Extrapolating backward toward the moment of the Big Bang, the temperatures and densities in the universe exceed those where the familiar building blocks of matter can exist. Quantum chromodynamics, the theory of the strong force, predicts that above energy densities of about 1 GeV/fm**3, protons and neutrons will melt into a hot soup containing their constituents -- quarks and gluons. Recently it has become possible to re-create such densities in the laboratory, namely by colliding heavy ions -- gold or copper nuclei -- at energies of up to 200 GeV per nucleon-nucleon collision. In this colloquium I will review recent experiments carried out at the Relativistic Heavy-Ion Collider at Brookhaven National Laboratory and discuss the present understanding of matter at these energy densities.
ANL Physics Division Colloquium Schedule