# “QFT, Strings and Beyond”

## Autumn Semester 2023

Organised by: Niklas Beisert, Johannes Broedel, Sibylle Driezen, Matthias Gaberdiel, Edward Mazenc

day

date

date

time

venue

venue

speaker

title

title

Wed

27.09.

14:00

HIT E41.1

Upamanyu Moitra (ICTP)

“Finite Entanglement Entropy in String Theory”

abstract (click to view)
The finiteness of the entanglement entropy in string theory has crucial implications for the information paradox, quantum gravity, and holography. In my talk, I will describe the recent progress made on establishing this finiteness. I will describe the orbifold construction appropriate for analyzing entanglement in string theory, the tachyonic divergences that one encounters and how we get a finite and calculable answer for the entanglement entropy. Time permitting, I will also discuss some additional examples where this method bears fruits. (Based on works in collaboration with Atish Dabholkar) (click to hide)

Wed

04.10.

14:00

HIT E41.1

Edward Mazenc (ETH)

“Strings from Feynman Diagrams”

abstract (click to view)
To understand Holography is to understand precisely how large N gauge theories can be recast as a sum over two-dimensional worldsheets. In the simplest case, where the gauge theory reduces to a matrix integral, we can carry out this program explicitly. This talk will sketch how strings concretely emerge from gauge theory Feynman diagrams - both at the level of the worldsheet, as well as its embedding into the target space. Along the way, we will see how several open string descriptions can give rise to the same closed string background. Finally, by reinterpreting the worldsheet directly as a 2d „bulk“ spacetime, we discuss how the story of Stanford-Shenker-Saad can be upgraded away from the double-scaling limit, giving rise to new mathematical ideas such as „discrete Weil-Petersson volumes“ and exact bulk descriptions of the low-energy sector of double-scaled SYK. (Works in progress with R. Gopakumar, P. Maity, S. Komatsu, R. Kaushik & D. Sarkar) (click to hide)

Wed

11.10.

14:00

HIT E41.1

Shota Komatsu (CERN)

“Worldsheet for 2d Yang-Mills and symmetric product orbifold”

abstract (click to view)
Two-dimensional Yang-Mills theory is arguably the simplest confining gauge theory and its large N expansion can be interpreted as a genus expansion of string theory. Nevertheless its worldsheet description has not been fully understood. I will propose a bosonic string dual to a chiral sector of two-dimensional Yang-Mills. The worldsheet theory consists of $\beta \gamma$-system deformed by a chiral analog of the Polchinski-Strominger term, and can be viewed as a noncritical version of nonrelativistic string theory introduced by Gomis and Ooguri. I will then comment on extensions of our worldsheet theory which are candidate duals of symmetric product orbifolds of arbitrary seed CFTs and their $T\bar{T}$ deformation. (click to hide)

Wed

18.10.

14:00

HIT E41.1

Frank Coronado (ETH)

“Correlator/amplitude duality and ten-dimensional symmetry.”

abstract (click to view)
In planar N=4 SYM, correlation functions of the stress tensor are dual to massless scattering amplitudes when the operators form a light-like polygon. I will present a (conjectured) generalization of this duality which equates correlators of determinant operators, in a special ten-dimensional null limit, with massive scattering amplitudes in the Coulomb branch of N=4. By using a novel twistor space representation for the determinant operators and matrix duality we compute the planar loop-integrands of these correlators. We show these planar integrands contain only ten-dimensional distances which combine the spacetime and R-charge space distances. This is amenable to the null limit that corresponds to the amplitude. (click to hide)

Wed

25.10.

14:00

HIT E41.1

Martijn Hidding (ETH)

“Special Functions in String Amplitudes and Feynman Integrals”

abstract (click to view)
In this talk, I'll cover various aspects of my research exploring the role of special functions in string amplitudes and quantum field theory. First, I'll review the appearance of multiple polylogarithms and elliptic multiple polylogarithms in (open-)string amplitudes of genus 0 and 1 and Feynman integrals relevant to massive QCD. The next part of the talk will focus on elliptic modular graph forms, based on the treatment in arXiv:2208.11116, which show up in closed-string amplitudes at genus-one. I'll explain how these functions can be expressed in terms of iterated integrals, and discuss their modular covariance properties. Next, I will introduce the function space of equivariant iterated Eisenstein integrals, which we studied in arXiv:2209.06772, and will examine what combinations of iterated integrals are modular covariant. I'll conclude by briefly mentioning work on new polylogarithmic functions on Riemann surfaces of arbitrary genus (arXiv:2306.08644) and its role in amplitude computations. (click to hide)

Wed

01.11.

14:00

HIT E41.1

Priyadarshi Paul (ICTS)

abstract (click to view)
We obtain solutions of the Wheeler-DeWitt equation with positive cosmological constant for a closed universe in the large-volume limit. We argue that this space of solutions provides a complete basis for the Hilbert space of quantum gravity in an asymptotically de Sitter spacetime. Our solutions take the form of a universal phase factor multiplied by distinct diffeomorphism invariant functionals, with simple Weyl transformation properties, that obey the same Ward identities as a CFT partition function. Each functional can be thought of as specifying a “theory” and, in this sense, the space of solutions is like “theory space”.

We propose that the norm on this space of solutions is obtained by integrating the squared wavefunctional over field configurations and dividing by the volume of the diff-and-Weyl group. We apply our formalism to cosmological correlators and propose that they should be understood as gauge-fixed observables. In a theory of quantum gravity, we demonstrate a version of the principle of holography of information: cosmological correlators in an arbitrarily small region suffice to completely specify the state. (click to hide)

We propose that the norm on this space of solutions is obtained by integrating the squared wavefunctional over field configurations and dividing by the volume of the diff-and-Weyl group. We apply our formalism to cosmological correlators and propose that they should be understood as gauge-fixed observables. In a theory of quantum gravity, we demonstrate a version of the principle of holography of information: cosmological correlators in an arbitrarily small region suffice to completely specify the state. (click to hide)

Wed

08.11.

