“QFT, Strings and Beyond”
Spring Semester 2025
Organised by: Niklas Beisert, Johannes Broedel, Sibylle Driezen, Matthias Gaberdiel, Edward Mazenc
day
date
date
time
venue
venue
speaker
title
title
Tue
11.02.
16:00
HIT E41.1
Carlo Maccaferri (Turin U)
“On Open-Closed Duality in String Field Theory”
abstract (click to view)
I approach the problem of open-closed duality through the perspective of
string field theory (SFT) where I provide a description of the backreaction
of a large N stack of D-branes as a new closed string background, possibly
without D-branes anymore. To achieve this, I first of all give a new
convenient formulation of open-closed SFT based on a single open-closed
nilpotent structure which captures the consistency of the theory. Then I
perform the 't Hooft large N limit, obtaining at the leading order a
classical closed SFT coupled to a quantum (but planar) open SFT. I then
discuss the integration-out of the open string sector, ending up with a
purely classical closed SFT. The obtained closed string theory however has
tadpoles whose strength is controlled by the 't Hooft coupling. To get rid
of the tadpoles I perform a vacuum shift, which finally describes closed
strings in the back-reacted new background, without D-branes anymore. I
show that several obstructions (related to open and closed string
degeneration) can hinder this geometric transition and I finally discuss
this construction in an example where obstructions are absent, in the
context of minimal string theory. There, following well-known results by
Gaiotto and Rastelli, I show how we can move in the space of $(2,p)$ purely
closed string backgrounds by computing the backreaction of a large N number
of FZZT branes originally placed in the $(2,1)$ background.
This talk is based on 2305.02843, 2305.02844 and work in progress. (click to hide)
string field theory (SFT) where I provide a description of the backreaction
of a large N stack of D-branes as a new closed string background, possibly
without D-branes anymore. To achieve this, I first of all give a new
convenient formulation of open-closed SFT based on a single open-closed
nilpotent structure which captures the consistency of the theory. Then I
perform the 't Hooft large N limit, obtaining at the leading order a
classical closed SFT coupled to a quantum (but planar) open SFT. I then
discuss the integration-out of the open string sector, ending up with a
purely classical closed SFT. The obtained closed string theory however has
tadpoles whose strength is controlled by the 't Hooft coupling. To get rid
of the tadpoles I perform a vacuum shift, which finally describes closed
strings in the back-reacted new background, without D-branes anymore. I
show that several obstructions (related to open and closed string
degeneration) can hinder this geometric transition and I finally discuss
this construction in an example where obstructions are absent, in the
context of minimal string theory. There, following well-known results by
Gaiotto and Rastelli, I show how we can move in the space of $(2,p)$ purely
closed string backgrounds by computing the backreaction of a large N number
of FZZT branes originally placed in the $(2,1)$ background.
This talk is based on 2305.02843, 2305.02844 and work in progress. (click to hide)
Tue
18.02.
16:00
HIT E41.1
Lorenz Eberhardt (Amsterdam U)
“A microscopic description of dS3”
abstract (click to view)
I will explain a proposal that relates pure Einstein gravity with positive cosmological constant in 2+1d to a matrix model. The matrix model is capable of reproducing all the integrated cosmological correlators in the bulk as well as the exact de Sitter entropy. Based on 2501.01486 in collaboration with Scott Collier and Beatrix Mühlmann. (click to hide)
Tue
25.02.
16:00
HIT E41.1
Matthias Wilhelm (Odense U)
“Functions in Scattering Amplitudes: From QFT to Gravitational Waves”
abstract (click to view)
Multi-loop Feynman integrals for collider physics are known to contain intricate geometries and to evaluate to complicated transcendental numbers and functions. In this talk, I will investigate Feynman integrals contributing to the emission of gravitational waves. I will provide a full classification of the contributing Feynman diagrams up to fifth order in the post-Minkowskian expansion, identifying new geometries that lead to new transcendental functions. (click to hide)
Thu
27.02.
