The model describes the interplay of three forces — electromagnetic, weak and strong — and 12 elementary matter particles. (Gravity is not included.) Each force is mediated by the exchange of carrier particles: the photon, W or Z boson, or the gluon, as shown. Matter particles are divided into leptons and quarks, and, according to their mass hier archy, line up into three 'generations'. Matter particles also have antimatter equivalents — such as the positron, which is an antielectron. For quarks, the antiparticles are typically represented by a bar placed over the letter that symbolizes them (for example, ū is the antiparticle of the u quark). Collections of quarks and antiquarks form other, composite particles known as hadrons, a selection of which are shown. Hadrons are divided into mesons and baryons: mesons comprise a quark and an antiquark; baryons (including the proton and the neutron) are three-quark states.

Figure 1
figure 1