Playlist.
| Sergei Alexandrov | D-instantons in Calabi-Yau compactifications |
Euclidean D-branes wrapped on non-trivial cycles of a Calabi-Yau threefold are known to affect the metric on the hypermultiplet moduli space determining the effective action of type II strings on such manifolds. After reviewing the existing results on the instanton corrected metric following from a combination of constraints imposed by supersymmetry and dualities, I'll show how the D-instanton effects can be computed by a direct worldsheet approach. This required solving several conceptual and technical problems, which also paved the way for extension of this approach to compactifications with lower supersymmetry. The talk is based on joint work with A. Sen and B. Stefanski. Slides. Recording.
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| Teresa Bautista | Timelike Liouville gravity and its sphere partition function |
Liouville conformal field theory models two-dimensional gravity with a cosmological constant and coupled to conformal matter. In its timelike regime, it reproduces the characteristic negative kinetic term of the conformal factor of the metric in the Einstein-Hilbert action, the sign which infamously makes the gravity path integral ill-defined. In this talk, I will first review a proposal for the spectrum of physical energies of timelike Liouville gravity, and argue that the resulting 4-point function is finite and crossing symmetric. Then, I will discuss the perturbative computation of the timelike Liouville partition function around the sphere saddle and propose an all-orders result. Time permitting, I will also discuss some work in progress on timelike Liouville with a conformal boundary. Slides. Recording.
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| Ted Erler | Lightcone gauge in covariant SFT |
We discuss the nature of string interactions when covariant SFT is fixed to lightcone gauge. We find that the solution of the geometrical BV equation of the original covariant SFT is projected by the lightcone gauge condition to a solution characterized by Mandelstam diagrams. Slides. Recording.
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| Atakan Hilmi Firat | Characterizing 4-string contact interaction using machine learning |
In this talk I will show how the geometry of 4-string contact interaction of closed SFT can be characterized using machine learning. First, I will describe how one can obtain Strebel differential on a 4-punctured sphere as a neural network. This allows one to solve for local coordinates and compute their associated mapping radii numerically. Then I will present a neural network distinguishing vertex from Feynman regions which simplifies the integration over the moduli. As a check, the computation for 4-tachyon contact term will be presented. Lastly I will emphasize how the techniques in this study can be uplifted to characterizing n-string contact interactions. Slides. Recording.
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| Manki Kim | Important problems in string phenomenology, and how string field theory may help us (cancelled) |
In this talk, I will list and explain a few very important problems in string phenomenology that are difficult to solve in traditional methods in supergravity approximation and worldsheet description of string theory. For some of the problems, I will explain why I think string field theory can be very useful to understand these problems. In the last part of the talk, I will provide an example of such, where string field theory helps us tame the IR-divergence associated to the D-instanton contribution to the superpotential
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| Matej Kudrna | Marginal deformations in OSFT |
I will discuss marginal deformations in OSFT in the level truncation approach. I will review marginal deformations in the free boson theory using three different approaches: the traditional marginal approach, the tachyon approach and the perturbative approach. Then I will present new results regarding marginal deformations of nontrivial D-brane solutions in the SU(2)_k WZW model. Slides. Recording.
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| Carlo Maccaferri | The classical cosmological constant of open-closed string field theory |
I consider deformations of D-brane systems induced by a change in the closed string background in the framework of bosonic open-closed string field theory. A closed string classical solution changes the closed string background and induces a tadpole for the open strings which shifts the open string vacuum, generating a cosmological constant. Background independence implies that this observable is universally proportional to the shift in the world-sheet disk partition function between the starting D-brane in the starting background and the final D-brane in the deformed background, which may also include a change of the string coupling constant. We test this conjecture by considering a perturbative closed string solution describing deformations of a Narain compactification and we reproduce the expected shift in the g-function of various D-branes living in the compactification. In doing this we identify a change in the string coupling constant at second order in the deformation. The talk is based on https://arxiv.org/abs/2208.00410. Slides. Recording.
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| Raghu Mahajan | Normalization of ZZ instanton amplitudes in minimal string theory |
We use insights from string field theory to analyze and cure the divergences in the cylinder diagram in minimal string theory with both boundaries lying on a ZZ brane. (Minimal string theory refers to the theory of worldsheet gravity coupled to a minimal model CFT that serves as the matter sector.) The string field theory procedure gives a finite normalization constant for non-perturbative effects in minimal string theory. We find precise agreement with the prediction from the dual double-scaled matrix integrals. Slides. Recording.