11:30
HIT E41.1
Sylvain Lacroix (LPTHE - Sorbonne)
“Integrable structure of the WZW model”
abstract (click to view)
In this talk, I will discuss some preliminary results on the quantum integrable structure of the Wess-Zumino-Witten model. Focusing on the group SU(2), I will give evidence for the existence of an infinite number of commuting higher-spin local charges built from the current algebra underlying this CFT. In the second half of the talk, I will discuss the diagonalisation of these commuting operators on the Hilbert space of the theory, formed by highest-weight representations of the current algebra. In particular, I will review a conjecture relating the spectrum of these operators to the properties of specific ODEs (within the so-called ODE/IM correspondence). This is based on work in progress with Adrien Molines. (click to hide)
Tue
04.03.
16:00
HIT E41.1
Jesse Van Muiden (Imperial College London)
“Localisation of the M2-brane”
abstract (click to view)
In this talk, we explore how M-theory observables in holographic backgrounds admit localisation, mirroring their gauge theory counterparts. Focusing on backgrounds of the form M_4 x S^7/Z_k, we demonstrate that the M-theory path integral can be systematically expressed as an infinite sum over saddle points, with each saddle localised according to target-space supersymmetry. We identify the subleading saddles with M2-instantons that we explicitly quantise, and provide compelling evidence that this quantisation is one-loop exact. Our results hold universally for any M_4 that has asymptotic AdS boundary conditions, and as such offer new insights into the non-perturbative structure of various ABJM partition functions—including the topologically twisted index, the superconformal index, and the squashed sphere partition function. (click to hide)
Tue
11.03.
16:00
HIT E41.1
Alexandre Belin (Milan Bicocca U)
“Symmetric Orbfiolds, thermal universality and stringy von Neumann algebras”
abstract (click to view)
Symmetric orbifolds are 2d CFTs with universal properties in the large N limit, mimicking many desired properties of holographic CFTs. In particular, the thermal partition function is universal and agrees with that of 3D gravity. I will present some new results for the universality of symmetric orbiflds: the thermal correlation functions at large N agree with those on the BTZ background. Along the way, I will also discuss other results for correlation functions in heavy states which, even though they never dominate the canonical ensemble, still display universality. I will then interpret these results in terms of von Neumann algebras, in a context where the bulk dual is known to be a tensionless string in AdS_3. (click to hide)
Tue
18.03.
16:00
HIT E41.1
Davide Lai (DESY)
“Clustering and the Five-Point Function”
abstract (click to view)
The hexagonalisation technique arose in the context of N=4 SYM theory to address the tessellation of the effective world-sheet describing three-point functions of single trace operators. Despite its apparent technical difficulty, exploiting the knowledge of all its ingredients at finite 't Hooft coupling one can perform some non-trivial limits, like the strong-coupling one. In this case, a phenomenon called "clustering" allows the complete resummation of finite-size effects leading to the semi-classical string answer. I will introduce the general background and then discuss the generalization to some particular class of five-point functions, where the clustering phenomenon becomes more involved, involving cross-ratios and some analytic continuation of the ingredients to unusual kinematic regimes. In the end, I will speculate about some tentative directions that one could take. (click to hide)
Tue
25.03.
16:00
HIT E41.1
Victor Mishnyakov (Nordita)
“How integrability informs perturbation theory: conformal Yangian and GKZ systems”
abstract (click to view)
We remind that Yangian symmetry property of fishnet Feynman integrals can be extended to the so-called Loom graphs. Further, we describe the general structure of the corresponding differential operators. In particular we show, that they are almost (and in some cases exactly) equivalent to the so-called Gelfand-Kapranov-Zelevisnky systems, which have already appeared in the context of Feynman integrals, but also in the study of mirror symmetry in string theory. Moreover, it appears that the equations themselves impose certain consistency conditions, which constrain the types of graph topologies that could support Yangian symmetry. (click to hide)
Tue
08.04.
16:00
HIT E41.1
Xinan Zhou (KITS - UCAS)
“Giant Graviton Correlators as Defect Systems”
abstract (click to view)
In this talk, I will discuss correlation functions in 4d N = 4 SYM involving two maximal giant gravitons and two light 1/2-BPS operators. I will argue that it is most natural to view them as two-point correlators in the presence of a zero dimensional defect. Using this picture together with analytic bootstrap techniques, I will show how all infinitely many such correlators can be fully fixed just from symmetries and consistency conditions. Moreover, I will point out a hidden higher dimensional symmetry which repackages these correlators into a simple generating function. I will also present evidence that the same symmetry holds at weak coupling for loop correction integrands. (click to hide)
Mon
14.04.
