- Thursday 26 January 2023 :
**Aron Wall**(Cambridge U.)

Title: The Off-shell String Effective Action

Abstract: Taking string theory off shell requires breaking Weyl invariance on the worldsheet, i.e. the worldsheet theory is now a QFT instead of a CFT. I will explain Tseytlin’s approach to taking the worldsheet theory off-shell in a consistent manner, with a particular emphasis on the subtleties involved in calculating the sphere amplitude. This approach allows for the derivation of a classical string action which gives rise to the correct equations of motion and S-matrix, to all orders in perturbation theory. I will also attempt to explain how the resulting off-shell effective tree amplitudes are related to string field theory. Based on arXiv:2211.08607

- Thursday 09 February 2023 :
**Amr Ahmadain**(Cambridge U.)

Title: Off-shell Strings and Black Hole Entropy

Abstract: In this talk, after reviewing Tseytlin’s prescriptions for constructing classical (sphere) off-shell effective actions, I’ll explain the underlying conceptual structure of the Susskind and Uglum black hole entropy argument. There I will show explicitly how the classical (tree-level) effective action and entropy $S = A/4G_N$ may be calculated from the sphere diagrams. We also discuss the behaviour of the Susskind and Uglum entropy under RG flow. Although the conical manifold smooths out under RG flow, moving towards an on-shell configuration, the entropy doesn’t change.

I will also compare these off-shell results with the much more popular orbifold method for calculating entropy from the on-shell $C/Z_N$ background. By considering processes involving twisted string states (basically, the orbifold conserves twist and hence does not allow twisted strings to pinch off at the tip, while the off-shell NLSM of a slightly smoothed-out cone obviously does allow this process), I will conclude that the orbifold backgrounds are fundamentally different from conical backgrounds at the same value of inverse temperature $\beta$—unless one allows tachyons to condense on the orbifold.

- Thursday 23 February 2023 :
**Hong Liu**(MIT)

Title: Subalgebra-subregion duality: emergence of space and time in holography

Abstract: In holographic duality, a higher dimensional quantum gravity system emerges from a lower dimensional conformal field theory (CFT) with a large number of degrees of freedom. We propose a formulation of duality for a general causally complete bulk spacetime region, called subalgebra-subregion duality, which provides a framework to describe how geometric notions in the gravity system, such as spacetime subregions, different notions of times, and causal structure, emerge from the dual CFT. Subalgebra-subregion duality generalizes and brings new insights into subregion-subregion duality (or equivalently entanglement wedge reconstruction). It provides a mathematically precise definition of subregion-subregion duality and gives an independent definition of entanglement wedges without using entropy. Geometric properties of entanglement wedges, including those that play a crucial role in interpreting the bulk as a quantum error correcting code, can be understood from the duality as the geometrization of the additivity anomaly of certain algebras.

Using general boundary subalgebras rather than those associated with geometric subregions makes it possible to find duals for general bulk spacetime regions, including those not touching the boundary. Applying subalgebra-subregion duality to a boundary state describing a single-sided black hole also provides a precise way to define mirror operators.

- Thursday 09 March 2023 :
**Nobuyuki Ishibashi**(Tsukuba U.)**Unusual time**: 7:00 (EST) – 13:00 (CET) – 17:30 (IST) – 21:00 (JST)

Title: The Fokker-Planck formalism for closed bosonic strings

Abstract: Every Riemann surface with genus $g$ and $n$ punctures admits a hyperbolic metric, if $2g-2+n>0$. Such a surface can be decomposed into pairs of pants whose boundaries are geodesics. We construct a string field theory for closed bosonic strings based on this pants decomposition. In order to do so, we derive a recursion relation satisfied by the off-shell amplitudes, using the Mirzakhani’s scheme for computing integrals over the moduli space of bordered Riemann surfaces. The recursion relation can be turned into a string field theory via the Fokker-Planck formalism. The Fokker-Planck Hamiltonian consists of kinetic terms and three string vertices. Unfortunately, the worldsheet BRST symmetry is not manifest in the theory thus constructed. We will show that the invariance can be made manifest by introducing auxiliary fields.

