S^3 x T^4 at k = 1 and their holographic duals

Abstract: Following the recent work of Eberhardt, Gaberdiel and Gopakumar, exact comparison between various quantities living on the two sides of the AdS/CFT duality has become a possibility. The goal of this talk will be to extend the existing holographic dictionary to include some non-perturbative vacua on both sides. I will start by reviewing the original, purely closed-string setup, giving arguments that string theory on AdS_3 x S^3 x T^4 with minimal k=1 NS-NS flux is exactly dual to the symmetric-product orbifold CFT with the T^4 as the seed. I will then construct various D-branes of this string theory and calculate their associated cylinder amplitudes. We will observe that these amplitudes match with the cylinder correlators of certain boundary states of the dual CFT, thus suggesting a direct correspondence between these boundary conditions. I will also show that the disk amplitudes of these D-branes localise to those points in the worldsheet moduli space where the worldsheet disk holomorphically covers the spacetime disk. This talk is based on 2110.05509.

### António Antunes

08 December 2021, 13.45

Location: Zoom https://uu-se.zoom.us/j/68817284142

Title: Lightcone Bootstrap at higher-points

Abstract: Higher-point functions of scalar operators are a rich observable in CFTs, as they contain OPE data involving multiple spinning operators. In this talk, I will derive the lightcone blocks for five- and six-point functions in the snowflake channel and use them to bootstrap these correlators in the lightcone limit. As a result, I will determine the large spin expansion of OPE coefficients involving two or three spinning operators, which can then be checked in simple theories. Finally, I will point out many open directions of research involving CFT higher-point functions.

### Joshua Kames-King (Bonn U. and Santa Barbara, KITP)

Tuesday 30 November 2021, 15.45 (Nordic HET)

Location: Zoom https://uu-se.zoom.us/j/62123265765

Title: No Page curves for de Sitter

Abstract: The nature of de Sitter horizon and a possible microscopic interpretation are still a mystery. In this talk I will explain how we investigated the question of unitary evaporation for the de Sitter horizon in [2108.09318]. More concretely, in the out-of-equilibrium Unruh de Sitter state, analogous to black hole evaporation for cosmological horizons we study the time-dependent radiation entropy for both the meta-observer and the static patch observer. I will conclude by commenting on work in progress relating to recent approaches to static patch holography and quantum information in cosmological spacetimes.

### Mathew Bullimore (Durham U.)

17 November 2021, 13.45

Location: Zoom https://uu-se.zoom.us/j/65324164463

Title: 3d Supersymmetric Gauge Theories on an Elliptic Curve

Abstract: TBA

### Matteo Bertolini (SISSA)

Tuesday 09 November 2021, 14.15 (Nordic HET)

Location: Zoom https://uu-se.zoom.us/j/68952081910

Title: On fixed-points and phase transitions in five dimensions

Abstract: Supersymmetric gauge theories in five dimensions, although power counting non-renormalizable, are known to be in some cases UV completed by a superconformal field theory (SCFT). Similar results are not available without supersymmetry and it is in fact an open question whether non-supersymmetric CFT exist at all in five dimensions. A concrete way to try and answer this question is to deform known superconformal fixed points by non-supersymmetric relevant deformations and study the end-point of the RG-flow. Recently, a supersymmetry-breaking deformation of one such SCFT, the so-called E_1 theory, was proposed which, at weak gauge coupling, leads to pure SU(2) Yang-Mills and which was conjectured to lead to an interacting CFT at strong coupling. Using both field theory and brane-web techniques, I will show that for large enough gauge coupling a global symmetry is spontaneously broken and the theory enters a new phase which, at infinite coupling, displays an instability. The Yang-Mills and the symmetry broken phases are separated by a phase transition, which can be first or second order. Quantum corrections to this analysis are discussed, as well as possible outlooks.

### Adam Chalabi

03 November 2021, 13.45

Location: Zoom https://uu-se.zoom.us/j/66345096567

Title: Defect Central Charges

Abstract: Conformal defects can be characterised by their contributions to the Weyl anomaly. The coefficients of these terms, often called defect central charges, depend on the particular defect insertion in a given conformal field theory. I will review what is currently known about them across dimensions, and present novel results. I will discuss many examples where they can be computed exactly without requiring any approximations or limits. These include defects in free theories, and recently developed tools for defects in superconformal field theories.

