In 2005 the four experimental groups at Brookhaven National Laboratory’s Relativistic Heavy Ion Collider (RHIC) announced that collisions of gold nuclei at ultrarelativistic energies produced a “perfect liquid” of quarks and gluons. That’s something quite different from the gaseous quarkgluon state theorists and experimenters were expecting from quantum chromodynamics, the standard theory of the strong interaction.
Nucleons and all other hadrons are made up of quarks bound together by the strong interaction, which is mediated by gluon exchange. Unlike the chargeless photons that mediate the electromagnetic interactions, gluons carry color charge, the QCD analogue of electric charge. Therefore gluons, unlike photons, attract each other, which gives rise to the confinement of quarks inside hadrons.
A central consequence of QCD is the remarkable property of “asymptotic freedom,” the gradual vanishing of the strong forces between partons (quarks and gluons) with increasing proximity (see Physics Today, December 2004, page 21)....
