We report the beam energy ( √ sNN = 7.7 -200 GeV) and collision centrality dependence of the mean (M ), standard deviation (σ), skewness (S), and kurtosis (κ) of the net-proton multiplicity distributions in Au+Au collisions. The measurements are carried out by the STAR experiment at midrapidity (|y| < 0.5) and within the transverse momentum range 0.4 < pT < 0.8 GeV/c in the first phase of the Beam Energy Scan program at the Relativistic Heavy Ion Collider. These measurements are important for understanding the Quantum Chromodynamic (QCD) phase diagram.

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Vector mesons may be photoproduced in relativistic heavy-ion collisions when a virtual photon emitted by one nucleus scatters from the other nucleus, emerging as a vector meson. The STAR Collaboration has previously presented measurements of coherent ρ 0 photoproduction at center of mass energies of 130 GeV and 200 GeV in AuAu collisions. Here, we present a measurement of the cross section at 62.4 GeV; we find that the cross section for coherent ρ 0 photoproduction with nuclear breakup is 10.5 ± 1.5 ± 1.6 mb at 62.4 GeV. The cross-section ratio between 200 GeV and 62.4 GeV is 4.4 ± 0.6, less than is predicted by most theoretical models. It is, however, proportionally much larger than the previously observed 15% ± 55% increase between 130 GeV and 200 GeV.

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The STAR Collaboration at the Relativistic Heavy Ion Collider presents a systematic study of high-transversemomentum charged-di-hadron correlations at small azimuthal pair separation φ in d + Au and central Au + Au collisions at √ s NN = 200 GeV. Significant correlated yield for pairs with large longitudinal separation η is observed in central Au + Au collisions, in contrast to d + Au collisions. The associated yield distribution in η× φ can be decomposed into a narrow jet-like peak at small angular separation which has a similar shape to that found in d + Au collisions, and a component that is narrow in φ and depends only weakly on η, the "ridge." Using two systematically independent determinations of the background normalization and shape, finite ridge yield is found to persist for trigger p t > 6 GeV/c, indicating that it is correlated with jet production. The transverse-momentum spectrum of hadrons comprising the ridge is found to be similar to that of bulk particle production in the measured range (2 < p t < 4 GeV/c).