Proton And Anti-Proton Production At Mid-Rapidity From ～(197)Au+～(197)Au Collisions At SNN～(1/2)=200 GeV

The mid-rapidity proton and anti-proton yields are presented for the SNn = 200 GeV Au+Au data sets which were taken by the Solenoidal Tracker at RHIC (STAR) in 2001 run. The results are from transverse momentum range 0.4 < pt < 1.05 GeV/c and rapidity range |y| < 0.5 by using the energy loss in the Time Projection Chamber (TPC). The measured transverse momentum distribution becomes more convex from peripheral to central collisions for both proton and anti-proton implying the strong collective expansion at the early stage of the collision. The measured rapidity distributions of both proton and anti-proton are fiat within |y| < 0.5 indicating a boost invariant region around mid-rapidity. p/p≈0.8 is independent of the measured rapidity region |y| < 0.5 and decreases slightly from peripheral collision (≈ 0.85) to central collisions (≈ 0.80). It's still not net baryon free at RHIC energy. The slight decrease in the p/p ratio reflects the rich collision dynamics at RHIC: both initial baryon transfer and final stage hadronic rescatterings are important for the observation.The kinetic freeze-out conditions are extracted by applying a thermal + radial flow fit to the proton data and are calculated by extrapolating the measured spectra with the model. The kinetic freeze-out temperature decreases from peripheral (≈135 MeV) to most central collision (≈89 MeV). They are all smaller than the chemical freeze-out temperature (≈160±5 MeV) indicating an additional hadronic rescattering phase after the chemical freeze-out in Au + Au collisions at RHIC. The transverse flow velocity increases from peripheral to central collision. The central-ity dependence of (pt) for different particles (π, K, P) confirm this conclusion. The difference between the of different particles (π, K, P) increases as centrality increases, this indicates that the development of collective flow is stronger in central collisions than in peripheral collisions.The same results are also calculated by employing a transport model: Rela-tivistic Quantum Molecular Dynamics (RQMD). The spectra of different particles (π, K, P) are not following the so called m_t scaling. While the of them show similar trend with the experiment data despite of the underestimation of the absolute value. Furthermore, the underestimation of the absolute value might indicate early flow development. The earlier freeze-out of multi-strange particles (φ, Ξ, Ω) is demonstrated with this model from the freeze-out time and radius distribution of these particles. The measurements of these particles are necessary to confirm this. By applying thermal model fit to the spectra from this model, the same trend of kinetic freeze-out condition as in data is observed. The effects of resonance decay on the thermal fit parameters are also studied by letting the resonance particles decay with Pythia. The effect is small under this model's framework. Afterswitch off the rescattering in this model, the violation of mt scaling and central-Ity dependence of (pt) disappear. This indicates the importance of rescattering in heavy-ion collisions. However, only hadronlc interactions are included in RQMD. Such hadronlc interaction does not generate enough collective flow comparing to data. This demonstrates that partonlc collectivity is needed In the heavy ion collisions at RHIC. The realization of the partonlc collectivity is important toward the understanding of the partonlc equation of state in high-energy nuclear collisions.