Lattice QCD for flavour physics

Abstract

Quarks are the fundamental particles making up 99.9 per cent of ordinary matter. They are bound together by the strong nuclear force, mediated by the exchange of gluons. The theory of quark and gluon interactions is Quantum Chromodynamics, or QCD. The strong force is actually weak when the quarks are close together but grows as you try to separate them, making it impossible to isolate a single quark, a property known as ’confinement’. This means that in experiments we do not detect quarks and gluons directly but instead see particles which are complicated bound states. It is thus very hard to determine the basic properties of the six types or flavours of quark, such as their masses and the strengths of the interactions which turn one flavour of quark into another. The flavour-changing interactions are related to the tiny difference between matter and antimatter, called CP violation, which may help explain why our Universe is dominated by matter (and why we can exist at all).
Supercomputer simulations allow us to discover whether our current theories can explain this or if there is some new physics at work. The simulations are the vital link between fundamental theories and the particles observed in high energy physics experiments. They enable scientists to ’look inside’ quark and gluon bound states, such as the proton and a plethora of other states known collectively as hadrons. The calculations are performed by constructing a discrete four dimensional space-time grid (the lattice) and then solving the fundamental QCD equations on this grid. Such lattice QCD simulations are the only known first-principles method for studying hadronic interactions.