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.
The diphoton invariant mass distributions using the EMC-TPC method in 0-20% Au+Au collisions at $\sqrt{s_{NN}}=200$ GeV.
The diphoton invariant mass distributions using the EMC-TPC method in 0-20% Au+Au collisions at $\sqrt{s_{NN}}=200$ GeV.
The diphoton invariant mass distributions using the EMC-EMC method in 0-20% Au+Au collisions at $\sqrt{s_{NN}}=200$ GeV.
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The reaction π − p→ π 0 π 0 n has been measured with a 648 channel hodoscope spectrometer for the detection of the four γ's from the π 0 decays. The π 0 π 0 D-wave is fully compatible with the f 0 contribution as it is determined in high-statistics π + π − experiments. The magnitude of the π 0 π 0 S-wave and the cosinus of its phase angle (relative to the known D-wave) are determined from fits to the π 0 π 0 angular distributions. Argand diagrams for the I = 0 amplitude S 0 are given for the range 1000 to 1500 MeV/ c 2 . Two solutions exist. One exceeds the unitarity limit above 1200 MeV/ c 2 . The other remains within the unitarity limit and is nearly elastic up to 1450 MeV/ c 2 . It indicates an S 0 wave resonance around 1300 MeV/ c 2 .
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