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Tatsuya Chujo
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The baryon production mechanism at the intermediate $p_T$ (2 - 5 GeV/$c$) at RHIC is still not well understood. The beam energy scan data in Cu+Cu and Au+Au systems at RHIC may provide us a further insight on the origin of the baryon anomaly and its evolution as a function of $sqrt{s_{NN}}$. In 2005 RHIC physics program, the PHENIX experiment accumulated the first intensive low beam energy data in Cu+Cu collisions. We present the preliminary results of identified charged hadron spectra in Cu+Cu at $sqrt{s_{NN}}$ = 22.5 and 62.4 GeV using the PHENIX detector. The centrality and beam energy dependences of (anti)proton to pion ratios and the nuclear modification factors for charged pions and (anti)protons are presented.
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The intermediate $p_T$ region (2 - 5 GeV/$c$) in central Au+Au collisions at RHIC has a rich physics content. The (anti)proton to pion ratio at the intermediate $p_T$ gives us a powerful tool to investigate the bulk properties of the hot and dense matter created at RHIC and their hadronization processes. We present the preliminary results of identified charged hadron spectra at the lower beam energies at RHIC. The excitation function of (anti)proton to pion ratios from SPS to RHIC are shown. We also discuss the onset of the baryon enhancement at the high energy heavy ion collisions.
The latest PHENIX results for particle production are presented in this paper. A suppression of the yield of high p_t (transverse momentum) hadrons in central Au+Au collisions is found. In contrast, direct photons are not suppressed in central Au+Au collisions and no suppression of high p_t particles can be seen in d+Au collisions. This leads to the conclusion that the dense medium formed in central Au+Au collisions is responsible for the suppression. It is as well found, that the properties of this medium are similar to the one of a liquid. Further measurements provide information about the chiral dynamics of the system.
The J/psi is considered to be among the most important probes for the deconfined quark gluon plasma (QGP) created by relativistic heavy ion collisions. While the J/psi is thought to dissociate in the QGP by Debye color screening, there are competing effects from cold nuclear matter (CNM), feed-downs from excited charmonia (chi_c and psi) and bottom quarks, and regeneration from uncorrelated charm quarks. Measurements that can provide information to disentangle these effects are presented in this paper.
The first results from Au-Au collisions at $sqrt{s_{NN}}$=130 GeV obtained with the PHENIX detector in the Year 2000 run at RHIC are presented. The mid-rapidity charged particle multiplicity and transverse energy per participating nucleon rise steadily with the number of participants, such that transverse energy per charged particle remains relatively constant as a function of centrality. Identified charged hadron spectra as well as $bar{p}/p$ and $K^+/K^-$ ratios are discussed. Charged particle and neutral pion transverse momentum distributions in peripheral nuclear collisions are consistent with point-like scaling. The spectra at high $p_t$ from central collisions are significantly suppressed when compared to a simple superposition of binary nucleon-nucleon collisions.
A plastic scintillator paddle detector with embedded fiber light guides and photomultiplier tube readout, referred to as the Reaction Plane Detector (RXNP), was designed and installed in the PHENIX experiment prior to the 2007 run of the Relativistic Heavy Ion Collider (RHIC). The RXNPs design is optimized to accurately measure the reaction plane (RP) angle of heavy-ion collisions, where, for mid-central $sqrt{s_{NN}}$ = 200 GeV Au+Au collisions, it achieved a $2^{nd}$ harmonic RP resolution of $sim$0.75, which is a factor of $sim$2 greater than PHENIXs previous capabilities. This improvement was accomplished by locating the RXNP in the central region of the PHENIX experiment, where, due to its large coverage in pseudorapidity ($1.0<|eta|<2.8$) and $phi$ (2$pi$), it is exposed to the high particle multiplicities needed for an accurate RP measurement. To enhance the observed signal, a 2-cm Pb converter is located between the nominal collision region and the scintillator paddles, allowing neutral particles produced in the heavy-ion collisions to contribute to the signal through conversion electrons. This paper discusses the design, operation and performance of the RXNP during the 2007 RHIC run.
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