**Timothy Adamo**, Imperial College London

*Amplitudes on plane waves from the worldsheet *

Unlike conventional string theories, ambitwistor strings can be coupled to background fields exactly -- that is, without having to resort to worldsheet perturbation theory. This raises the possibility of using ambitwistor strings to compute scattering amplitudes perturbatively around non-trivial backgrounds. I'll describe how this works for the simplest of amplitudes (3-point, tree-level) on the simplest non-trivial backgrounds (plane waves) in gauge theory and gravity.

**Thales Azevedo**, Uppsala University

*(DF)^2 gauge theories and (ambitwistor) strings*

Recently, a gauge theory built out of dimension-six operators such as (DF)^2 appeared in the double-copy construction of conformal supergravity. In this talk, I will show how theories of that kind are related to conventional and ambitwistor string theories.

**Nathan Berkovits**, ICTP-SAIFR

*On pure spinor superfield actions in ten and eleven dimensions*

In collaboration with Max Guillen, we show how to derive the interacting equations of motion from Martin Cederwall's pure spinor superfield actions for d=10 super-Born-Infeld and d=11 supergravity.

**Eduardo Casali**, University of Oxford

*Scattering on planes waves and the double-copy*

**Humberto Gomez**, Universidad Santiago de Cali

*Double-cover scattering equations and Yang-Mills*

In this talk, we describe the double-cover (DC) extension of the scattering equation approach formulated by Cachazo, He and Yuan (CHY). So far, the DC prescription has only been developed for the $\phi^3$ theory. Here, we introduce the DC representation for the Yang-Mills theory and, as a by-product, we obtain a new recurrence relation.

**Song He**, ITP-Chinese Academy of Sciences

*Scattering Forms and the Positive Geometry of Kinematics, Color and the Worldsheet*

**Hadleigh Frost**, University of Oxford

*Bi-adjoint phi^3 amplitudes*

We discuss new ideas for writing biadjoint scalar amplitudes. We relate the tree amplitudes to a fan of cones in "dual kinematic space"---building on work by Arkani-Hamed, Bai, He and Yan last year. We describe how some of these ideas could be extended to loop integrands.

**Sebastian Mizera**, Perimter Institute

*Scattering Amplitudes as Intersection Pairings of Twisted Cohomology Groups*

**Ricardo Monteiro**, Queen Mary University & CERN

*Scattering Amplitudes, String Models and Gravitational Waves*

The study of scattering amplitudes in field theory connects a wide range of problems, from the mathematics of string perturbation theory to computations related to gravitational waves. I will discuss a couple of topics that keep scattering amplitudes researchers busy, and that motivate an ongoing workshop at PI. First, I will give an overview of recent progress in describing interactions in particle theories in terms of "ambitwistor strings", a new type of field theory model inspired by string theory. The result is an elegant formalism for scattering amplitudes in certain field theories, based on the "scattering equations". This formalism brings a new light into the "double copy relation", discovered in string theory, that expresses perturbative gravity in terms of perturbative gauge theory. I will review the double copy for scattering amplitudes, and then I will discuss the recent application of this idea to classical solutions. One of the aims is to export to gravitational phenomenology the dramatic simplifications provided by the double copy for scattering amplitudes.

**Oliver Schlotterer**, Perimeter Institute & Max Planck Institute

*The double-copy structure of one-loop open-string amplitudes from pure spinors*

**Yong Zhang**, ITP-Chinese Academy of Sciences

*New BCJ representations for one-loop amplitudes in gauge theories and gravity*

We explain a procedure to manifest the Bern–Carrasco–Johansson duality between color and kinematics in n-point one-loop amplitudes of a variety of supersymmetric gauge theories. Explicit amplitude representations are constructed through a systematic reorganization of the integrands in the Cachazo–He–Yuan formalism. Our construction holds for any nonzero number of supersymmetries and does not depend on the number of space-time dimensions. The cancellations from supersymmetry multiplets in the loop as well as the resulting power counting of loop momenta are manifested along the lines of the corresponding superstring computations. The setup is used to derive the one-loop version of the Kawai–Lewellen–Tye formula for the loop integrands of gravitational amplitudes. Some discussions about one-loop amplitudes with zero supersymmetry are included.