“QFT, Strings and Beyond”

Spring Semester 2021

Tuesday, 16:00, zoom 940 4378 1498

Organised by: Niklas Beisert, Johannes Broedel, Matthias Gaberdiel, Marc-Antoine Fiset, Pietro Longhi

day
date
time
venue
speaker
title
Tue
23.02.
16:00
zoom 940 4378 1498
Ashoke Sen (Harish-Chandra Research Institute)
“D-instanton amplitudes in string theory”
abstract (click to view) D-instantons give non-perturbative contribution to string theory
amplitudes which can be computed using world-sheet techniques. However the
integrals that appear in this computation often have divergences from
corners of the moduli spaces which cannot be tamed by the usual procedure
of analytic continuation. We show how using insights from string field
theory we can extract finite unambiguous results from these apparently
divergent integrals. (click to hide)
Tue
02.03.
16:00
zoom 940 4378 1498
Edoardo Lauria (École Polytechnique, CPHT)
“Bootstrappping defect-localized interactions”
abstract (click to view) Is there any room for non-trivial unitary and conformal boundary conditions in the theory of a single free massless scalar field? And what about defects? In this talk I will discuss how the free scalar equations of motion and the structure of the bulk-to-boundary operator expansion lead us to a non-trivial system of crossing equations that we analyze numerically for boundaries in four bulk dimensions. We show that large regions of parameter spaces are excluded, but a ‘kink’ in the numerical bounds obeys all our consistency checks and might be an indication of a new conformal boundary condition. For higher co-dimensions, for example a line defect in three bulk dimensions, these constraints are enough to prove that the only unitary conformal defects are essentially trivial. Based on 2005.02413, 2009.03336 and 2012.07733. (click to hide)
Tue
09.03.
15:45
zoom 940 4378 1498
Christoph Nega (Bonn)
“Analytic Structure of Banana Feynman Integrals”
abstract (click to view) In this talk I will start with a short motivation why Calabi-Yau varieties and therefrom derived geometric tools are useful in Feynman graph computations. These tools will be exemplified on the so called banana Feynman integrals culminating in understanding their analytic properties, first in two dimensions and then in the dimensional regularization prescription. These Feynman integrals in two dimensions have a natural interpretation as relative periods of a complete intersection Calabi-Yau manifold, whose dimension is the loop order minus one of the banana integral. In particular, we find that the leading logarithmic structure in the high energy regime, which corresponds to the point of maximal unipotent monodromy, is determined by a novel Gamma-class evaluation in the ambient space of the mirror CY and the mirror CY itself. In dimensional regularization we use a Mellin-Barnes and a Bessel function representation of the banana Feynman integral to solve them. I will extend these ideas also shortly to the non-equal mass case which can be described similarly. (click to hide)
Tue
16.03.
16:00
zoom 940 4378 1498
Ömer Gürdoğan (Southampton)
“From integrability to the coaction properties of Feynman periods”
abstract (click to view) Results in Quantum Field Theories, such as anomalous dimensions or scattering amplitudes are known to exhibit rich properities under the motivic coaction that acts on period integrals. I will show how these phenomena arise directly in an integrability setup describing the anomalous dimensions of a four-dimensional scalar model. (click to hide)
Tue
23.03.
16:00
zoom 940 4378 1498
Till Bargheer (DESY and Hannover)
“Deconstructing AdS/CFT with integrability”
abstract (click to view) I will describe the integrability-based "hexagonalization" approach to
correlation functions in AdS/CFT, with an emphasis on the connection
between the gauge-theory perturbative expansion and the string-theory
moduli space integral, which is nicely captured by the construction. (click to hide)
Tue
13.04.
14:30
zoom 940 4378 1498
David Skinner (Cambridge)
“Ambitwistor Strings on AdS3 x S3”
abstract (click to view) Ambitwistor strings are purely chiral worldsheet theories that, in the simplest case, provide a worldsheet description of supergravity. Around flat space, they underpin the CHY description of perturbative scattering amplitudes in terms of scattering equations. In this talk, I’ll construct an anomaly-free ambitwistor string describing supergravity on AdS3 x S3 at the NS point. The problem of computing n-point boundary correlation functions in this theory is transformed into a certain Gaudin integrable system. The scattering equations on AdS3 amount to the statement that all eigenvalues of the SL(2) Gaudin Hamiltonians should simultaneously vanish. (click to hide)
Tue
20.04.
16:00
zoom 940 4378 1498
Mathew Bullimore (Durham)
“Towards a Mathematical Definition of the 3d Superconformal Index”
