The CHY formulae are a set of remarkable formulae describing the scattering amplitudes of a variety of massless theories, as certain worldsheet integrals, localized on the solutions to certain polynomial equations (scattering equations). These formulae arise from a new class of holomorphic strings called Ambitwistor strings that encode exactly the dynamics of the supergravity (Yang-Mills) modes of string theory. Despite some recent progress by W. Siegel and collaborators, it remains as an open question as to what extent this theory was connected to the full string theory. The most mysterious point being certainly that the localization equations of the ambitwistor string also appear in the zero tension limit of string theory (alpha’ to infinity), which is the opposite limit than the supergravity one (alpha’ to zero). In this talk, I’ll report on some work in progress with E. Casali (Math. Inst. Oxford) and argue that the ambitwistor string is actually a tensionless string. Using some forgotten results on the quantization of these objects, we explain that the quantization of tensionless strings is ambiguous, and can lead either to a higher spin theory, or to the ambitwistor string, hence solving the previously mentioned paradox. In passing, we see that the degenerations of the tensile worldsheet that lead to tensionless strings make connection with Galilean Conformal Algebras and the (3d) BMS algebra.

# Past Relativity Seminar

Loop computations put the 'quantum' into quantum field theory. Much effort has focused on their structure and properties, with most spectacular progress in maximally supersymmetric gauge theories in the planar limit. These theories are however quite far from reality as described for instance in the standard model of particle physics. In this talk I'll report on ongoing work using BCFW on-shell recursion to obtain loop amplitude integrands in a much more realistic theory, pure Yang-Mills theory, using methods which apply directly to the standard model.

Black holes are one of the few available laboratories for testing theoretical ideas in fundamental physics. Since Hawking's result that they radiate a thermal spectrum, black holes have been regarded as thermodynamic objects with associated temperature, entropy, etc. While this is an extremely beautiful picture it has also lead to numerous puzzles. In this talk I will describe the two-loop correction to scalar correlation functions due to \phi^{^4} interactions and explain why this might have implications for our current view of semi-classical black holes.

I will summarize recent (arXiv:1603.05262) and upcoming work with Igor Buchberger and Oliver Schlotterer. We construct a map from n-point 1-loop string amplitudes in maximal supersymmetry to n-3-point 1-loop amplitudes in minimal supersymmetry. I will outline a few implications for the quantum string effective action.

In this talk, a concrete realization of the Bern-Carrasco-Johansson (BCJ) duality between color and kinematics in non-abelian gauge theories is presented. The method of Berends-Giele to package Feynman diagrams into currents is shown to yield classical solutions to the non-linear Yang-Mills equations. We describe a non-linear gauge transformation of these perturbiner solutions which reorganize the cubic-diagram content such that the kinematic dependence obeys the same Jacobi identities as the accompanying color factors. The resulting tree-level subdiagrams are assembled to kinematic numerators of tree-level and one-loop amplitudes which satisfy the BCJ duality.

In the last few years, it has become clear that there are striking connections between supersymmetry and geometric representation theory. In this talk, I will discuss boundary conditions in three dimensional gauge theories with N = 4 supersymmetry. I will then outline a physical understanding of a remarkable conjecture in representation theory known as `symplectic duality.

In this talk I will present some recent work on the amplituhedron formulation of scattering amplitudes. Very recently it has been conjectured that amplitudes in planar N=4 sYM are nothing else but the volume of a completely new mathematical object, called amplituhedron, which generalises the positive Grassmannian. After a review of the main ingredients which will be used, I will discuss some of the questions which remain open in this framework. I will then describe a new direction which promises to solve these issues and compute the volume of the amplituhedron at tree level.

Modular invariance is ubiquitous in string theory. This is the symmetry of genus-one amplitudes, as well as the non-perturbative duality symmetry of type IIb superstring in ten dimensions. The alpha’ expansion of string theory amplitudes leads to interesting new modular forms. In this talk we will describe the properties of the new modular forms. We will explain that the modular forms entering the alpha’ expansion of genus one type-II superstring amplitude are naturally expressed as particular values of single valued elliptic multiple polylogarithm. They are natural modular generalization of the single valued elliptic multiple-zeta introduced by Francis Brown.

Recent results showed that the low energy expansion of closed superstring amplitudes can be expressed in terms of single-valued multiple elliptic polylogarithms. I will explain how these functions may be defined as iterated integrals on the torus and sketch how they arise from Feynman integrals.

About 10 years ago Minahan and Zarembo made a remarkable discovery: the one-loop Dilatation Operator in the SO(6) sector of planar N=4 SYM can be identified with the Hamiltonian of an integrable spin chain. This one-loop Dilatation operator was obtained by computing a two-point correlation function at one loop, which is a completely off-shell quantity. Around the same time, Witten proposed a duality between N=4 SYM and twistor string theory, which initiated a revolution in the field of on-shell objects like scattering amplitudes. In this talk we illustrate that these techniques that have been sucessfully used for on-shell quantities can also be employed for the computation of off-shell quantities by computing the one-loop Dilatation Operator in the SO(6) sector. The first half of the talk will be dedicated to doing this calculation using MHV diagrams and the second half of the talk shows the computation in twistor space.

These two short talks will be followed by an informal afternoon session for those interested in further details of these approaches, and in form factors in Class Room C2 from 2-4.30 pm then from 4.30pm in N3.12. All are welcome.