14:00

HIT E41.1

Anthony Ashmore (Sorbonne)

“Machine learning for geometry and string compactifications”

abstract (click to view)
Understanding Calabi-Yau metrics and hermitian Yang-Mills connections has long been a challenge in mathematics and theoretical physics. These geometric objects play a crucial role in constructing realistic models of particle physics in string theory. However, with no closed-form expressions for them, we are unable to compute basic quantities in top-down string models, such as particle masses and couplings.

Breakthroughs in machine learning have opened a new path to tackle this problem. After recalling the relationship between these geometric ingredients and 4d effective field theory, I will review recent progress in using machine learning to calculate these metrics and connections numerically. Finally, I will highlight how this newly available geometric data can be used, including studying the spectrum of Laplace-type operators on a Calabi-Yau in the presence of a background gauge field. (click to hide)

Breakthroughs in machine learning have opened a new path to tackle this problem. After recalling the relationship between these geometric ingredients and 4d effective field theory, I will review recent progress in using machine learning to calculate these metrics and connections numerically. Finally, I will highlight how this newly available geometric data can be used, including studying the spectrum of Laplace-type operators on a Calabi-Yau in the presence of a background gauge field. (click to hide)

Wed

15.11.

14:00

HIT E41.1

Tim Meier (Humboldt U. Berlin)

abstract (click to view)
Applying the Yang-Baxter (YB) deformations to the famously integrable AdS5 x S5 string give rise to a variety of new integrable models. In the context of the AdS/CFT correspondence, these models are conjectured to be dual to gauge theories on various noncommutative spacetimes obtained via Drinfel’d twists. To date, however, it was unclear how to formulate such noncommutative gauge theories precisely beyond the simplest case of constant noncommutativity. In my talk, I will show how to construct gauge invariant noncommutative Yang-Mills actions for a broad class of noncommutative structures, relying on a deformed version of the Hodge star operation. I will also show how to include matter fields and hence how to construct noncommutative versions of N=4 SYM which give promising candidates for the dual theory to YB deformations of the AdS5 x S5 string. I will construct gauge invariant operators for the deformed models. Finally, I will comment on the connection to a possible spin chain picture for the one loop anomalous dimension of such operators. (click to hide)

Wed

22.11.

11:30

HIT E41.1

Erez Urbach (Weizmann Institute)

abstract (click to view)
String stars, or Horowitz-Polchinski solutions, are string theory saddles with normalizable condensates of thermal-winding strings. In the past, string stars were offered as a possible description of stringy (Euclidean) black holes in asymptotically flat spacetime, close to the Hagedorn temperature. I will discuss the thermodynamic properties of string stars in asymptotically (thermal) anti-de Sitter background (including AdS3 with NS-NS flux), their possible connection to small black holes in AdS, and their implications for holography. I will also present new ``winding-string gas'' saddles for confining holographic backgrounds such as the Witten model, and their relation to the deconfined phase of 3+1 pure Yang-Mills. (click to hide)

Wed

22.11.

14:00

HIT E41.1

Fiona Seibold (Imperial College London)

abstract (click to view)
The low-energy dynamics of confining strings is captured by the Nambu-Goto area action, which in critical dimension is related to an integrable TTbar deformation of a free theory. In this talk I will consider the setup of a membrane compactified on a circle, leading to a tower of massive modes. I will present the perturbative worldsheet S-matrix, compare with the Nambu-Goto case, and discuss the integrability of the model. (click to hide)

Wed

06.12.

11:30

HIT E41.1

Anthony Houppe (ETH)

“Microstate geometries without supersymmetry”

abstract (click to view)
Using supersymmetry, large families of microstate geometries have been constructed in AdS3. The holographic dictionary of these solutions is now well established, and has been extensively used to compute vevs and correlators of the states of the D1-D5 CFT directly from the bulk, with a remarkable matching with the CFT values. The situation is however very different away from the BPS regime: few bulk geometries are known, and the matching is complicated by the absence of protected operators. In this talk, I will present recent advances on the construction of non-supersymmetric microstate geometries in AdS3, and on the identification of their CFT dual. (click to hide)

Wed

06.12.

14:00

HIT E41.1

Paul Balduf (University of Waterloo)

abstract (click to view)
The Feynman period is the contribution of a subdivergence-free log-divergent graph to the beta function, or equivalently the simple pole of its amplitude in dimensional regularization. We have used an algorithm recently developed by Borinsky to compute numerical approximations to the periods of 1.3 million distinct graphs in phi^4 theory, including all subdivergence-free graphs up to and including 13 loops. On the one hand, we obtain an accurate numerical value for the primitive beta function of phi^4 theory at 13 loops. On the other hand, and more importantly, this large data set allows us to accurately determine distribution parameters, growth rates, and correlations between the period and numerous graph-theoretic properties.

In my talk, I will show some of these numerical and qualitative results. A special focus will be the distribution of periods, the presence of exotic "outlier" amplitudes, and the implications of these results for non-complete samples of Feynman graphs at large loop order. (click to hide)

In my talk, I will show some of these numerical and qualitative results. A special focus will be the distribution of periods, the presence of exotic "outlier" amplitudes, and the implications of these results for non-complete samples of Feynman graphs at large loop order. (click to hide)

last modified: Tue, 28 Nov 2023, 15:17 CET