The results of mid-rapidity ($0 < y < 0.8$) neutral pion spectra over an extended transverse momentum range ($1 < p_T < 12$ GeV/$c$) in $\sqrt{s_{NN}}$ = 200 GeV Au+Au collisions, measured by the STAR experiment, are presented. The neutral pions are reconstructed from photons measured either by the STAR Barrel Electro-Magnetic Calorimeter (BEMC) or by the Time Projection Chamber (TPC) via tracking of conversion electron-positron pairs. Our measurements are compared to previously published $\pi^{\pm}$ and $\pi^0$ results. The nuclear modification factors $R_{\mathrm{CP}}$ and $R_{\mathrm{AA}}$ of $\pi^0$ are also presented as a function of $p_T$ . In the most central Au+Au collisions, the binary collision scaled $\pi^0$ yield at high $p_T$ is suppressed by a factor of about 5 compared to the expectation from the yield of p+p collisions. Such a large suppression is in agreement with previous observations for light quark mesons and is consistent with the scenario that partons suffer considerable energy loss in the dense medium formed in central nucleus-nucleus collisions at RHIC.
We present the results of charged particle fluctuations measurements in Au + Au collisions at $\sqrt{s_{NN}}=130$ GeV using the STAR detector. Dynamical fluctuations measurements are presented for inclusive charged particle multiplicities as well as for identified charged pions, kaons, and protons. The net charge dynamical fluctuations are found to be large and negative providing clear evidence that positive and negative charged particle production is correlated within the pseudorapidity range investigated. Correlations are smaller than expected based on model-dependent predictions for a resonance gas or a quark gluon gas which undergoes fast hadronization and freeze-out. Qualitative agreement is found with comparable scaled p+p measurements and a HIJING model calculation based on independent particle collisions, although a small deviation from the 1/N scaling dependence expected from this model is observed.
Data from the first physics run at the Relativistic Heavy-Ion Collider at Brookhaven National Laboratory, Au+Au collisions at $\sqrt{s_{NN}}=130$ GeV, have been analyzed by the STAR Collaboration using three-pion correlations with charged pions to study whether pions are emitted independently at freezeout. We have made a high-statistics measurement of the three-pion correlation function and calculated the normalized three-particle correlator to obtain a quantitative measurement of the degree of chaoticity of the pion source. It is found that the degree of chaoticity seems to increase with increasing particle multiplicity.
The measurement of direct photons from Au$+$Au collisions at $\sqrt{s_{_{NN}}}=39$ and 62.4 GeV in the transverse-momentum range $0.4<p_T<3$ Gev/$c$ is presented by the PHENIX collaboration at the Relativistic Heavy Ion Collider. A significant direct-photon yield is observed in both collision systems. A universal scaling is observed when the direct-photon $p_T$ spectra for different center-of-mass energies and for different centrality selections at $\sqrt{s_{_{NN}}}=62.4$ GeV is scaled with $(dN_{\rm ch}/d\eta)^{\alpha}$ for $\alpha=1.21{\pm}0.04$. This scaling also holds true for direct-photon spectra from Au$+$Au collisions at $\sqrt{s_{_{NN}}}=200$ GeV measured earlier by PHENIX, as well as the spectra from Pb$+$Pb at $\sqrt{s_{_{NN}}}=2760$ GeV published by ALICE. The scaling power $\alpha$ seems to be independent of $p_T$, center of mass energy, and collision centrality. The spectra from different collision energies have a similar shape up to $p_T$ of 2 GeV/$c$. The spectra have a local inverse slope $T_{\rm eff}$ increasing with $p_T$ of $0.174\pm0.018$ GeV/$c$ in the range $0.4<p_T<1.3$ GeV/$c$ and increasing to $0.289\pm0.024$ GeV/$c$ for $0.9<p_T<2.1$ GeV/$c$. The observed similarity of low-$p_T$ direct-photon production from $\sqrt{s_{_{NN}}}= 39$ to 2760 GeV suggests a common source of direct photons for the different collision energies and event centrality selections, and suggests a comparable space-time evolution of direct-photon emission.
The PHENIX experiement has measured the electron-positron pair mass spectrum from 0 to 8 GeV/c^2 in p+p collisions at sqrt(s)=200 GeV. The contributions from light meson decays to e^+e^- pairs have been determined based on measurements of hadron production cross sections by PHENIX. They account for nearly all e^+e^- pairs in the mass region below 1 GeV/c^2. The e^+e^- pair yield remaining after subtracting these contributions is dominated by semileptonic decays of charmed hadrons correlated through flavor conservation. Using the spectral shape predicted by PYTHIA, we estimate the charm production cross section to be 544 +/- 39(stat) +/- 142(syst) +/- 200(model) \mu b, which is consistent with QCD calculations and measurements of single leptons by PHENIX.
