Forthcoming events in this series


Tue, 14 Mar 2017

14:45 - 15:45
L4

The topology of the Wilsonahedron: A small case study

Susama Agarwala
(USNA)
Abstract

In this talk, I discuss the positive geometry of the Wilson Loop Diagrams appearing in SYM N-4 theory. In particular, I define an algorithm for associating Wilson Loop diagrams to convex cells of the positive Grassmannians. Using combinatorics of these cells, I then consider the geometry of N^2MHV diagrams on 6 points.

Tue, 28 Feb 2017

12:00 - 13:15
L4

Critical L-values from multi-loop Feynman diagrams

David Broadhurst
(Open University)
Abstract


I shall report on recent progress, in Australia and Germany, on the empirical evaluation of special values of L-functions by minors of period matrices whose elements include Feynman integrals from diagrams with up to 20 loops. Previously such relations were known only for diagrams with up to 6 loops.
 

Tue, 07 Feb 2017

12:00 - 13:00
L4

Geometric scattering for linear quantum fields

Dr Michal Wrochna
(Grenoble)
Abstract

An essential ingredient of AdS/CFT, dS/CFT and other dualities is a geometric notion of scattering that refers to asymptotics rather than, say, infinite time limits. Though one expects non-perturbative versions to exist in the case of linear quantum fields (and non-linear classical fields), this has been rigorously implemented in Lorentzian settings only relatively recently. The goal of this talk will be to give an overview in different geometrical setups, including asymptotically Minkowski, de Sitter and Anti-de Sitter spacetimes. In particular I will discuss recent results on classical scattering and particle interpretations, compare them with the setup of conformal scattering and explain how they can be used to construct "in-out" Feynman propagators (based on joint works with Christian Gérard and András Vasy).

Tue, 17 Jan 2017

12:00 - 13:15
L4

Polylogarithmic Polygon Origami

Lance Dixon
(Stanford)
Abstract

Amplitudes in planar N=4 SYM are dual to light-like polygonal Wilson-loop expectation values. In many cases their perturbative expansion can be expressed in terms of multiple polylogarithms which also obey certain single-valuedness conditions or branch cut restrictions. The rigidity of this function space, together with a few other conditions, allows one to construct the six-point amplitude -- or hexagonal Wilson loop -- through at least five loops, and the seven-point amplitude through 3.5 loops. Then one can "fold" the polygonal Wilson loops into multiple degenerate configurations, expressing the limiting behavior in terms of simpler function spaces, and learning in the process about how amplitudes factorize.
 

Tue, 29 Nov 2016

12:00 - 13:15
L4

Finite BMS transformations

Glenn Barnich
(ULB Brussells)
Abstract

After a brief review of holographic features of general relativity in 3 and 4 dimensions, I will show how to derive the transformation laws of the Bondi mass and angular momentum aspects under finite supertranslations, superrotations and complex Weyl rescalings.
 

Tue, 22 Nov 2016

12:00 - 13:00
L4

The number theory of superstring scattering amplitudes

Federico Zerbini
(Bonn)
Abstract

The Feynman diagram expansion of scattering amplitudes in perturbative superstring theory can be written (for closed strings) as a series of integrals over compactified moduli spaces of Riemann surfaces with marked points, indexed by the genus. Therefore in genus 0 it is reasonable to find, as it often happens in QFT computations, periods of M_{0,N}, which are known to be multiple zeta values. In this talk I want to report on recent advances in the genus 1 amplitude, which are related to the development of 2 different generalizations of classical multiple zeta values, namely elliptic multiple zeta values and conical sums.

Tue, 08 Nov 2016

12:00 - 13:15
L4

Ten-dimensional light-like lines, smooth Wilson loops in N=4 super Yang-Mills and twistors

Dr Christian Vergu
(Kings College London)
Abstract

In this talk I will present a class of super-Wilson loops in N=4 super Yang-Mills theory. The expectation value of these operators has been shown previously to be invariant under a Yangian symmetry. I will show how the kinematics of such super-Wilson loops can be described in a twistorial way and how this leads to compact, manifestly super-conformal invariant expressions for some two-point functions.
 

