Particle Physics Theory


I lead the lattice QCD team at Glasgow and am a co-leader of the international High Precision QCD (HPQCD) collaboration.

Quantum Chromodynamics (QCD) is the theory that, we believe, describes the strong force of Nature that operates between quarks and gluons inside the protons and neutrons of the atom and is therefore fundamental to the structure of matter. It is very important to be able to solve the theory of QCD because the strong interactions have a key effect - they confine quarks into bound states called hadrons (such as the proton and neutron) and they are never seen as free particles. We can only study hadrons in experiments and must infer from those results, and our theoretical understanding, the properties of the component quarks. Lattice QCD is a numerical method to solve the theory, needed because it is so complicated. My work is focussed on high precision calculations in lattice QCD in which we can test QCD at the highly nontrivial 1% level and in which we can use lattice QCD results in combination with experiment to determine the properties of quarks accurately. These are all needed for our understanding of where and how the Standard Model of particle physics will fail.

A hadron consists of either 3 valence quarks (a baryon) or a valence quark and antiquark (a meson) living in a soup of strongly interacting gluons and other quark-antiquark pairs, known as sea quarks. The valence quarks give the hadron its basic properties but the background soup is also an important component. In particular, it is only recently that lattice QCD calculations have been able to include the sea quarks fully. This work, "High precision lattice QCD confronts experiment", CTH Davies et al, HPQCD/FNAL/MILC collaborations, gave accurate lattice QCD results in agreement with experiment for the first time and has started a revolution of accurate calculations in this field.

Since then the Glasgow team and HPQCD have predicted the masses of several mesons (some subsequently confirmed by experiment) and the rates of annihilation into the W bosons of the weak force for others as well as key decay rates for experiments at CERN's Large Hadron Collider. We have determined quark masses and the QCD coupling constant to a precision of 1%. A summary of our current results for meson masses is shown below:

gold-plated mesons

Some of this work is described in my article "Colourful calculations" that appeared in Physics World in December 2006. An earlier article "Joining up the dots with the strong force" appeared in the CERN Courier in 2004. For a recent update on the status of lattice QCD calculations see my recent review talk at the Lepton-Photon meeting in June 2013.

Please see our group's research page or the HPQCD collaboration page for more details.