We review the most important experimental results from the first three years of nucleus–nucleus collision studies at RHIC, with emphasis on results from the STAR experiment, and we assess their ...interpretation and comparison to theory. The theory-experiment comparison suggests that central Au + Au collisions at RHIC produce dense, rapidly thermalizing matter characterized by: (1) initial energy densities above the critical values predicted by lattice QCD for establishment of a quark–gluon plasma (QGP); (2) nearly ideal fluid flow, marked by constituent interactions of very short mean free path, established most probably at a stage preceding hadron formation; and (3) opacity to jets. Many of the observations are consistent with models incorporating QGP formation in the early collision stages, and have not found ready explanation in a hadronic framework. However, the measurements themselves do not yet establish unequivocal evidence for a transition to this new form of matter. The theoretical treatment of the collision evolution, despite impressive successes, invokes a suite of distinct models, degrees of freedom and assumptions of as yet unknown quantitative consequence. We pose a set of important open questions, and suggest additional measurements, at least some of which should be addressed in order to establish a compelling basis to conclude definitively that thermalized, deconfined quark–gluon matter has been produced at RHIC.
Annihilation of antiprotons and protons at rest into neutral particles has been studied with the Crystal Barrel detector at LEAR. Annihilation frequencies are determined for final states containing
π
...0,
η,
η′ and
ω mesons using a liquid and a room temperature, 12 bar, gaseous hydrogen target. Including annihilation frequencies for production of neutral kaons from other experiments, the identified reactions for annihilation in liquid hydrogen add up to a branching fraction of (3.56±0.28)% per annihilation compared to the frequency of (3.50±0.30)% with which we observe the all-neutral decay modes inclusively. Since the exclusive final states are normalized to the Crystal Barrel measurement of the
π
0
π
0 branching ratio, the latter result is strongly supported by this present study.
We present the first measurements of charge-dependent correlations on angular difference variables η1−η2 (pseudorapidity) and ϕ1−ϕ2 (azimuth) for primary charged hadrons with transverse momentum ...0.15⩽pt⩽2 GeV/c and |η|⩽1.3 from Au–Au collisions at sNN=130 GeV. We observe correlation structures not predicted by theory but consistent with evolution of hadron emission geometry with increasing centrality from one-dimensional fragmentation of color strings along the beam direction to an at least two-dimensional hadronization geometry along the beam and azimuth directions of a hadron-opaque bulk medium.
We present the first study of the energy dependence of p{sub t} angular correlations inferred from event-wise mean transverse momentum p{sub t} fluctuations in heavy ion collisions. We compare our ...large-acceptance measurements at CM energies {radical}s{sub NN} = 19.6, 62.4, 130 and 200 GeV to SPS measurements at 12.3 and 17.3 GeV. p{sub t} angular correlation structure suggests that the principal source of p{sub t} correlations and fluctuations is minijets (minimum-bias parton fragments). We observe a dramatic increase in correlations and fluctuations from SPS to RHIC energies, increasing linearly with ln {radical}s{sub NN} from the onset of observable jet-related p{sub t} fluctuations near 10 GeV.
We study the energy dependence of the transverse momentum (pT) spectra for charged pions, protons and anti-protons for Au+Au collisions at sNN=62.4 and 200 GeV. Data are presented at mid-rapidity ...(|y|<0.5) for 0.2<pT<12GeV/c. In the intermediate pT region (2<pT<6GeV/c), the nuclear modification factor is higher at 62.4 GeV than at 200 GeV, while at higher pT (pT>7GeV/c) the modification is similar for both energies. The p/π+ and p¯/π− ratios for central collisions at sNN=62.4GeV peak at pT≃2GeV/c. In the pT range where recombination is expected to dominate, the p/π+ ratios at 62.4 GeV are larger than at 200 GeV, while the p¯/π− ratios are smaller. For pT>2GeV/c, the p¯/π− ratios at the two beam energies are independent of pT and centrality indicating that the dependence of the p¯/π− ratio on pT does not change between 62.4 and 200 GeV. These findings challenge various models incorporating jet quenching and/or constituent quark coalescence.
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