# “QFT, Strings and Beyond”

## Spring Semester 2022

Tuesday, 14:00, HIT E41.1

Organised by: Niklas Beisert, Johannes Broedel, Matthias Gaberdiel, Marc-Antoine Fiset, Francesco Galvagno

day

date

date

time

venue

venue

speaker

title

title

Tue

01.03.

14:00

HIT E41.1

Ondřej Hulík (Vrije U., Brussels)

“G-Algebroids: a unified framework for exceptional and generalized geometry”

abstract (click to view)
I will describe the concept of G-algebroid, which generalizes both Lie and

Courant algebroids, as well as the Leibniz algebroids used in the

description of exceptional generalized geometry. I will discuss a

classification results and possible applications of G-algebroids in

formulating Poisson-Lie U-duality. The talk is based on a joint work with

Mark Bugden, Fridrich Valach and Daniel Waldram. (click to hide)

Courant algebroids, as well as the Leibniz algebroids used in the

description of exceptional generalized geometry. I will discuss a

classification results and possible applications of G-algebroids in

formulating Poisson-Lie U-duality. The talk is based on a joint work with

Mark Bugden, Fridrich Valach and Daniel Waldram. (click to hide)

Tue

08.03.

14:00

HIT E41.1

Marco Gualtieri (U. of Toronto)

“Recent advances in the geometry of the N=(2,2) supersymmetric 2-dimensional sigma model”

abstract (click to view)
I will explain some of the more intriguing recent developments in Generalized Kähler geometry, the general target space geometry of 2-dimensional sigma models with (2,2) supersymmetry. We will focus on the unexpected way that the Kähler potential function arises in the general case, and how this leads, through quantization, to a surprising relation with noncommutative algebraic geometry. (click to hide)

Tue

15.03.

14:00

Zoom: 669 6805 3963

Dalimil Mazáč (Institute for Advanced Study)

“Bootstrapping Boundaries and Branes”

abstract (click to view)
I will discuss bootstrap constraints on conformal boundary conditions in 2D CFTs. In particular, I will explain how to derive bounds on the boundary entropy starting from the consistency of the annulus partition function. In the context of string theory, these imply a universal upper bound on the tension of stable D-branes. It turns out that our bounds are often saturated by interesting boundary conditions in rational CFTs. In some cases, the exact saturation can be proven using analytic functionals, which is closely related to the solution of the sphere packing problem in 8 and 24 dimensions. Our bounds at large central charge have potential implications for end-of-the-world branes in pure gravity on AdS_3. The talk will be based on https://arxiv.org/pdf/2112.00750.pdf, which is joint work with Scott Collier and Yifan Wang. (click to hide)

Tue

22.03.

14:00

Zoom: 669 6805 3963

Eduardo Casali (Harvard U.)

“Celestial OPE from twistor strings”

abstract (click to view)
Celestial conformal field theory (CCFT) is a conjectured theory living on the celestial sphere of the asymptotic boundary of Minkowski. In analogy to the usual AdS/CFT dictionary, CCFTs would be dual to gravitational theories in the bulk, with bulk scattering amplitudes being dual to correlation functions on the celestial sphere. OPE coefficients are basic building blocks of CFTs which should also have an analogue in CCFTs. It has been shown that CCFT OPEs can be extracted from amplitudes with appropriate wavefunctions for external states, but there's still no direct computation using the CCFT itself since we lack a first principles definition. I will talk about recent work I have done computing these OPEs directly by using twistor strings, circumventing the issue of the target space theory. I will show how the worldsheet CFT of the twistor string gives a realization of the algebra of operators of the CCFT, reproducing the known leading OPE terms, as well as how it can be used to compute further regular terms in the OPE, beyond what is currently known. As a bonus, the worldsheet OPE also organizes the spectrum naturally in terms of the infinite dimensional symmetry algebras of CCFTs. (click to hide)

Tue

29.03.

