Recent results in relativistic heavy ion collisions: from 'a new state of matter' to 'the perfect fluid'

Experimental physics with relativistic heavy ions dates from 1992 when a beam of Au-197 of energy greater than 10A GeV/c first became available at the Alternating Gradient Synchrotron at Brookhaven National Laboratory (BNL) soon followed in 1994 by a Pb-208 beam of 158A GeV/c at the Super Proton Synchrotron at CERN (European Center for Nuclear Research). Previous pioneering measurements at the Berkeley Bevalac (Gutbrod et al 1989 Rep. Prog. Phys. 52 1267-132) in the late 1970s and early 1980s were at much lower bombarding energies (<= 1AGeV/c) where nuclear breakup rather than particle production is the dominant inelastic process in A+A collisions. More recently, starting in 2000, the relativistic heavy ion collider at BNL has produced head-on collisions of two 100A GeV beams of fully stripped Au ions, corresponding to nucleon-nucleon centre-of-mass (cm) energy, root s(NN) = 200 GeV, total cm energy 200A GeV. The objective of this research program is to produce nuclear matter with extreme density and temperature, possibly resulting in a state of matter where the quarks and gluons normally confined inside individual nucleons (r < 1 fm) are free to act over distances an order of magnitude larger. Progress from the period 1992 to the present will be reviewed, with reference to previous results from light ion and proton-proton collisions where appropriate. Emphasis will be placed on the measurements which formed the basis for the announcements by the two major laboratories: 'A new state of matter', by CERN on Febraury 10 2000 and 'The perfect fluid' by BNL on April 19 2005.