Tue, 01 Nov 2016

12:00 - 13:30
L4

Integrable Statistical Mechanics in Mathematics

Paul Fendley
(Oxford)
Abstract


I will survey of some of the many significant connections between integrable many-body physics and mathematics. I exploit an algebraic structure called a fusion category, familiar from the study of conformal field theory, topological quantum field theory and knot invariants. Rewriting statistical-mechanical models in terms of a fusion category allows the derivation of combinatorial identities for the Tutte polynomial, the analysis of discrete ``holomorphic'' observables in probability, and to defining topological defects in lattice models. I will give a little more detail on topological defects, explaining how they allows exact computations of conformal-field-theory quantities directly on the lattice, as well as a greatly generalised set of duality transformations.
 

Tue, 19 Jul 2016

12:00 - 13:15
L5

Doubled Geometry and $\alpha'$ Corrections

Dr Olaf Hohm
(Stonybrook)
Abstract

I review work done in collaboration with Siegel and Zwiebach,  in which a doubled geometry is developed that provides a spacetime  action containing the standard gravity theory for graviton, Kalb-Ramond field and dilaton plus higher-derivative corrections. In this framework the T-duality O(d,d) invariance is manifest and exact to all orders in $\alpha'$.  This theory by itself does not correspond to a standard string theory, but it does encode the Green-Schwarz deformation characteristic of heterotic string theory  to first order in $\alpha'$ and a Riemann-cube correction to second order in  $\alpha'$. I outline how this theory may be extended to include arbitrary string theories. 

 

Tue, 24 May 2016

10:30 - 11:30
L4

On the null string origin of the ambitwistor strings

Dr Piotr Tourkine
(Cambridge DAMTP)
Abstract
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.
Tue, 17 May 2016

12:00 - 13:15
L4

On-shell recursion at one loop in pure Yang-Mills theory, to an extent.

Dr Rutger Boels
(DESY, Hamburg)
Abstract

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.

Tue, 10 May 2016

12:00 - 13:15
L4

Quantum corrections to Hawking radiation

Dr Hadi Godazgar
(Cambridge DAMTP)
Abstract

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.
 

Wed, 27 Apr 2016

12:15 - 13:15
L4

From maximal to minimal supersymmetry in string loop amplitudes

Dr Marcus Berg
(Karlstadt University)
Abstract
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.
Tue, 19 Apr 2016

14:00 - 15:00
L4

A non-linear gauge transformation towards the BCJ duality

Dr Oliver Schlotterer
(AEI Golm)
Abstract
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.

Tue, 08 Mar 2016

12:00 - 13:15
L4

Boundary Conditions, Mirror Symmetry and Symplectic Duality

Dr Mat Bullimore
(Oxford)
Abstract

 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.

Tue, 23 Feb 2016

12:00 - 13:15
L4

The amplituhedron for tree-level scattering amplitudes in N=4 sYM

Dr Livia Ferro
(LMU-Muenchen and Max Planck Institut fuer Physik)
Abstract

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.

 

Tue, 09 Feb 2016

12:00 - 13:15
L4

Single Valued Elliptic Multizetas and String theory

Pierre Vanhove
(IHES & Cambridge)
Abstract

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. 

Tue, 26 Jan 2016

12:00 - 13:15
L4

Elliptic polylogarithms and string amplitudes

Dr Erik Panzer
(Oxford)
Abstract
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.
Tue, 24 Nov 2015

12:00 - 13:15
L4

From MHV diagrams and Twistors to the one-loop Dilatation Operator in the SO(6) sector

Brenda Penante and Laura Koster
(Humboldt and Queen Mary)
Abstract

 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.

 

Tue, 20 Oct 2015

12:00 - 13:30
L4

Recent progress in Ambitwistor strings

Yvonne Geyer
(Oxford)
Abstract

New ambitwistor string models are presented for a variety of theories and older models are shown to work at 1 loop and perhaps higher using a simpler formulation on the Riemann sphere.