14:00

Zoom: 669 6805 3963

Gabriel Larios (Madrid, IFT and Michigan U., LCTP)

“Lessons and surprises from Kaluza-Klein spectra”

abstract (click to view)
Infinite towers of massive modes arise for every compactification of higher dimensional theories. Understanding the properties of these Kaluza-Klein towers on non-trivial solutions with an AdS factor has been a longstanding issue with clear holographic interest, as they describe the spectrum of single-trace operators of the dual CFTs at strong coupling and large N. In this talk, I will focus on two classes of solutions of such kind. The first class consists of AdS4 S-fold solutions of Type IIB supergravity that can be obtained from maximal gauged supergravity in D=4. For the latter part, I will describe new families of solutions in N=(1,1) supergravity in D=6 which uplift from half-maximal supergravity in D=3. In both cases, the spectra can be computed using recent techniques from Exceptional Field Theory, and the information thus obtained leads to several questions from the CFT side and unexpected conclusions. (click to hide)

Tue

05.04.

14:00

HIT E41.1

Gabriel Cuomo (Stony Brook U., New York)

“Taxonomy of Wilson lines”

abstract (click to view)
I will discuss some results on the space of Wilson lines in

conformal (and quasi-conformal) gauge theories. I will first discuss 4d

theories, focussing on the illustrative examples of massless scalar and

fermionic QED. I will show in particular that lines with sufficiently large

charge are subject to an instability towards charge screening from matter

fields. I will discuss the physical consequences of this instability and

its relation with a novel class of Wilson lines. Similar results apply in

nonAbelian conformal gauge theories, such as N = 4 SYM. I will finally

mention the generalization of these results to 3d conformal gauge theories,

and discuss possible applications in condensed matter systems. Based on a

work in progress with Ofer Aharony, Zohar Komargodski, Ma ́rk Mezei and

Avia Raviv-Moshe. (click to hide)

conformal (and quasi-conformal) gauge theories. I will first discuss 4d

theories, focussing on the illustrative examples of massless scalar and

fermionic QED. I will show in particular that lines with sufficiently large

charge are subject to an instability towards charge screening from matter

fields. I will discuss the physical consequences of this instability and

its relation with a novel class of Wilson lines. Similar results apply in

nonAbelian conformal gauge theories, such as N = 4 SYM. I will finally

mention the generalization of these results to 3d conformal gauge theories,

and discuss possible applications in condensed matter systems. Based on a

work in progress with Ofer Aharony, Zohar Komargodski, Ma ́rk Mezei and

Avia Raviv-Moshe. (click to hide)

Tue

12.04.

14:00

HIT E41.1

Federico Zerbini (IPhT (CEA-Saclay))

“String amplitudes, polylogarithms and KZB connections”

abstract (click to view)
The perturbative expansion of string theory amplitudes is an important testing ground for double-copy relations between gravity and gauge theories, and the AdS/CFT correspondence. It is indexed by an integer which can be interpreted as the genus of a surface. In the last decade, the combined effort of mathematicians and physicists led to great progress in our understanding of the genus-zero and genus-one coefficients. I will report on this progress, with special focus on the role of Knizhnik-Zamolodchikov-Bernard (KZB) connections, which arise from Wess-Zumino-Witten models, and of polylogarithm functions. At the end I will report on higher-genus perspectives, based on a joint work with Benjamin Enriquez. (click to hide)

Tue

03.05.

14:00

HIT E41.1

Shota Komatsu (CERN)

“Large Charge and Coulomb Branch of N=4 SYM”

abstract (click to view)
We explore the relationship between the large charge sector and

the Coulomb branch of N=4 super-Yang-Mills (SYM) both in the large N limit

and in a theory with the SU(2) gauge group. In particular, we propose

concrete formulae relating the correlation functions of the large charge

operators and the correlation functions on the Coulomb branch and discuss

their interplay with the OPE and the form factor expansion. We also argue

that the spectrum of non-BPS operators in the large charge sector of N=4

SYM with the SU(2) gauge group is governed by the centrally-extended

SU(2|2) symmetry---the symmetry which played a crucial role in the analysis

of the large N limit. (click to hide)

the Coulomb branch of N=4 super-Yang-Mills (SYM) both in the large N limit

and in a theory with the SU(2) gauge group. In particular, we propose

concrete formulae relating the correlation functions of the large charge

operators and the correlation functions on the Coulomb branch and discuss

their interplay with the OPE and the form factor expansion. We also argue

that the spectrum of non-BPS operators in the large charge sector of N=4

SYM with the SU(2) gauge group is governed by the centrally-extended

SU(2|2) symmetry---the symmetry which played a crucial role in the analysis

of the large N limit. (click to hide)

Tue

17.05.

