Strongly interacting particles (hadrons) come in two classes: mesons made of quarks and antiquarks and baryons consisting of three quarks. Quarks come in six varieties or flavours: up, down, strange, charm (u, d, s, c) and much heavier bottom and top quarks (not considered here).
Symmetries between the different quark flavours mean that particles group together in families called multiplets. The proton and neutron are part of a multiplet with 20 members, shown in the left figure. Particles with no charm quark are in the bottom layer, followed by a layer of particles with one charm quark and then with two charm quarks. At present there is very little experimental knowledge about the doubly-charmed particles in the top layer.
We want to understand how particle masses are related to their position in the multiplet, and the masses of the quarks they contain. In computer simulations we are not limited to the quark masses nature has given us – we can see what would happen if the quarks had quite other masses. As an example, in the second figure we show how the masses of the particles in the lower slice of the multiplet change as we move between a situation with all quark masses equal (the crossing point of the fan) and the physical point (at the left edge of the graph).
While the main force between the quarks and antiquarks comes from QCD there is also a contribution from the electromagnetic force, (quantum electrodynamics, QED), which is usually left out of lattice calculations. We are also doing calculations with both forces included, to see what the effects of QED are. We can see clearly that mesons in which the quark and antiquark have the same sign of electrical charge become heavier compared with mesons where the quark and antiquark have opposite charges, as you might expect from the fact that opposite charges attract, and like charges repel.
Simulating two forces needs more computing power than just looking at one force, so DiRAC II is important in making all aspects of this project possible.