Introduction to the PHENIX Experiment
At very high temperatures, the vacuum is not just "empty space" devoid of matter but is actually filled with virtual
quarks and gluons, the ultimate building blocks of matter. For example, at T~150 MeV - more than 10 000 times hotter than the interior of the sun - our normal hadronic world that has quarks and gluons confined inside protons and neutrons is expected to "melt" into a phase of free quarks and gluons, the so-called "quark gluon plasma."
In the early Universe, about 1-10 usec after the big bang, the reverse process has led to the formation of hadrons. At the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory (BNL) we study the properties of the vacuum at high energy densities by colliding heavy nuclei, such as gold, and then reconstructing the collision products. In an exciting link, it appears that our understanding of proton sub-structure is closely connected to the physics of the quantum chromodynamics (QCD) vacuum at high energy densities: the properties of the proton cannot be understood without taking into account the complex "sea" of virtual quark, anti-quark pairs and gluons inside the proton. At RHIC, we study the sub-structure of protons in proton-nucleus and polarized proton-proton collisions leading to a very broad and rigorous program in the physics of hadrons and QCD as the theory describing their interactions and structure.