### Hans Jockers (Hamburg U.)

27 October 2021, 13.45

Location: Zoom https://uu-se.zoom.us/j/69185015007

Title: Quantum K-Theory and Wilson Loop Algebras

Abstract: In this talk we review certain aspects of Wilson line operators in 3d N=2 supersymmetric gauge theories with a Higgs branch that is geometrically described by complex projective varieties. We discuss the relationship between Wilson loop algebras of the gauge theory and the quantum K-theoretic ring of the associated complex projective variety.

### Eric Sharpe (Virginia Tech.)

06 October 2021, 16.00

Location: Zoom https://uu-se.zoom.us/j/62354444529

Title: Decomposition of quantum field theories

Abstract: In this talk, I will outline higher-form symmetries in quantum field theories, focusing on two-dimensional theories with one-form symmetries. A fundamental QFT result (first worked out in 2006, but recently becoming more widely understood) is that such theories are equivalent to disjoint unions of other quantum field theories, an equivalence known as `decomposition'. We will outline some basic examples, as well as applications to Gromov-Witten theory, gauged linear sigma models, elliptic genera, and anomaly resolution in orbifolds.

### Dmitry Galakhov

29 September 2021, 13.45

Location: Zoom https://uu-se.zoom.us/j/69225249112

Title: Quiver BPS Algebras

Abstract: In this talk we will discuss BPS algebras associated to BPS states in systems of D-branes wrapping toric Calabi-Yau threefolds. These algebras are labeled by framed quivers and are usually referred to as shifted quiver Yangians generalizing a notion of affine Yangians introduced in integrable models. Accordingly, quiver BPS algebras appear nested naturally in rational-trigonometric-elliptic hierarchies. I will derive these algebras from an effective gauge field description of D-brane systems as well as their representations, usually referred to as MacMahon modules, or molten crystal models. The analysis of these theories suggests that there exist even richer classes of algebras associated with generalized cohomology theories going beyond the traditional rational-trigonometric-elliptic hierarchy. This talk is based on arXiv:2108.10286 with Wei Li and Masahito Yamazaki.

### Markus Dierigl (Munich U., ASC)

15 September 2021, 13.45

Location: Zoom https://uu-se.zoom.us/j/66983852282

Title: The anomaly that was not meant IIB

Abstract: In this talk I will discuss possible quantum anomalies in the duality group of type IIB string theory. Quite surprisingly, these do not vanish, which would signal an inconsistency of F-theory. A subtle modification of the type IIB Chern-Simons term cancels the anomalies in a highly non-trivial way. However, in principle there are alternative ways to cancel the anomaly which might point towards the existence of a discrete string theory landscape.

Slides: pdf

## Spring 2021

### Massimo Taronna (Naples U.)

Tuesday 01 June 2021, 14.15 (Nordic HET)

Location: Zoom 674 0966 1179

Title: Bootstrapping Inflationary Correlators in Mellin Space

Abstract: The bootstrap philosophy has found remarkable successes in the context of Conformal Field Theories. These advances were mostly consequences of powerful non-perturbative implementations of causality and unitarity which translated into key results in the context of the dual gravitational theories in Anti de Sitter space. In this talk I will discuss recent progress towards the analogous problem in de Sitter space. Making use of a recently introduced Mellin-Barnes representation for momentum space correlators, I will discuss how to express dS exchange amplitudes in terms of AdS ones and how to employ conformal bootstrap methods like the spectral representation, Mellin Amplitudes and crossing decomposition in de Sitter space.

### Viraf Mehta (Göttingen U.)

19 May 2021, 13:45

Location: Zoom https://uu-se.zoom.us/j/62886323854

Title: Superradiance in the String Axiverse

Abstract: This work uses state-of-the-art computational geometric tools to directly manipulate the topological information contained in the infamous Kreuzer-Skarke database to derive axion data for compactifications of Type IIB string theory, including systems at the maximal Hodge number, 491. We discuss the physics of these multiaxion models focussing, in particular, on the constraints derived from observed spinning black holes.

