Research in the Particle Theory group

(Simon Hands, Biagio Lucini)

Numerical simulations of theories discretised on a space-time lattice allow us to understand from first principles the physics of strongly interacting systems. Among those systems, a central role is played by Quantum Chromodynamics (QCD), the theory describing the strong nuclear force. A possible way to understand QCD is to consider it as a member of the wider class of SU(N) gauge theories. In the limit for the number of colours N going to infinity, the theory still retains the key features of non-perturbative QCD (i.e. confinement and chiral symmetry breaking), but with the advantage of having a simpler structure. Thanks to the simplifications resulting from the large-N limit, the large-N theory is amenable to analytical calculations with field theory or gauge-string duality techniques. Through the extrapolation of physical observables to infinite N, lattice calculations enable us to connect analytical results to the real world and hence to gain insights about non-perturbative phenomena in QCD.

Another class of strongly interacting gauge theories could explain the origin of electroweak symmetry breaking (and hence elucidate the nature of the Higgs field) in the Standard Model. Theories in this class are called near-conformal or walking. Although at first sight very similar to QCD, those theories have very different physical properties (e.g. weak dependence of the coupling from the energy for a wide range of energy scales) that need to be studied non-perturbatively. Our group pioneered numerical studies of near-conformal gauge theories beyond the Standard Model (BSM) and is still at the forefront of the field. This work has also resulted in a general purpose benchmarking and diagnostic tool, BSMBench.

Finally, we also have an interest in lower-dimensional models that are strongly coupled and may or may not undergo chiral symmetry breaking.  In one theory, Nf = 2 relativistic flavours interacting via a contact between conserved charges, we identified a strongly-coupled phase where chiral symmetry is spontaneously broken; the resulting quantum critical point offers a strongly-coupled paradigm for electron transport in graphene. We have also studied a voltage-biased bilayer graphene, formally similar to QED3 with a nonzero isospin chemical potential, which opens the possibility of studying strong interaction effects at a Fermi surface using orthodox Monte Carlo techniques.