For many years the Plymouth Particle Theory group has performed internationally leading research on quantum field theory and its applications. The group has recently been reinforced by new appointments of young researchers and currently consists of seven permanent staff along with several postdocs and PhD students.

The research of the group centres around quantum field theory, which may be viewed as the unification of quantum mechanics and special relativity. Hence, quantum field theory 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 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 saw its final confirmation in 2012 with the discovery of the elusive Higgs particle.

# The main research fields are:

**Quantum Field Theory on the Lattice (Drach, McNeile, Rago)**

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 such as condensates and the origin of mass. A powerful first-principles approach giving 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:

- Quantum critical phenomena and the density of states in gauge theories.
- Properties of matter under extreme conditions such as in the Early Universe or compact star matter
- Physics beyond the Standard Model and the origin of mass as a result of strong interactions
- Precision calculation of the parameters of the standard model, such as CKM matrix elements or quark masses.
- Hadron spectroscopy of novel QCD bound states such as exotic hybrid mesons and glueballs.

We collaborate with many groups around the world, such as the Swansea Particle Physics and Cosmology Theory (PPCT) group, the HPQCD collaboration and the CERN theory group.

**Quantum Field Theory in external fields (Heinzl, King)**

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 mainly been performed in the well-understood perturbative regime, dominated by few-particle interactions. When the density of particles is made high, though, then arbitrarily large numbers can interact with equal influence, which takes us to a non-perturbative regime. Such a situation may be realised by in the presence of ultra-strong electromagnetic fields.

These strong fields give access to diverse non-linear and non-perturbative processes that can be probed at existing and future laser facilities and particle-acceleration facilities, and understanding this physics requires novel techniques which go beyond standard perturbative approaches.

Research interests within the group include:

- Non-perturbative pair creation from the vacuum (vacuum decay)
- Electron-positron-photon cascades
- Exactly solvable models of light-matter interactions (superintegrability)
- Vacuum polarisation and birefringence (photon-photon scattering)
- Classical and quantum dynamics (radiation reaction) in laser fields

A 2015 colloquium talk by Dr Tom Heinzl on “*Extreme light and quantum fields’*‘ may be seen here on iTunes.

There is more information about this research team here.

**Quantum Field Theory and Infra-Red Physics (Lavelle, McMullan)
**

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 field