Towards the 1-loop effective action of type IIB orientifolds

I will review the status of 1-loop corrections to the effective action of certain string orientifolds. Concretely, I will mainly focus on the moduli sector of 4-dimensional toroidal type IIB orientifolds with N=1 supersymmetry. Emphasis will be put on recent results concerning field redefinitions of the moduli fields which are enforced at string 1-loop level in order to comply with supersymmetry.

Recursion Relations for Anomalous Dimensions of the 6d (2,0) Theory

I will describe recursion relations for the anomalous dimensions of double-trace operators occurring in the conformal partial wave expansion of four-point stress tensor correlators in the 6d (2,0) theory, which encode higher-derivative corrections to supergravity in AdS7xS4 arising from M-theory.

Massive Automorphic Green's Functions

I review some work in progress on string loop amplitudes with physicists and mathematicians: one way to think of worldsheet Green's functions in nontrivial backgrounds is to construct a massive propagator for the plane wave (pp wave) gravitational background as an automorphic function, then assemble "massive" modular graph functions.

Asymptotic safety versus string theory

I discuss issues around the possibility of defining asymptotic safety within a theory such as string theory that has a messy UV, by considering amplitudes in certain kinematic limits. In some respects the theory can be thought of as being asymptotically free, and the attenuation of amplitudes can be understood using an effective world line description. Motivated by this I present a class of world-line theories that have the same property. It turns out that these theories are related to a class of ghost-free infinite derivative theories inspired by string field theory.

Non-geometric Calabi-Yau backgrounds and heterotic/type II duality

Non-geometric compactifications are believed to be rather generic in string theory, yet explicit constructions are rare. In this talk I will present recent work about non-geometric type IIA models based on Calabi-Yau compactifications, that can be understood as mirror-folds with K3 fibers. I will also discuss their heterotic duals, the vector and hypermultiplets moduli spaces and their quantum corrections

Field theory actions for ambitwistors strings

I will review the ambitwistor string and show that the full spectrum for bosonic and Heterotic models include non-unitary states, which are expected since the amplitudes in these models have higher powers of momenta. In the end, the quadratic action for these theories will be contracted.

Pure spinors and ambitwistor strings

In this talk I will review the pure spinor versions of the Type IIA/IIB and heterotic ambitwistor strings. Unlike the RNS description, this pure spinor approach is manifestly spacetime supersymmetric which makes it attractive for computing N-point correlation functions. After replacing the current algebra of the pure spinor heterotic model by two holomorphic supersymmetry currents, the new pure spinor model will describe tree-level interactions of N=1 D=10 Born-Infeld physical states in a manifestly supersymmetric way.

Untwisting the Pure spinor in the non-minimal formalism

I will review the procedure of untwisting the pure spinor in the non-minimal formulation and show that, as in the minimal formulation, the fields combine naturally into N=1 worldsheet superfields that are constrained by a twistor-like variable. I am going to show that this constraints could be interpreted as a gauge fixing condition for a bilinear action that contains both the N=1 superconformal supersymmetry and the target space supersymmetry manifest. The goal is to related RNS and Pure Spinor formalism.

Pure Spinor String in Hamiltonian Form

For the Pure Spinor string in a general curved background, the nilpotency and holomorphicity of the BRST currents were shown to be a key component of the formalism. These conditions imply that the background fields satisfy supergravity constraints. However, the computations leading to this result didn't show a clear geometrical meaning. We will show that by transforming the original Pure Spinor sigma model to its Hamiltonian form, we get an action that treats matter and ghost fields on the same footing and is characterized by a "generalized metric". Finally, the nilpotency and holomorphicity conditions of the BRST currents are translated to equations in the context of generalized geometry.

Topological amplitudes in the pure spinor superstring

I will review the relation between some superstring scattering amplitudes in a Calabi-Yau compactification with the topological string partition function at any genus, using the RNS and the hybrid formalisms. Then I will describe how these same partition functions can be obtained from the non-minimal pure spinor formulation of the superstring with a suitable choice of b-ghost.

6d superamplitudes and the polarized scattering equations

The last years have seen tremendous progress in our understanding of the S-matrix of massless quantum field theories. However, this progress is usually confined to two areas: amplitudes not manifesting supersymmetry, or massless theories in 4d. In this talk, I show that the simplicity observed in 4d carries over to 6d superamplitudes, and discuss \the resulting formulas for complete tree-level superamplitudes in a variety of massless theories. The construction is motivated by a chiral worldsheet model, known as the ambitwistor string, which gives rise to the supersymmetry representation and the localization conditions.

