Probing the Dynamics of Nuclear Collisions with "Precision" Proton-Nucleus Measurements

Theoretical studies of the thermodynamics of strongly interacting matter at high temperatures and/or baryon densitites unambiguously predict the existence of the "quark-gluon plasma", a state of matter in which hadrons have dissolved into their constituent quarks and gluons. It has been long thought that high energy nuclear collisions may produce matter hot enough or dense enough to form the QGP in the laboratory. The recently completed Relativistic Heavy Ion Collider (RHIC) may provide the best opportunity for creating and studying the QGP. Nonetheless, experiments studying lower energy fixed-target collisions at the Brookhaven National Laboratory AGS and CERN SPS accelerators have already produced results that are claimed to provide evidence for production of a thermalized QGP state. Important questions remain, however, regarding the role of "pre-equilibrium" dynamics in these collisions and a poor understanding of this physics has prevented unambiguous interpretation of the existing data. Proton-nucleus (p-A) collisions potentially provide the solution to this problem since they allow the multiple scattering of a single energetic nucleon in a large nucleus to be studied in isolation. A new generation of electronic, large-acceptance p-A experiments is producing data that is revolutionizing our understanding of this physics by providing the ability to experimentally control the number of interactions of the proton in a target nucleus. The first such experiment, E910 at the BNL AGS was designed and led by Columbia.

I will review one of the claimed experimental "signatures" for QGP formation, the enhanced production of strange hadrons, particularly multi-strange baryons, and will use results from E910 to demonstrate the important contributions of non-equilibrium dynamics in producing the observed effect. I will argue that the strangeness enhancement does not result from QGP formation per se but that it likely results from the quark structure of the baryon and the direct role played by quarks in the pre-equilibrium dynamics of heavy ion collisions. I will show how lessons from this analysis are relevant to RHIC and will stress the importance of precision p-A data at RHIC. I will also discuss the potential role of future fixed-target p-A experiments.