14:00

HIT E41.1

Andrew Mcleod (CERN)

“A Three-Point Form Factor through Eight Loops, and a New Form Factor/Amplitude Duality”

abstract (click to view)
Traditional methods for computing quantities such as scattering amplitudes and form factors in quantum field theory become intractable at high perturbative orders. However, a great deal is now known about the mathematical properties of these quantities, especially in supersymmetric gauge theories. In this talk, I will describe how this knowledge can be leveraged to 'bootstrap' amplitudes and form factors directly, by constructing an ansatz with the appropriate mathematical structure and requiring it to have certain expected behavior in special kinematic limits. I will focus on the example of three-point form factors in maximally supersymmetric gauge theory, which have recently been bootstrapped through eight loops. I will then describe a remarkable new duality between this form factor and certain six-particle amplitudes in the same theory, which holds order by order in perturbation theory. (click to hide)

Tue

24.05.

14:00

Zoom: 669 6805 3963

Silvia Nagy (Queen Mary, U. of London)

“Different approaches to gravity from Yang-Mills squared”

abstract (click to view)
The idea of writing various quantities in gravity as double copies of the analogous objects in Yang-Mills gauge theory has been gaining a lot of traction in recent years - I will give an overview of the numerous different formulations that have arisen from this drive. Then I will focus on 2 particular ones: the first is based on certain double copy replacement rules in the self-dual sector, and the second links with twistor theory. These have allowed for a recent expansion of the remit of the double copy in the context of symmetries and classical solutions. Finally, I will make some comments on a more ambitious question: is it possible to unify the various different formulations into a single framework? (click to hide)

Tue

31.05.

14:00

HIT E41.1

Ben Hoare (Durham U.)

“Integrable deformations of Z2 and Z4 permutation cosets”

abstract (click to view)
Integrable 2d sigma models on Z2 and Z4 permutation cosets, with G x G symmetry group, describe strings on AdS3 space-times. The product structure of the symmetry group leads to a richer space of integrable deformations. In this talk I will outline recent progress in mapping out this space, before discussing a special class of deformations of AdS3 x S3 x T4 that preserve half the supersymmetry. Interestingly, these deformations can either be found via Yang-Baxter deformations, proving their integrability, or by U-duality transformations. (click to hide)

Tue

07.06.

11:30

HIT E41.1

Jnan Maharana (Institute of Physics, Bhubaneswar)

“Is the scattering amplitude analytic in a field theory with a compact spatial coordinate?”

abstract (click to view)
N.N. Khuri, in 1995, investigated analytcity property of scattering

amplitude in potential scattering where one spatial dimension was

compactified on a circle of radius R. He showed that the forward

scattering amplitude, in this case does not satisfy dispersion relations

whereas in potentials without a compact coordinate the amplitude is

analytic, known for a long time. If this result were valid for a

relativistic QFT with a compact spatial coordinate then it will be

a matter of serious concern. I consider a massive scalar field theory in flat

5-dimensional space and then compactify one spatial cordinate on a circle.

I employ LSZ formalism and show that forward scattering amplitude satisfies

dispersion relations. Properties of nonforward amplitude are discussed

which goes beyond Khuri's result. Possible consequences of decompactification

at LHC energies will be discussed. (click to hide)

amplitude in potential scattering where one spatial dimension was

compactified on a circle of radius R. He showed that the forward

scattering amplitude, in this case does not satisfy dispersion relations

whereas in potentials without a compact coordinate the amplitude is

analytic, known for a long time. If this result were valid for a

relativistic QFT with a compact spatial coordinate then it will be

a matter of serious concern. I consider a massive scalar field theory in flat

5-dimensional space and then compactify one spatial cordinate on a circle.

I employ LSZ formalism and show that forward scattering amplitude satisfies

dispersion relations. Properties of nonforward amplitude are discussed

which goes beyond Khuri's result. Possible consequences of decompactification

at LHC energies will be discussed. (click to hide)

last modified: Wed, 1 Jun 2022, 09:24 CEST