- Thursday 23 March 2023 :
**Piotr Tourkine**(Université de Savoie)

Title: Integrands, monodromies and loop momentum in string amplitudes

Abstract: Monodromy relations relate different orderings of open string amplitudes at tree-level. These relations originate from the mutlivaluedness of the open-string integrand and are fundamental to understand the famous KLT relations, which allow to explain of closed-string amplitudes factorise into product of open-string amplitudes. Upon the field theory limit, this relates to the double-copy formalism, which has been instrumental in the progress in the calculations of gravitational scattering amplitudes over the last 15 years.

At loop-level, we showed how the monodromy relations are naturally expressed in terms of loop-momentum, and they relate graphs with different orderings and topologies. This raises some immediate problems: from the Feynman diagram intuition, loop momentum in different graphs are not supposed to have anything to do with each other, and therefore the monodromy relations might be ambiguous.

In this talk, I will explain that this is not the case, and that the formalism used to introduced loop momentum, chiral splitting, defines a global loop momentum. One consequence is that, upon the field theory limit, a similar global choice exists in field theory as well. I will start by reviewing the monodromy relations, chiral splitting, and then I’ll elaborate on this definition of the loop momentum and relations to the field theory limit and double-copy procedure.

- Thursday 06 April 2023 :
**Cumrum Vafa**(Harvard)

Title: String Theory and Our Universe

Abstract: String theory landscape of vacua point to new consistency conditions that a quantum gravitational system must satisfy. There are only a small number of quantum field theories that satisfy these conditions and all the rest belong to the `Swampland’ which cannot be consistently coupled to gravity. In this talk I review some of these conditions and their implications for cosmology and particle physics.

- Thursday 20 April 2023 :
**Keith R. Dienes**(Arizona U.)

Title: UV/IR Mixing, EFTs, Hidden Cancellations, and Origami: Calculating the Higgs Mass and Gauge-Coupling Running in String Theory

Abstract: In this talk, we shall present a non-technical method of understanding UV/IR mixing from a field-theoretic perspective. We will then discuss how these ideas are ultimately realized in string theory, providing a self-contained introduction to relevant string ideas as we proceed. Finally, we shall present a fully string-theoretic framework for calculating one-loop Higgs masses directly from the first principles in perturbative closed string theories. Notably, using our framework, we find that a gravitational modular anomaly generically relates the Higgs mass to the one-loop cosmological constant, thereby yielding a string-theoretic connection between the two fundamental quantities, which are known to suffer from hierarchy problems in the absence of spacetime supersymmetry. We also discuss a number of crucial issues involving the use and interpretation of regulators in UV/IR-mixed theories, such as string theory, and the manner in which one can extract an EFT description from such theories. Finally, we analyze the running of the Higgs mass within such an EFT description and uncover the existence of a “dual IR” region which emerges at high energies as the consequence of an intriguing scale-inversion duality symmetry. We also identify a generic stringy effective potential for the Higgs fields in such theories. Our results can therefore serve as the launching point for a rigorous investigation of gauge hierarchy problems in string theory.

- Thursday 04 May 2023 :
**Manki Kim**(MIT)

Title: Some difficult problems in string compactifications and string phenomenology.

Abstract: In this talk, I will describe a few important open problems in string phenomenology. I will describe how some of the problems may benefit from recent developments in string field theory.

- Thursday 25 May 2023 :
**Kevin Costello**(Perimeter Institute) (rescheduled from 12 January 2023)

Title: Twisted supergravity and string field theory

Abstract: Twisted supergravity is ordinary supergravity in a background where the superghost has a vev. I will review what is known about twisted supergravity. I will also discuss some joint work with Si Li, where we give a string field theory inspired argument for how to quantize twisted IIB supergravity