# Research

## Quantum Field Theory

Quantum field theory is the language in which all of modern physics is formulated. It represents the marriage of quantum mechanics with special relativity and provides the mathematical framework in which to describe the creation and destruction of hoards of particles as they pop in and out of their ethereal existence and interact. Whether you want to understand the collisions of protons at the LHC, how electrons co-operate inside solids, or how black holes evaporate, you need to work with quantum field theory. Moreover, it has also proven to be a remarkably subtle and rich subject for mathematicians, providing insights into many new areas of mathematics.

When the interactions between particles are weak, quantum field theory can be well understood using perturbative techniques of Feynman diagrams. However, when the interactions between particles become strong, these methods no longer work and one must develop new mathematical tools and techniques. Such tools are necessary to understand phenomena such as the confinement of quarks inside protons and neutrons, or the fractionalization of electrons in the quantum Hall effect.

Members of DAMTP are actively engaged in understanding various aspects of strongly coupled field theories through lattice field theory, supersymmetry, integrability, gauge/gravity duality and other techniques

Those working in these areas include :

- Professor Nick Dorey
- Dr Jonathan Evans
- Professor R. R. Horgan
- Professor H. Osborn
- Dr David Skinner
- Dr Christopher Thomas
- Professor David Tong
- Dr Matthew Wingate

## Particle Phenomenology

The Large Hadron Collider (LHC) is currently at the world's high energy frontier. The discovery of a particle that looks and behaves very much like a Higgs boson as an inferred test of the Bose-Englert-Higgs mechanism of mass generation, is a triumph for 20th century theoretical physics. However, many questions remain unanswered, such as: why is the Higgs boson stable with respect to quantum fluctuations? What is dark matter? What caused the baryon asymmetry of the universe?

Speculative effective field theory models such as supersymmetric models or large extra dimensions solve some of these problems and their phenomenology and their LHC signals are the subject of active research. There is close collaboration and contact on the LHC new physics signals with experimentalists and theorists in the High Energy Physics Group at the Cavendish Laboratory. The emphasis is on what can be done with the experiments.

String theory model building connects the high energy scales of string theory to weak scale phenomenology. The ambitious goal is to construct a predictive and testable string model. String theory models can have an impact on the expected gauge groups and particles in the effective quantum field theory.

Members of DAMTP interested in these areas include :

## String Theory

String theory is an ambitious project. It purports to be an all-encompassing theory of the universe, unifying the forces of nature, including gravity, in a single quantum mechanical framework. The theory involves many elegant mathematical ideas, woven together to form a rich and beautiful tapestry of unprecedented sophistication. Moreover, through the AdS/CFT correspondence, string theory also offers new mathematical tools to understand aspects of strongly coupled field theories.

Members of the DAMTP have played a leading role in the development of superstring theory and M-theory. Current research is focussed in a number of directions, including developing a better understanding of the underlying structure of string theory, extracting predictions for particle physics and cosmological scenarios, and exploring the consequences of the AdS/CFT correspondence. There is also close collaboration between members of the High Energy Group and the Relativity Group.

Members of DAMTP interested in these areas :

- Professor Nick Dorey
- Professor Michael Green
- Professor Malcolm Perry
- Professor Fernando Quevedo
- Professor Harvey Reall
- Dr Jorge Santos
- Dr David Skinner
- Professor David Tong
- Professor Paul Townsend

## Lattice Field Theory

Lattice field theory combines techniques from quantum field theory and statistical field theory to study systems with strongly interacting particles. "The lattice" is both mathematically beautiful and computationally practical: quantum field theories formulated in a discrete spacetime are finite and mathematically well-defined; furthermore the theories are amenable to numerical solution without recourse to having a small expansion parameter.

Members of our group are using "the lattice" primarily to study the physics of quarks through a combination of effective field theory and numerical calculations. For example, the details of how quarks change their flavour provide clues to the physics we expect to find beyond our current Standard Model of particle physics. Precise lattice QCD calculations connect experimental measurements of hadron flavour interactions with the structure of quark flavour interactions in fundamental Lagrangians.

We also maintain active interest in applying similar theoretical methods to the flow and diffusion in random media, to the thermodynamics of ultracold atoms, as well as other areas for which ideas of field theory and statistical physics are relevant.

Members of the group participate in the UKQCD collaboration, which involves 7 UK Universities, and are part of the international HPQCD lattice collaboration.

Members of DAMTP interested in these areas :

## Cosmology

The last three decades have seen growing links develop between particle physics and astronomy, mainly centered on the cosmological implications of theories of high energy physics. The cosmological models which arose from the particle-cosmology connection in the 80's provided the motivation for the development of accurate computational techniques to address the problem of structure formation, suggested early universe inflation, and also encouraged the launch of spacecraft to acquire extensive data on the cosmic microwave background (CMB).

Recent observations of the CMB are putting the theory to a stringent test. The microwave map of the sky, at high resolution, is providing vast amounts of unprecedentedly clean data, and is revolutionising our understanding of the universe.

DAMTP has a very strong track record in this area, in which both the High Energy Physics Group, and the Relativity Group participate.

DAMTP members who are actively involved in this area include :

## Solitons

Solitons are novel objects that appear in systems due to correlated behaviour of the underlying constituents. Familiar examples are vortices in fluids. Other examples include domain walls, magnetic monopoles and instantons.

The group studies various aspects of the physics and mathematics of solitons, including the relationship to integrability, the geometry of soliton moduli spaces and the relationship to supersymmetry. A current interest is the application of Skyrmions to the modelling of nuclei.

Those working in this area are :