High-energy scattering and the eikonal in field and string theory

The day is focused on the study of gravitational phenomena derived from an amplitude approach (both in string and field theory). There are four brief talks followed by an open discussion (at 15:00) coordinated by Gabriele Veneziano

Soft theorems for the gravity dilaton and the Nambu-Goldstone boson dilaton

We study the soft behaviour of tree level amplitudes involving two seemingly different particles that are both referred to as dilatons in the literature, namely the one that appears in theories of gravity and in string theory and the Nambu-Goldstone boson of spontaneously broken conformal invariance. The main result is the discovery of a soft theorem at subsubleading order for each dilaton, which in both cases contains the operator of special conformal transformations. Multiloop extension of these soft theorems will also be discussed in the framework of the Bosonic String theory

Strings on Celestial Sphere

Celestial amplitudes provide holographic imprints of four-dimensional scattering processes in terms of conformal correlation functions on a two-dimensional sphere describing Minkowski space at null infinity. The points on celestial sphere are associated to the asymptotic directions of (light-like) momenta of external particles, with the Lorentz group realized as the SL(2,C) conformal symmetry of the sphere. The energies are dualized through Mellin transforms into the parameters that determine dimensions of the primaries. We transform superstring scattering amplitudes into the correlation functions of primary conformal fields on two-dimensional celestial sphere. We focus on four-point amplitudes involving gauge bosons and gravitons in type I open superstring theory and in closed heterotic superstring theory at the tree-level. In the high-energy limit we find a direct relation between the celestial sphere, the string world-sheet and the solutions of scattering equations.

Superstring n-point 1-loop amplitudes

In 2011, the superstring n-point amplitudes at tree level have been explicitly computed using the pure spinor formalism. In this talk I will discuss the recent work on extending these calculations to one loop. Several very interesting patterns emerge in this quest. One of them is a general Lie-polynomial structure of superstring correlators. There is also a certain double-copy duality between inematic factors in pure spinor superspace and generalized elliptic integrands whose systematic construction in terms of objects satisfying shuffle-symmetries may suggest an underlying combinatoric explanation.

A proposal for the CFT dual for superstrings on ADS3xS1xS3xS3 at the string radius

After a review of the spectrum of superstrings on the AdS3 WZW background, I will review evidence for a phase transition when the AdS radius becomes of order of the string length. Next I will propose a holographic dual CFT that matches exactly the entire continuous spectrum of the superstring. I will conclude with a few observations on the role of interactions and discrete, short-string states.

Stability in open strings with broken supersymmetry

We construct type I string models where supersymmetry is spontaneously broken, which have exponentially small cosmological constant and are tachyon free at 1-loop. These theories have rigid Wilson lines associated with stacked branes. Models with positive effective potentials of runaway type are also presented.

AdS3 holography and black hole microstates

The holographic duality between black holes in appropriate decoupling limits and a two-dimensional CFT is a useful tool to elucidate the structure of black hole microstates. Focusing on the paradigmatic example of the Strominger-Vafa black hole, I will describe the holographic computation of correlators between chiral primary operators and the heavy operators dual to the microstates. This will provide evidence for the correspondence between the CFT and the geometric pictures and it will test the consistency of the gravitational description of the microstates with unitarity.

Triality in Little String Theories

I discuss a class of little string theories (LSTs) with eight supercharges on the world-volume of N M5-branes probing a transverse Z_M orbifold. These M-brane configurations compactified on a circle are dual to M D5-branes intersecting N NS5-branes on T^2 x R^{7,1} as well as to F-theory compactified on a toric Calabi-Yau threefold X_{N,M}. I argue that the Kähler cone of X_{N,M} admits three regions associated with weakly coupled quiver gauge theories of gauge groups [U(N)]^M, [U(M)]^N and [U(NM/k)]^k where k=gcd(N,M). These provide low-energy descriptions of different LSTs. The duality between the first two gauge theories is well known and is a consequence of the S-duality between D5- and NS5-branes or the T-duality of the LSTs. The triality involving the third gauge theory is new and I demonstrate it using several examples. Finally, I argue that triality implies a dihedral symmetry for the theories with M=1, which acts intrinsically non-perturbative from a gauge theory perspective.

Symmetries of near extremal black holes

The near horizon region of any black hole looks like flat space and displays an approximate Poincare symmetry. We study the way these symmetries are realized for near extremal black holes. We construct SL(2) generators that move matter fields relative to the boundaries. The construction is exact to all orders in the gravitational coupling. We also display an approximate realization of these generators in terms of simple operators defined at the boundary. These can also be realized in the boundary quantum mechanical theory. We also discuss the realization of these in the SYK model.