Here, the STAR Collaboration reports on the photoproduction of π+π- pairs in goldgold collisions at a center-of-mass energy of 200 GeV/nucleon-pair. These pion pairs are produced when a nearly-real ...photon emitted by one ion scatters from the other ion. We fit the π+π- invariant mass spectrum with a combination of 0 and ! resonances and a direct π+π- continuum. This is the first observation of the ! in ultra-peripheral collisions, and the first measurement of ρ - ω interference at energies where photoproduction is dominated by Pomeron exchange.
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We present measurements of second-order azimuthal anisotropy (v2) at midrapidity (|y| < 1.0) for light nuclei d,t,3He (for sNN=200, 62.4, 39, 27, 19.6, 11.5, and 7.7 GeV) and antinuclei $ ...\overline{d}\ $; (sNN=200, 62.4, 39, 27, and 19.6 GeV) and 3H$ \overline{e}\ $(sNN=200 GeV) in the STAR (Solenoidal Tracker at RHIC) experiment. The v2 for these light nuclei produced in heavy-ion collisions is compared with those for p and $ \overline{p}\ $. We observe mass ordering in nuclei v2(pT) at low transverse momenta (pT < 2.0 GeV/c). We also find a centrality dependence of v2 for d and $ \overline{d}\ $. The magnitude of v2 for t and 3He agree within statistical errors. Light-nuclei v2 are compared with predictions from a blast-wave model. Atomic mass number (A) scaling of light-nuclei v2(pT) seems to hold for pT/A < 1.5GeV/c. Results on light-nuclei v2 from a transport-plus-coalescence model are consistent with the experimental measurements.
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We report on the first measurements of J/ψ production at very low transverse momentum (pT<0.2 GeV/c) in hadronic Au+Au collisions at sNN=200 GeV and U+U collisions at sNN=193 GeV. Remarkably, the ...inferred nuclear modification factor of J/ψ at midrapidity in Au+Au (U+U) collisions reaches about 24 (52) for pT<0.05 GeV/c in the 60%–80% collision centrality class. This noteworthy enhancement cannot be explained by hadronic production accompanied by cold and hot medium effects. In addition, the dN/dt distribution of J/ψ for the very low pT range is presented for the first time. The distribution is consistent with that expected from the Au nucleus and shows a hint of interference. Comparison of the measurements to theoretical calculations of coherent production shows that the excess yield can be described reasonably well and reveals a partial disruption of coherent production in semicentral collisions, perhaps due to the violent hadronic interactions. Incorporating theoretical calculations, the results strongly suggest that the dramatic enhancement of J/ψ yield observed at extremely low pT originates from coherent photon-nucleus interactions. In particular, coherently produced J/ψ's in violent hadronic collisions may provide a novel probe of the quark-gluon plasma.
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We report high-precision measurements of the longitudinal double-spin asymmetry, ALL, for midrapidity inclusive jet and dijet production in polarized p p collisions at a center-of-mass energy of √ s ...= 200 GeV . The new inclusive jet data are sensitive to the gluon helicity distribution, Δg (x, Q2), for gluon momentum fractions in the range from x ≃ 0.05 to x ≃ 0.5, while the new dijet data provide further constraints on the x dependence of Δ g (x , Q2). The results are in good agreement with previous measurements at √ s = 200 GeV and with recent theoretical evaluations of prior world data. Our new results have better precision and thus strengthen the evidence that Δg(x,Q2) is positive for x > 0.05.
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Here, we present measurements of three-particle correlations for various harmonics in Au+Au collisions at energies ranging from √sNN=7.7 to 200 GeV using the STAR detector. The quantity < ...cos(mΦ1+nΦ2–(m+n)Φ3) >, with Φ being the azimuthal angles of the particles is evaluated as a function of √sNN, collision centrality, transverse momentum, pT, pseudorapidity difference, Δη, and harmonics (m and n). These data provide detailed information on global event properties such as the three-dimensional structure of the initial overlap region, the expansion dynamics of the matter produced in the collisions, and the transport properties of the medium. A strong dependence on Δη is observed for most harmonic combinations, which is consistent with breaking of longitudinal boost invariance. An interesting energy dependence is observed when one of the harmonics m,n, or m+n is equal to two, for which the correlators are dominated by the two-particle correlations relative to the second-harmonic event plane. These measurements can be used to constrain models of heavy-ion collisions over a wide range of temperature and baryon chemical potential.
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In this paper, we present high precision measurements of elliptic flow near midrapidity (|y|<1.0) for multistrange hadrons and φ meson as a function of centrality and transverse momentum in Au+Au ...collisions at center of mass energy $\sqrt{sNN}$=200 GeV. We observe that the transverse momentum dependence of φ and Ωv2 is similar to that of π and p, respectively, which may indicate that the heavier strange quark flows as strongly as the lighter up and down quarks. This observation constitutes a clear piece of evidence for the development of partonic collectivity in heavy-ion collisions at the top RHIC energy. Number of constituent quark scaling is found to hold within statistical uncertainty for both 0%-30% and 30%-80% collision centrality. Finally, there is an indication of the breakdown of previously observed mass ordering between φ and proton v2 at low transverse momentum in the 0%-30% centrality range, possibly indicating late hadronic interactions affecting the proton v2.
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We present results from a harmonic decomposition of two-particle azimuthal correlations measured with the STAR detector in Au+Au collisions for energies ranging from sqrts_{NN}=7.7 to 200 GeV. The ...third harmonic v_{3}^{2}{2}=⟨cos3(ϕ_{1}-ϕ_{2})⟩, where ϕ_{1}-ϕ_{2} is the angular difference in azimuth, is studied as a function of the pseudorapidity difference between particle pairs Δη=η_{1}-η_{2}. Nonzero v_{3}^{2}{2} is directly related to the previously observed large-Δη narrow-Δϕ ridge correlations and has been shown in models to be sensitive to the existence of a low viscosity quark gluon plasma phase. For sufficiently central collisions, v_{3}^{2}{2} persist down to an energy of 7.7 GeV, suggesting that quark gluon plasma may be created even in these low energy collisions. In peripheral collisions at these low energies, however, v_{3}^{2}{2} is consistent with zero. When scaled by the pseudorapidity density of charged-particle multiplicity per participating nucleon pair, v_{3}^{2}{2} for central collisions shows a minimum near sqrts_{NN}=20 GeV.
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