J/Psi production in p+p collisions at sqrt(s) = 200 GeV has been Measured in the PHENIX experiment at the Relativistic Heavy Ion Collider (RHIC) over a rapidity range of -2.2 < y < 2.2 and a transverse momentum range of 0 < pT < 9 GeV/c. The statistics available allow a detailed measurement of both the pT and rapidity distributions and are sufficient to constrain production models. The total cross section times branching ratio determined for J/Psi production is B_{ll} sigma_pp^J/psi = 178 +/- 3(stat) +/- 53(syst) +/- 18(norm) nb.
The PHENIX Collaboration at the Relativistic Heavy Ion Collider has measured open heavy flavor production in Cu$+$Cu collisions at $\sqrt{s_{_{NN}}}$=200 GeV through the measurement of electrons at midrapidity that originate from semileptonic decays of charm and bottom hadrons. In peripheral Cu$+$Cu collisions an enhanced production of electrons is observed relative to $p$$+$$p$ collisions scaled by the number of binary collisions. In the transverse momentum range from 1 to 5 GeV/$c$ the nuclear modification factor is $R_{AA}$$\sim$1.4. As the system size increases to more central Cu$+$Cu collisions, the enhancement gradually disappears and turns into a suppression. For $p_T>3$ GeV/$c$, the suppression reaches $R_{AA}$$\sim$0.8 in the most central collisions. The $p_T$ and centrality dependence of $R_{AA}$ in Cu$+$Cu collisions agree quantitatively with $R_{AA}$ in $d+$Au and Au$+$Au collisions, if compared at similar number of participating nucleons $\langle N_{\rm part} \rangle$.
The results from the STAR Collaboration on directed flow (v_1), elliptic flow (v_2), and the fourth harmonic (v_4) in the anisotropic azimuthal distribution of particles from Au+Au collisions at sqrtsNN = 200 GeV are summarized and compared with results from other experiments and theoretical models. Results for identified particles are presented and fit with a Blast Wave model. Different anisotropic flow analysis methods are compared and nonflow effects are extracted from the data. For v_2, scaling with the number of constituent quarks and parton coalescence is discussed. For v_4, scaling with v_2^2 and quark coalescence is discussed.
We present the directed flow ($v_1$) measured in Au+Au collisions at $\sqrt{s_{_{NN}}}$ = 62.4 GeV in the mid-pseudorapidity region $|\eta|<1.3$ and in the forward pseudorapidity region $2.5 < |\eta| < 4.0$. The results are obtained using the three-particle cumulant method, the event plane method with mixed harmonics, and for the first time at the Relativistic Heavy Ion Collider (RHIC), the standard method with the event plane reconstructed from spectator neutrons. Results from all three methods are in good agreement. Over the pseudorapidity range studied, charged particle directed flow is in the direction opposite to that of fragmentation neutrons.
The PHENIX experiment has measured the production of neutral pions in Au+Au collisions at sqrt(s_NN)=200 GeV. The new data offer a fourfold increase in recorded luminosity, providing higher precision and a larger reach in transverse momentum, p_T, to 20 GeV/c. The production ratio of eta/pi^0 is 0.46+/-0.01(stat)+/-0.05(syst), constant with p_T and collision centrality. The observed ratio is consistent with earlier measurements, as well as with the p+p and d+Au values. The production of pi^0 is suppressed by a factor of 5, as in earlier findings. However, with the improved statistical precision a small but significant rise of the nuclear modification factor, R_AA, vs p_T, with a slope of 0.0106+/-^(0.0034)_(0.0029)[GeV/c]^-1, is discernible in central collisions. A phenomenological extraction of the average fractional parton energy loss shows a decrease with increasing p_T. To study the path length dependence of suppression, the pi^0 yield was measured at different angles with respect to the event plane; a strong azimuthal dependence of the pi^0 R_AA is observed. The data are compared to theoretical models of parton energy loss as a function of the path length, L, in the medium. Models based on pQCD are insufficient to describe the data, while a hybrid model utilizing pQCD for the hard interactions and AdS/CFT for the soft interactions is consistent with the data.
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