### Rob Klabbers (Nordita and Royal Inst. Tech., Stockholm)

12 May 2021, 13:45

Location: Zoom https://uu-se.zoom.us/j/68211574846

Title: How coordinate Bethe ansatz works for Inozemtsev model

Abstract: Three decades ago, Inozemtsev discovered an isotropic long-range spin chain with elliptic pair potential that interpolates between the Heisenberg and Haldane–Shastry spin chains while admitting an exact solution throughout, based on a connection with the elliptic quantum Calogero–Sutherland model. Though Inozemtsev’s spin chain is widely believed to be quantum integrable, the underlying algebraic reason for its exact solvability is not yet well understood. As a step in this direction in arxiv:2009.14513 we introduced a refinement of Inozemtsev’s ‘extended coordinate Bethe ansatz’ and clarify various aspects of the model’s exact spectrum and its limits. I will discuss this refinement and show how it improves our control over the model. For example, in the new coordinates, the energy becomes close to additive. Moreover, both the Bethe ansatz equations and the energy become elliptic functions, allowing for a lifting of the spectral problem to the elliptic curve, effectively rationalising it as one might expect for an isotropic spin chain. Direct comparison with the limiting models is now also possible: I will showcase the relation with the regular Bethe ansatz for the XXX chain and the Haldane–Shastry spectrum. In particular, I will show that the Inozemtsev model links the scattering states of the Heisenberg model to the Yangian highest-weight states of Haldane Shastry, while Heisenberg bound states become affine descendants in the Haldane–Shastry spectrum.

### Matthew Buican (Queen Mary, U. of London)

Tuesday 27 April 2021, 14:15 (Nordic HET)

Location: Zoom https://uu-se.zoom.us/j/66168579767

Title: aXb=c in 2+1D TQFT

Abstract: I will start by giving a gentle introduction to 2+1D topological quantum field theory (TQFT), the fusion of line operators in these theories, and how 1-form symmetry arises. I will then discuss recent work in which we study fusions that are, in a sense I will explain, generalizations of 1-form symmetry and reveal much about global properties of TQFT. I will illustrate the discussion with various examples from 2+1D discrete gauge theories to Chern-Simons theories with continuous gauge groups and mention various open problems.

### Yasunori Lee (Tokyo U., IPMU)

Tuesday 06 April 2021, 14:15 (Nordic HET)

Location: Zoom https://uu-se.zoom.us/j/65178150280

Title: Some comments on 6d global gauge anomalies

Abstract: Given a G gauge theory, there can be global (non-perturbative) gauge transformations under which the partition function is not invariant. In 6d, relevant cases include G = SU(2), SU(3), and G2, and the old computations utilizing homotopy groups affirmed that the anomalous phases can indeed arise in all three cases. On the other hand, from the modern point of view utilizing bordism groups, there should not be such global gauge anomalies in the first place. In this talk, I will describe how this apparent conflict is resolved by carefully examining the cancellation of perturbative gauge anomalies via 6d Green-Schwarz mechanism.

Slides: pdf

### Simone Giacomelli (Oxford U.)

03 March 2021, 13:45

Location: Zoom https://uu-se.zoom.us/j/63833131892

Title: Superconformal theories from S-fold geometries

Abstract: The term S-folds denotes F-theory compactifications which involve non-trivial S-duality transformations. In this talk I will discuss 4d N=2 preserving S-folds and the worldvolume theories on D3-branes probing them. They consist of two new infinite series of superconformal theories whose distinction lies in the discrete torsion carried by the S-fold and in the difference in the asymptotic holonomy of the gauge bundle on the 7-brane. These models are connected by an interesting web of RG flows and their Higgs branches provide new examples of instanton moduli spaces.

### Yegor Zenkevich (SISSA)

03 February 2021, 13:45

Location: Zoom https://uu-se.zoom.us/j/61908309181

Title: Networks of branes and intertwining operators

Abstract: BPS particles in supersymmetric theories form a subspace of the Hilbert space. Moreover, when one scatters a pair of BPS particles there is an amplitude for getting a new BPS particle. This endows the space of BPS particles with the structure of an algebra, while extended objects (e.g. branes) to which the particles can bound should furnish a representation of this algebra. Brane junctions in this picture become intertwining operators between representations of the BPS algebra.

I will argue that BPS states of (p,q)-strings of Type IIB string theory in a certain Omega-background form Ding–Iohara–Miki algebra, and that three- and fivebranes indeed correspond to representations of this algebra. I will introduce some brane junctions and show how they are related to 3d gauge theories living in the worldvolume of D3 branes ending on 5-branes.

Based on 1812.11961, 1912.13372, 2012.15563