- Friday 29 October 2021:
**Barton Zwiebach**(CTP, MIT)

Title: Nonlocal field theory, rolling tachyons, and initial value problem (recording, slides)

Abstract: We consider a nonlocal scalar field theory inspired by the tachyon action in open string field theory and p-adic string models. We study in detail field redefinitions that re-cast the theory in a form with a clear initial value problem. We discuss rolling tachyons and issues of causality.

- Friday 12 November 2021 :
**Carlo Maccaferri**(Università di Torino)

Title: D-branes potentials and closed string deformations in open string field theory (recording, slides)

Abstract: I discuss recent results on the effective potential of various D-branes systems in both bosonic string theory and Type II superstring. I focus on open string field theories based on Witten’s star product for which we know how to construct exact classical solutions and where the coupling with deforming closed strings can be obtained via a simple open/closed gauge invariant vertex. The obtained effective algebraic couplings can be understood as zero-momentum amplitudes where all possible infrared divergences arising from open string degeneration are systematically tamed by the open string propagator. I also discuss how one can possibly handle similar divergences from closed string degeneration, when a closed string deformation is added to the pure open string theory.

- Friday 3 December 2021 :
**Nathan Berkovits**(ICTP-SAIFR, IFT-UNESP)

Title: Applications of the $N=1, d=4$ spacetime-supersymmetric version of open superstring field theory (recording)

Abstract: I will first review the version of open superstring field theory based on the hybrid formalism which has manifest $N=1, d=4$ spacetime supersymmetry. I will then discuss two recent applications including the equations of motion for higher-spin massive fields in an electromagnetic background and the computation of stringy corrections to the self-dual $d=4$ instanton solution.

- Thursday 27 January 2022 :
**Ashoke Sen**(ICTS)

Title: D-instanton amplitudes (recording)

Abstract: I’ll describe the general procedure for computing D-instanton amplitudes in string theory, with special focus on type II string theories compactified on Calabi-Yau manifolds.

- Thursday 10 February 2022 :
**Yuji Okawa**(Tokyo University)

Title: Renormalization of one-loop scattering amplitudes in scarlar field theories via homotopy algebras

Abstract: 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. In this talk we describe renormalization of one-loop scattering amplitudes in $\phi^3$ theory using an $A_\infty$ algebra.

- Thursday 24 February 2022 :
**Ivo Sachs**(LMU)

Title: The BV-theory of the Point Particle (recording, slides)

Abstract: I will review recent and ongoing work on the BV-field theory of the relativistic spinning particle as a tool to explore aspects of string field theory, including: background independence, non-linear field equations, massive states and “closed string” vacuum solutions. I will also propose a notion of spin fields for the point particle and their application to Ramond-Ramond backgrounds.

- Thursday 10 March 2022 :
**Edward Witten**(IAS Princeton)

Title: No Ensemble Averaging Below the Black Hole Threshold (recording)

Abstract: In the AdS/CFT correspondence, amplitudes associated to connected bulk manifolds with disconnected boundaries have presented a longstanding mystery. A possible interpretation is that they reflect the effects of averaging over an ensemble of boundary theories. But in examples in dimension D≥3, an appropriate ensemble of boundary theories does not exist. Here we sharpen the puzzle by identifying a class of “sub-threshold” observables that we claim do not show effects of ensemble averaging. These are amplitudes that do not involve black hole states. To support our claim, we explore the example of D=3, and show that connected solutions of Einstein’s equations with disconnected boundary never contribute to sub-threshold observables. To demonstrate this requires some novel results about the renormalized volume of a hyperbolic three-manifold, which we prove using modern methods in hyperbolic geometry. Why then do any observables show apparent ensemble averaging? We propose that this reflects the chaotic nature of black hole physics and the fact that the Hilbert space describing a black hole does not have a large N limit.

- Thursday 24 March 2022 :
**Spring break**

- Thursday 7 April 2022 :
**Raghu Mahajan**(Stanford University)

Title: Semiclassical evaluation of the vacuum sphere and the FZZT disk path integrals in noncritical string theory (recording, slides)

Abstract: We compute the vacuum sphere and the FZZT disk partition functions in noncritical string theory in the semiclassical limit. The worldsheet theory consists of the (2,p) minimal model, the Liouville field and the usual bc ghosts. The semiclassical limit is the limit of large p, in which the central charge of the Liouville sector goes to plus infinity, and the path integral can be done via the saddle point method. The main conceptual point is that the unfixed PSL(2,C) and PSL(2,R) gauge groups are spontaneously broken by the saddle point configuration of the Liouville field, which removes the usual obstacles in computing these path integrals in critical string theory. The quantity sphere/disk^2 is free of all normalization ambiguities and we find a precise numerical match between the string theory answer and the answer from the dual double-scaled one-matrix integral. (This talk is based on work with Douglas Stanford and Cynthia Yan.)

- Thursday 21 April 2022 :
**Nima Arkani-Hamed**(IAS)

Title: On Unitarity of Tree-Level String Amplitudes (recording)

Abstract: Four-particle tree-level scattering amplitudes in string theory are magically consistent with unitarity, reflected in the non-trivial fact that beneath the critical dimension, the residues of the amplitudes on massive poles can be expanded in partial waves with all positive coefficients. While this follows (rather indirectly) from the no-ghost theorem, the simplicity of the statement and its fundamental importance for the physical consistency of string theory begs for a more direct and elementary understanding. In this note we take a step in this direction by presenting a new expression for the partial wave coefficients of string amplitudes, given by surprisingly simple double/triple contour integrals for open/closed strings. This representation allows us to directly prove unitarity of all superstring theories in $D \leq 6$ spacetime dimensions, and can also be used to determine various asymptotics of the partial waves at large mass levels.

- Thursday 5 May 2022 :
**Atakan Hilmi Fırat**(MIT)

Title: D-instanton Induced Superpotential

Abstract: In this talk, I will explain how one can fix the normalization of the D-instanton contribution to the superpotential in type II string theory on Calabi-Yau orientifolds using string field theory and show such normalization is holomorphic function of the moduli. Along the way I will sketch the proof for a folk theorem relating D-instantons to space-filling D-branes on Calabi-Yau’s. Lastly, I will argue for the absence of multi-instanton contribution to the superpotential in these scenarios. Based on 2204.02981.

- Thursday 19 May 2022 :
**Oliver Schlotterer**(Uppsala University)

Title: Recent developments in superstring loop amplitudes

Abstract: In this talk, I will review motivations, results and strategies for loop amplitudes in superstring theories. Based on chiral splitting of correlation functions of closed-string vertex operators, a simplified result will be presented for two-loop five-point amplitudes involving massless type-II states.

- Thursday 2 June 2022 :
**Hiroshi Kunitomo**(Kyoto University)**Special time**: 8:00 (EST), 14:00 (CEST), 17:30 (IST), 21:00 (JST)

Title: Open-closed homotopy algebra in superstring field theory

Abstract: We construct open-closed superstring interactions based on the open-closed homotopy algebra structure. It provides a classical open superstring field theory on general closed-superstring field backgrounds described by classical solutions of the nonlinear equations of motion of the closed superstring field theory.