In this talk, we discuss an ongoing calculation of the

four-loop form factors in massless QCD. We begin by discussing our

novel approach to the calculation in detail. Of particular interest

are a new polynomial-time integration by parts reduction algorithm and

a new method to algebraically resolve the IR and UV singularities of

dimensionally-regulated bare perturbative scattering amplitudes.

Although not all integral topologies are linearly reducible for the

more non-trivial color structures, it is nevertheless feasible to

obtain accurate numerical results for the finite parts of the complete

four-loop form factors using publicly available sector decomposition

programs and bases of finite integrals. Finally, we present first

results for the four-loop gluon form factor Feynman diagrams which

contain three closed fermion loops.

# Past Quantum Field Theory Seminar

The $\phi^4$ model in statistical physics describes the

continous phase transition in the liquid-vapour system, transition to

the superfluid phase in helium, etc. Experimentally measured values in

this model are critical exponents and universal amplitude ratios.

These values can also be calculated in the framework of the

renormalization group approach. It turns out that the obtained series

are divergent asymptotic series, but it is possible to perform Borel

resummation of such a series. To make this procedure more accurate we

need as much terms of the expansion as possible.

The results of the recent six loop analitical calculations of the

anomalous dimensions, beta function and critical exponents of the

$O(N)$ symmetric $\phi^4$ model will be presented. Different technical

aspects of these calculations (IBP method, R* operation and parametric

integration in Feynman representation) will be discussed. The

numerical estimations of critical exponents obtained with Borel

resummation procedure are compared with experimental values and

results of Monte-Carlo simulations.

The big phase space is an infinite dimensional manifold which is the arena

for topological quantum field theories and quantum cohomology (or

equivalently, dispersive integrable systems). tt*-geometry was introduced by

Cecotti and Vafa and is a way to introduce an Hermitian structure on what

would be naturally complex objects, and the theory has many links with

singularity theory, variation of Hodge structures, Higgs bundles, integrable

systems etc.. In this talk the two ideas will be combined to give a

tt*-geometry on the big phase space.

(joint work with Liana David)

This talk will start with a brief historical review of the classification of solids by their symmetries, and the more recent K-theoretic periodic table of Kitaev. It will then consider some mathematical questions this raises, in particular about the behaviour of electrons on the boundary of materials and in the bulk. Two rather different models will be described, which turn out to be related by T-duality. Relevant ideas from noncommutative geometry will be explained where needed.

I'll explain the formalism of extended QFT, while

focusing on the cases of two dimensional conformal field theories,

and three dimensional topological field theories.

In this talk, I will review an inverse scattering construction of interacting integrable quantum field theories on two-dimensional Minkowski space and its ramifications. The construction starts from a given two-body S-matrix instead of a classical Lagrangean, and defines corresponding quantum field theories in a non-perturbative manner in two steps: First certain semi-local fields are constructed explicitly, and then the analysis of the local observable content is carried out with operator-algebraic methods (Tomita-Takesaki modular theory, split subfactor inclusions). I will explain how this construction solves the inverse scattering problem for a large family of interactions, and also discuss perspectives on extensions of this program to higher dimensions and/or non-integrable theories.

Amplitudes in quantum field theory have discontinuities when regarded as

functions of

the scattering kinematics. Such discontinuities can be determined from

Cutkosky rules.

We present a structural analysis of such rules for massive quantum field

theory which combines

algebraic geometry with the combinatorics of Karen Vogtmann's Outer Space.

This is joint work with Spencer Bloch (arXiv:1512.01705).