Relativistic nucleus-nucleus collisions: from the BEVALAC to RHIC

I briefly describe the initial goals of relativistic nuclear collision research, focusing on the LBL Bevatron/Bevalac facility in the 1970s. An early concept of high hadronic density fireball formation, and subsequent isentropic decay (preserving information of the high-density stage), led to an outline of physics observables that could determine the nuclear matter equation of state at several times the nuclear ground state matter density. With the advent of QCD the goal of locating and characterizing the hadron-parton deconfinement phase transformation suggested the need for higher √s, the research thus shifting to the BNL AGS and CERN SPS, and finally to RHIC at BNL. A set of physics observables is discussed where present data span the entire √s domain, from Bevalac and SIS at GSI, to high RHIC energy. Referring, selectively, to data concerning bulk hadron production, the overall √s evolution of directed and radial flow observables, and of pion pair Bose-Einstein correlation is discussed. The hadronization process is studied in the grand canonical statistical model. The resulting hadronization points in the plane T versus μB converge onto the parton-hadron phase boundary predicted by finite μB lattice QCD, from high SPS to RHIC energy. At lower SPS and high AGS energy a steep strangeness maximum occurs at which the Wroblewski parameter λs∼ 0.6; a possible connection to the QCD critical point is discussed. Finally the unique new RHIC physics is addressed: high-pT hadron suppression and jet 'tomography'