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| Pronobesh Maity | From Fields to Strings (online) |
I will describe a mechanism to reorganize the Feynman diagrams of a perturbative large N gauge theory into localized integrals over closed string moduli space using the Strebel parametrization of the latter. This will be explicitly illustrated for free symmetric product orbifold CFTs where holomorphic covering maps play a key role. After recasting these covering maps in terms of dual worldsheet twistor fields in $AdS_3$, it will be generalized for $AdS_5$. Imposing appropriate reality conditions, I will discuss a proposed stringy incidence relation. For correlators of free $\mathcal{N}=4$ SYM with special kinematics, I will then show an explicit construction of these twistor covering maps, where the regularized "Strebel area" of the worldsheet corresponding to these maps, reproduces the Feynman propagator. Slides. Recording.
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| Yuji Okawa | Correlation functions of scalar field theories from homotopy algebras |
When actions are written in terms of homotopy algebras such as $A_\infty$ algebras and $L_\infty$ algebras, expressions of on-shell scattering amplitudes in perturbation theory are universal for both string field theories and ordinary field theories. We thus expect that homotopy algebras can be useful in gaining insights into quantum aspects of string field theories from ordinary field theories. In addition to on-shell scattering amplitudes we find that correlation functions can also be described in terms of homotopy algebras, and in this talk we explain explicit expressions for correlation functions of scalar field theories using quantum $A_\infty$ algebras presented in arXiv:2203.05366. Then we further discuss the application to the renormalization group. Slides. Recording.
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| Ivo Sachs | RR-fields for the RNS World-line |
I will describe an attempt to construct the RNS-worldline BRST operator in a RR-background with the help of “spin fields”. The construction of the latter is complicated by the absence of bosonisation in 1 dimension forcing us to resort to a non-Lie BRST operator algebra. The relation to the Brink-Schwarz particle will also be discussed. Slides. Recording.
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| Evgeny Skvortsov | Higher Spin Gravity: news and (possible) relations to string theory |
I will review the general idea of constructing higher spin extensions of gravity and give a (short) list of available higher spin gravities, stressing the most important hurdles and obstructions on this way. Next, I will review several ideas on how to identify higher spin gravities inside string theory via tensionless limits and AdS/CFT. Lastly, I will give some details on A- and L-infinity algebras that are behind certain higher spin gravities and sketch the relation to extensions of Kontsevich formality.
Slides. Recording.
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| David Turton | String worldsheet models of black hole microstates |
I will describe recent studies of bound states of NS5 branes carrying momentum and/or fundamental string charge, in the decoupling limits leading to little string theory and to AdS3/CFT2 duality. This work involves a class of exactly solvable worldsheet models that describe families of BPS and non-BPS black hole microstates. These models have enabled studies of string and D-brane probes of these microstates, yielding insight into their stringy structure in the gravitational bulk description. Slides. Recording.
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| Jakub Vosmera | Observables of open-closed superstring field theory |
As a consequence of the background independence conjecture, any two consistent classical backgrounds for a critical open-closed superstring perturbation theory can be connected by a solution to the $L_\infty$ closed SFT equation of motion together with a solution to the weak $A_\infty$ equation of motion describing the open SFT vacuum shift. We will first recognize on-shell tree-level amplitudes around the shifted background as gauge-invariant observables which can help with characterizing these classical solutions. Focusing on zero- and one-point amplitudes on the disk, we will outline an explicit construction of the corresponding observables (generalized action / cosmological constant and generalized Ellwood invariant, respectively) in the NSNS sector of the RNS superstring. This procedure requires a definition of tree-level off-shell superstring disk vertices (with a correct distribution of PCOs) containing purely closed-string punctures, for which we give a Munich-like construction: we will observe that this is most naturally formulated starting with "wrong-parity" (degree-even) bosonic products which are defined in terms of pure-surface states in the closed-string small Hilbert space enlarged by the $\xi_0^-$ ghost. The corresponding gauge products (obtained by suitably distributing $\xi_0^+$ modes) will then be degree-odd and act on the full closed-string large Hilbert space. We illustrate our results by considering marginal deformations of a toroidal compactification with a wrapped Dp-brane, highlighting simplifications due to the presence of an extended superconformal symmetry on the worldsheet. Slides. Recording.
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| Charles Wang | On the Relationship Between Super-Riemann Surfaces and PCOs |
We establish the equivalence between two formulations of superstring perturbation theory, one based on integration over the supermoduli space of super Riemann surfaces (SRS), the other based on integration over the bosonic moduli space with insertions of picture changing operators (PCO) on the worldsheet and the vertical integration prescription, by showing how the latter arises from a specific construction of the supermoduli integration contour. If time permits, we will also discuss an explicit case of the construction in genus two. Slides. Recording.
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