Yields for J/{psi} production in Cu+Cu collisions at {radical}(s{sub NN})=200 GeV have been measured over the rapidity range |y|<2.2 and compared with results in p+p and Au+Au collisions at the same ...energy. The Cu+Cu data offer greatly improved precision over existing Au+Au data for J/{psi} production in collisions with small to intermediate numbers of participants, in the range where the quark-gluon plasma transition threshold is predicted to lie. Cold nuclear matter estimates based on ad hoc fits to d+Au data describe the Cu+Cu data up to N{sub part}{approx}50, corresponding to a Bjorken energy density of at least 1.5 GeV/fm{sup 3}.
The production of low mass e+e- pairs for m_{e+e-} < 300 MeV/c^2 and 1 < p_T <5 GeV/c is measured in p+p and Au+Au collisions at sqrt(s_NN)=200 GeV. Enhanced e+e- pair yield above hadronic sources is ...observed in Au+Au collisions. Treating the excess as internal conversion of direct photons, the invariant yield of direct photons is deduced. In central Au+Au collisions, the excess over p+p is exponential in p_T}, with inverse slope T = 221 +/- 23 (stat) +/- 18 (syst) MeV. Hydrodynamical models with initial temperatures T_init ~ 300-600 MeV at times of 0.6 - 0.15 fm/c after the collision are in qualitative agreement with the data. Lattice QCD predicts a phase transition at ~ 170 MeV.
Fast parton probes produced by hard scattering and embedded within collisions of large nuclei have shown that partons suffer large energy loss and that the produced medium may respond collectively to ...the lost energy. We present measurements of neutral pion trigger particles at transverse momenta p^t_T = 4-12 GeV/c and associated charged hadrons (p^a_T = 0.5-7 GeV/c) as a function of relative azimuthal angle Delta Phi at midrapidity in Au+Au and p+p collisions at sqrt(s_NN) = 200 GeV. These data lead to two major observations. First, the relative angular distribution of low momentum hadrons, whose shape modification has been interpreted as a medium response to parton energy loss, is found to be modified only for p^t_T < 7 GeV/c. At higher p^t_T, the data are consistent with unmodified or very weakly modified shapes, even for the lowest measured p^a_T. This observation presents a quantitative challenge to medium response scenarios. Second, the associated yield of hadrons opposite to the trigger particle in Au+Au relative to that in p+p (I_AA) is found to be suppressed at large momentum (IAA ~ 0.35-0.5), but less than the single particle nuclear modification factor (R_AA ~0.2).
We present azimuthal angle correlations of intermediate transverse momentum (1-4 GeV/c) hadrons from dijets in Cu+Cu and Au+Au collisions at {radical}(s{sub NN})=62.4 and 200 GeV. The away-side dijet ...induced azimuthal correlation is broadened, non-Gaussian, and peaked away from {delta}{phi}={pi} in central and semicentral collisions in all the systems. The broadening and peak location are found to depend upon the number of participants in the collision, but not on the collision energy or beam nuclei. These results are consistent with sound or shock wave models, but pose challenges to Cherenkov gluon radiation models.
J/{psi} production in p+p collisions at {radical}(s)=200 GeV has been measured by the PHENIX experiment at the BNL Relativistic Heavy Ion Collider over a rapidity range of -2.2<y<2.2 and a transverse ...momentum range of 0<p{sub T}<9 GeV/c. The size of the present data set allows a detailed measurement of both the p{sub T} and the rapidity distributions and is sufficient to constrain production models. The total cross section times the branching ratio is B{sub ll}{sigma}{sub pp}{sup J/{psi}}=178{+-}3{sup stat}{+-}53{sup sys}{+-}18{sup norm} nb.
The PHENIX experiment at the BNL Relativistic Heavy Ion Collider (RHIC)has measured J/{psi} production for rapidities -2.2<y<2.2 in Au+Au collisions at {radical}(s{sub NN})=200 GeV. The J/{psi} ...invariant yield and nuclear modification factor R{sub AA} as a function of centrality,transverse momentum, and rapidity are reported. A suppression of J/{psi} relative to binary collision scaling of proton-protonreaction yields is observed. Models which describe the lower energy J/{psi} data at the CERN Super Proton Synchrotron invoking only J/{psi} destruction based on the local medium density predicta significantly larger suppression at RHIC and more suppression at midrapiditythan at forward rapidity. Both trends are contradicted by our data.
The beam-helicity asymmetry in associated electroproduction of real photons, ep arrow right e gamma pi N , in the Delta (1232)-resonance region is measured using the longitudinally polarized Hera ...positron beam and an unpolarized hydrogen target. Azimuthal Fourier amplitudes of this asymmetry are extracted separately for two channels, ep arrow right e gamma pi super(0) p and ep arrow right e gamma pi super(+) n, from a data set collected with a recoil detector. All asymmetry amplitudes are found to be consistent with zero.
The PHENIX experiment at the BNL Relativistic Heavy Ion Collider (RHIC) has measured electrons with 0.3<p{sub T}<9 GeV/c at midrapidity (vertical bar y vertical bar <0.35) from heavy-flavor (charm ...and bottom) decays in Au+Au collisions at {radical}(s{sub NN})=200 GeV. The nuclear modification factor R{sub AA} relative to p+p collisions shows a strong suppression in central Au+Au collisions, indicating substantial energy loss of heavy quarks in the medium produced at RHIC energies. A large azimuthal anisotropy v{sub 2} with respect to the reaction plane is observed for 0.5<p{sub T}<5 GeV/c indicating substantial heavy-flavor elliptic flow. Both R{sub AA} and v{sub 2} show a p{sub T} dependence different from those of neutral pions. A comparison to transport models which simultaneously describe R{sub AA}(p{sub T}) and v{sub 2}(p{sub T}) suggests that the viscosity to entropy density ratio is close to the conjectured quantum lower bound, i.e., near a perfect fluid.
Differential measurements of elliptic flow (v{sub 2}) for Au+Au and Cu+Cu collisions at {radical}(s{sub NN})=200 GeV are used to test and validate predictions from perfect fluid hydrodynamics for ...scaling of v{sub 2} with eccentricity, system size, and transverse kinetic energy (KE{sub T}). For KE{sub T}{identical_to}m{sub T}-m up to {approx}1 GeV the scaling is compatible with hydrodynamic expansion of a thermalized fluid. For large values of KE{sub T} mesons and baryons scale separately. Quark number scaling reveals a universal scaling of v{sub 2} for both mesons and baryons over the full KE{sub T} range for Au+Au. For Au+Au and Cu+Cu the scaling is more pronounced in terms of KE{sub T}, rather than transverse momentum.
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