Zoltan Bajnok (Wigner RCP)
Zurich Theoretical Physics Colloquium: “Resurgence in the Disk Capacitor, the Lieb-Liniger, and O(3) Sigma Models”
abstract (click to view)
The calculation of the capacity of the Maxwell-Kirchhoff disk capacitor is a historically significant and notoriously challenging problem that has captivated physicists for over a century. The renowned Love equation, which expresses the solution in terms of a linear integral equation, remarkably coincides with the Lieb equation describing the ground-state energy of the Lieb-Liniger model. The same equation also governs the free energy of the O(3) non-linear sigma model when one of its conserved charges is coupled to an external field.
Although this integral equation lacks an exact analytical solution, the Wiener-Hopf technique allows for the computation of the leading terms in the perturbative expansion, which determine the well-known mass-gap relation in the sigma model. However, extending beyond these leading terms is technically demanding. This challenge can be addressed using Volin’s ingenious method of matching asymptotics.
The resulting perturbative series is not convergent but only asymptotic, necessitating Borel resummation. The ambiguities that arise in this process signal the presence of non-perturbative corrections, requiring a double-series expansion that progresses in both perturbative and non-perturbative parameters. We fully determine this so-called trans-series, incorporating all non-perturbative corrections, and unveil its intrinsic resurgence structure.
----
Apéro after the Colloquium. (click to hide)
Although this integral equation lacks an exact analytical solution, the Wiener-Hopf technique allows for the computation of the leading terms in the perturbative expansion, which determine the well-known mass-gap relation in the sigma model. However, extending beyond these leading terms is technically demanding. This challenge can be addressed using Volin’s ingenious method of matching asymptotics.
The resulting perturbative series is not convergent but only asymptotic, necessitating Borel resummation. The ambiguities that arise in this process signal the presence of non-perturbative corrections, requiring a double-series expansion that progresses in both perturbative and non-perturbative parameters. We fully determine this so-called trans-series, incorporating all non-perturbative corrections, and unveil its intrinsic resurgence structure.
----
Apéro after the Colloquium. (click to hide)
Tue
15.04.
16:00
HIT E41.1
Zoltan Bajnok (Wigner RCP)
“Solving four-dimensional superconformal Yang-Mills theories with Tracy-Widom distribution”
abstract (click to view)
In my talk I will review our recent developments in calculating the strong coupling expansion of specific observables in N=2 and N=4 superconformal Yang-Mills theories. These observables can be written as determinants of semi-infinite matrices, the generalizations of the famous Tracy-Widom distribution. By exploiting this relationship we developed an efficient method to compute their strong coupling expansion, which is an asymptotic series. This necessitates the involvement of non-perturbative corrections, which we also determine together with their intriguing resurgence relations. (click to hide)
Tue
29.04.
16:00
HIT E41.1
Ludo Fraser-Taliente (U. of Oxford)
“F-extremization determines the melonic large-N CFTs”
abstract (click to view)
We show that the conformal data of a range of large-N conformal field theories, the melonic CFTs, are specified by constrained extremization of the universal part of the sphere free energy $F=-\log Z_{S^d}$, called $\tilde{F}$. This family includes the generalized SYK models, the vector models (O(N), Gross-Neveu, etc.), and the tensor field theories. The known F and a-maximization procedures in 3d/4d SCFTs are therefore extended to these non-supersymmetric CFTs in continuous dimension. We establish our result using the two-particle irreducible (2PI) effective action, and, equivalently, by Feynman diagram resummation. $\tilde{F}$ interpolates in continuous dimension between the known C-functions, so we interpret this result as an extremization of the number of IR degrees of freedom, in the spirit of the generalized c,F,a-theorems. The outcome is a complete classification of the melonic CFTs: they are the conformal mean field theories which extremize the universal part of the sphere free energy, subject to an IR marginality condition on the interaction Lagrangian. (click to hide)
Tue
06.05.
16:00
HIT E41.1
Shai Chester (Imperial College London)
t.b.a.
Tue
13.05.
16:00
HIT E41.1
Pietro Benetti Genolini (Geneva U)
t.b.a.
Tue
20.05.
16:00
HIT E41.1
Rob Klabbers (HU Berlin)
t.b.a.
last modified: Mon, 28 Apr 2025, 15:29 CEST