The fascinating new world inside the nucleon, of quarks, gluons and colour, the nuclear strong force. How quantum chromodynamics (QCD) was discovered: probing the nucleons with scattering experiments and with increasing energy e+e− colliders, where quarks and gluons appear as hadronic jets.
The colour charges are three. Being the gauge of QCD non-Abelian, the gluons, not only the quarks, are ‘coloured’. How colour charges bind three quarks or a quark–antiquark pair forming hadrons that have zero overall colour charges.
The QCD coupling constant runs as the fine-structure constant, but with increasing momentum transfer, it decreases, instead of growing. Quarks become ‘free’, when they are very close to each other. Only a very small fraction of the proton mass is due to the quark masses, 99% being the energy of the colour field. The QCD vacuum, the status of minimum energy, a very active medium indeed, beautiful to study.
When matter first appeared in the universe, in the first microsecond after the Big Bang, quarks and gluons moved freely in a hot ‘soup’, the quark–gluon plasma. It is created in the laboratory in the ultra-relativistic heavy ion colliders and theoretically analysed with lattice QCD