Where are we in the search for the Quark Gluon Plasma?

Abstract

The Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory was completed in 1999, with the first data-taking runs in the summer of 2000. Since then the measurements at RHIC have yielded a wealth of data from four independent detectors: BRAHMS, PHENIX, PHOBOS, and STAR. For the first time, collisions of heavy nuclei have been carried out with colliding-beam energies that have previously been accessible only to high-energy physics experiments. It is at these high energies that the predictions of QCD come into play and new phenomena are sought that may illuminate our view of the basic structure of matter.The RHIC experiments have recorded data from collisions of gold nuclei from Ô s NN 19 GeV up to the highest energy of 200 GeV, as well as reference data in proton-proton and deuteron-gold collisions. These collisions result in final states of unprecedented complexity, with thousands of produced particles radiating from the nuclear collision.The early measurements have revealed compelling evidence for the existence of a new form of nuclear matter at extremely high density and temperature. This medium allows for the predictions of QCD to be tested, and new phenomena explored, under conditions where the relevant degrees of freedom, over nuclear volumes, are expected to be those of quarks and gluons, rather than of hadrons. This is the realm of the quark gluon plasma.However, detailed analyses of the data also make it clear that this hot, dense medium has properties that are surprising, and not yet fully understood in terms of the early expectations for the quark gluon plasma. In this paper I review the major findings of the RHIC experiments to date and discuss the current understanding of the state of matter created.