abstract (click to view) The aim of this talk is to give a mathematical definition of the superconformal index counting local operators in gauge theories with 3d N = 2 supersymmetry. This can be computed exactly using supersymmetric localisation, which leads to an explicit contour integral formula involving infinite q-Pochhammer symbols. I will explain how this result can be interpreted as the Witten index of a supersymmetric quantum mechanics, or index of a twisted Dirac operator on a certain infinite-dimensional space. To illustrate the essential points, I will focus on a concrete example of supersymmetric Chern-Simons theory. (click to hide)
Tue
27.04.
16:00
zoom 940 4378 1498
Ana Retore (Trinity College Dublin)
“Constructing new solutions of Yang-Baxter equation”
abstract (click to view) There are several well known integrable models, like Hubbard model, Heisenberg spin chain and various versions of the AdS/CFT correspondence. The main ingredient of an integrable theory is an object called R-matrix which satisfies the so-called Yang-Baxter equation. However, constructing new integrable models can be very complicated. In this talk, I will introduce a new method to find solutions of the Yang-Baxter equation, using the so-called boost operator, and use it to classify R-matrices in various set-ups. Using this method we were able to find several new interesting integrable models, including three deformations of lower dimensional
AdS/CFT S-matrices. (click to hide)
Tue
04.05.
16:00
zoom 940 4378 1498
Miroslav Rapčák (Berkeley)
“M5-M2 Systems and the Affine Yangian”
abstract (click to view) Representation theory of affine Yangians is known to be closely related to the geometry of branes inside toric Calabi-Yau three-folds. M5-branes wrapping four-cycles give rise to W-algebra-like representations. M2-branes wrapping two-cycles lead to Coulomb-branch-like representations. In my talk, we are going to investigate the system of M2-branes ending on M5-branes. We are going to see a novel interpretation of the free-boson vertex operator, a generalization of the Gaberdiel-Li-Peng "high-wall" modules of the affine Yangian and an algebraic refinement of the relation between Donaldson-Thomas and Pandharipande-Thomas topological vertices. (click to hide)
Tue
11.05.
16:00
zoom 940 4378 1498
Ida Zadeh (ICTP, Trieste)
“Narain to Narnia”
abstract (click to view) Recently, a new holographic correspondence was discovered between an ensemble average of toroidal conformal field theories in two dimensions and an abelian Chern-Simons theory in three dimensions coupled to topological gravity. I will discuss a generalisation of this duality for three families of conformal field theories and show that the correspondence works for toroidal orbifolds but not for K3/Calabi-Yau sigma-models and not always for the minimal models. For toroidal orbifolds, the holographic correspondence is extended to correlation functions of twist operators by using topological properties of rational tangles in the three-dimensional ball. (click to hide)
Tue
18.05.
16:00
zoom 940 4378 1498
Brice Bastian (Utrecht)
“Modelling General asymptotic Calabi-Yau Periods”
abstract (click to view) I will discuss an approach to the general study of period vectors of Calabi-Yau threefolds in asymptotic regions of complex structure moduli space. These period vectors play an important role in string compacitifcations, making a better understanding of their general structure desirable. The strategy is to exploit constraints imposed by completeness, symmetry, and positivity, which are formalised in asymptotic Hodge theory. Together with the classification of all possible boundaries in moduli space, these principles allow for a systematic construction of models for the asymptotic period vectors. One also gains insight about the necessity of exponentially suppressed corrections to the polynomial part of the period vector when away from the well-studied large complex structure point. I will also present results for the cases of one- and two-moduli boundaries where this method has been carried out explicitly. (click to hide)
Tue
25.05.
16:00
zoom 940 4378 1498
André Kaderli (Humboldt and Max Planck Institute)
“Elliptic KZB associator and open-string corrections at genus one”
abstract (click to view) A method to recursively calculate the genus-one, open-string corrections to field-theory amplitudes is presented. For this purpose vectors of iterated integrals satisfying an elliptic Knizhnik-Zamolodchikov-Bernard (KZB) system on the punctured torus are constructed. These vectors interpolate between the genus-zero and genus-one, open-string corrections. The two boundary values, containing the open-string corrections, are related by a representation of the elliptic KZB associator, the generating series of elliptic multiple zeta values. This yields a relation which can be used to calculate genus-one, open-string corrections from the well-known genus-zero, open-string corrections solely using matrix operations of explicitly known matrices. Geometrically, two external states of the (n+2)-point, genus-zero worldsheet are glued together to form an n-point, genus-one worldsheet. (click to hide)

last modified: Mon, 17 May 2021, 08:31 CEST