Since many years the Plymouth Particle Theory group has been doing internationally leading research on quantum field theory and its applications. The group has recently been reinforced by new appointments of young researchers. It now consists of six permanent staff with a focus on numerical methods employing high-performance computing.

The research of the group centres around quantum field theory which may be viewed as the unification of quantum mechanics and special relativity. Hence, it allows for a coherent description of processes that occur both at microscopic scales and high energies, such as the creation of particle-anti-particle pairs. Historically, the first example of a quantum field theory was the quantum version of Maxwell’s theory of electromagnetism, now aptly called quantum electrodynamics (QED). This theory has since then been combined with the theory of weak and strong interactions describing radioactivity and (sub)nuclear binding, respectively. Together these theories form the Standard Model of particle physics which has seen its final confirmation this summer (2012) with the discovery of the elusive Higgs particle.

# The main research fields are:

Quantum Field Theory on the Lattice

The understanding of the Standard Model nowadays ranges from the sub-nuclear forces to the extreme properties of matter in the early universe. At low energies, the theory provides insights into such elusive quantum phenomena as the condensates and the origin of mass. Up to now, the only first principle theoretical access to such phenomena is based upon High-Performance-Computing simulations in the context of lattice gauge theories.

Within the group our research expertise covers:

- Vacuum structure including the Casimir effect
- Quantum critical phenomena and the density of states in gauge theories
- Mechanism of colour confinement in quantum chromodynamics (QCD), the theory of strong interactions
- Properties of matter under extreme conditions such as in the Early Universe or compact star matter
- Physics beyond the Standard Model and the orgin of mass as a result of strong interactions

Quantum Field Theory at High Intensities

QED is one of the most successful theories of the natural world, having been tested to one part in a billion. However, such precise tests have been mainly conducted in the well-understood perturbative regime, dominated by few-particle interactions. What happens when the density of particles is made so high, that arbitrarily large numbers can act with equal influence? A charge finds itself in such a situation when exposed to extremely intense laser pulses that comprise vast numbers of photons. Such a scenario is described by so-called “strong-field QED”, which predicts diverse non-linear and non-perturbative processes that can be probed at existing and future laser and particle-acceleration facilities. A 2015 colloquium talk given by Dr Tom Heinzl in Munich may be seen here on iTunes.

Important phenomena under study include:

- Radiation reaction
- Non-linear Compton scattering
- Electron-positron-photon cascades
- Vacuum polarisation (photon-photon scattering)
- Vacuum breakdown (spontaneous pair creation)
- (Classical) Charge Dynamics in Laser Fields

Quantum Field Theory and Infra-Red Physics

The Standard Model of particle physics is constructed from gauge theories with massless particles and long range interactions. Calculations of scattering processes are then found to include soft and collinear infra-red divergences. QED is the paradigm example of this. However, in QCD important physics such as colour confinement and the concomitant hadronisation processes are also dominated by infra-red physics. A fundamental understanding of this requires a better description of the physical (gauge invariant) fields in gauge theories. Areas under study include:

- the construction of gauge invariant (dressed) fields in the various theories of the Standard Model
- the onset of non-perturbative obstructions to constructing locally gauge invariant descriptions of quarks and gluons
- interplay between gauge invariance and infra-red finite cross-sections
- collinear and soft collinear divergences, disconnected processes and resummations
- infra-red motivated construction of the electron Green’s functions in an intense background