Display of a single Au + Au ion collision at an energy of 200 A-GeV, shown as an end view of the STAR detector. Outlines of detector elements are shown as well as the thousands of particle tracks from this one event. This particular event is one of the first of several million events acquired by the STAR experiment in 2001.

Doug Olson, Lawrence Berkeley National Laboratory
John Harris, Yale University

Research Objectives
The STAR detector (Solenoidal Tracker at RHIC) at Brookhaven National Laboratory is a large acceptance collider detector designed to study the collision of heavy nuclei at very high energy in the laboratory. Its goal is to investigate nuclear matter at extreme energy density and to search for evidence of the phase transition between hadronic matter and the deconfined quark-gluon plasma. The computations carried out at NERSC are focused around analysis of the processed data, comparison of these data with experimental models, and studies of the detector performance and acceptance.

Computational Approach
Physics results are derived from experimental relativistic heavy ion collisions by carrying out statistical analysis of large numbers of events (collisions of individual atomic nuclei). The theoretical models are implemented as Monte Carlo codes that describe the final state of each of the thousands of particles that are produced in these collisions. We use a number of these theoretical codes (VENUS, HIJING, RQMD, and others) to produce large samples of events. A simulation code called GEANT is used to propagate each of these thousands of particles through the material of the STAR detector and compute the reactions and energy deposition that occurs throughout the detector.

Accomplishments
Elliptic flow from nuclear collisions is a hadronic observable sensitive to the early stages of system evolution.

STAR reported first results on elliptic flow of charged particles at midrapidity in Au + Au collisions at = 130 GeV. The elliptic flow signal, v₂, averaged over transverse momentum, reaches values of about 6% for relatively peripheral collisions and decreases for the more central collisions. This can be interpreted as the observation of a higher degree of thermalization than at lower collision energies.

Results on the ratio of midrapidity antiproton-to-proton yields in Au + Au collisions show that the ratio is essentially independent of either transverse momentum or rapidity, within the rapidity and transverse momentum range of |y| < 0.5 and 0.4. From a pion interferometry analysis, the multidimensional identical pion correlation functions at midrapidity indicate that the source size grows with event multiplicity. The dependence of the correlations on transverse momentum, reflecting collective transverse flow, is qualitatively similar to that observed at lower energies. Anomalously large sizes or emission durations, which have been suggested as signals of quark-gluon plasma formation and rehadronization, are not observed. These results extend the weak dependence of the Hanbury-Brown-Twiss (HBT) parameters established for heavy ion collisions at lower energies.

The minimum bias multiplicity distribution and the transverse momentum and pseudorapidity distributions for central collisions have been measured for negative hadrons (h-) in Au + Au interactions. The multiplicity density per participant at midrapidity for the 5% most central interactions increases 38% relative to collisions at the same energy. The mean transverse momentum is larger than in central Pb + Pb collisions at lower energies. The scaling of the yield per participant is a strong function of p_t. The pseudorapidity distribution is almost constant within |h| < 1.

Significance
The existence of the quark-gluon plasma is predicted by lattice QCD calculations, and this state of matter is thought to be important in the dynamics of the early universe and the core of neutron stars. The most violent nuclear collisions at RHIC generate approximately ten thousand secondary particles. STAR detects and characterizes a large fraction of these secondaries in order to reconstruct a meaningful picture of each individual collision.

Publications
K. H. Ackermann et al., "Elliptic flow in Au + Au collisions at = 130 GeV," Phys. Rev. Lett. 86, 402 (2001).

C. Adler et al., "Midrapidity antiproton-to-proton ratio from Au + Au = 130 GeV," Phys. Rev. Lett. 86, 4778 (2001).

C. Adler et al., "Pion interferometry of = 130 GeV Au + Au collisions at RHIC," Phys. Rev. Lett. 87, 082301 (2001).

http://www.star.bnl.gov/