The production of e{sup +}e{sup -} pairs for m{sub e}{sup +}{sub e}{sup -}<0.3 GeV/c{sup 2} and 1<p{sub T}<5 GeV/c is measured in p+p and Au+Au collisions at {radical}(s{sub NN})=200 GeV. An enhanced ...yield above hadronic sources is observed. Treating the excess as photon internal conversions, the invariant yield of direct photons is deduced. In central Au+Au collisions, the excess of the direct photon yield over p+p is exponential in transverse momentum, with an inverse slope T=221{+-}19{sup stat{+-}}19{sup syst} MeV. Hydrodynamical models with initial temperatures ranging from T{sub init{approx}}300-600 MeV at times of {approx}0.6-0.15 fm/c after the collision are in qualitative agreement with the data. Lattice QCD predicts a phase transition to quark gluon plasma at {approx}170 MeV.
The second Fourier component v_2 of the azimuthal anisotropy with respect to the reaction plane was measured for direct photons at midrapidity and transverse momentum (p_T) of 1--13 GeV/c in Au+Au ...collisions at sqr(s_NN)=200 GeV. Previous measurements of this quantity for hadrons with p_T < 6 GeV/c indicate that the medium behaves like a nearly perfect fluid, while for p_T > 6 GeV/c a reduced anisotropy is interpreted in terms of a path-length dependence for parton energy loss. In this measurement with the PHENIX detector at the Relativistic Heavy Ion Collider we find that for p_T > 4 GeV/c the anisotropy for direct photons is consistent with zero, as expected if the dominant source of direct photons is initial hard scattering. However, in the p_T < 4 GeV/c region dominated by thermal photons, we find a substantial direct photon v_2 comparable to that of hadrons, whereas model calculations for thermal photons in this kinematic region significantly underpredict the observed v_2.
Exclusive rho super(0)-meson electroproduction is studied in the HERMES experiment, using a 27.6 GeV longitudinally polarized electron/positron beam and unpolarized hydrogen and deuterium targets in ...the kinematic region 0.5 GeV super(2)<Q super(2)<7.0 GeV super(2), 3.0 GeV <W<6.3 GeV, and -t'<0.4 GeV super(2). Real and imaginary parts of the ratios of the natural-parity-exchange helicity amplitudes T sub(11) ( $\gamma*}_{T} \rightarrow\rho_{T}$ ), T sub(01) ( $\gamma*}_{T} \rightarrow \rho_{L}$ ), T sub(10) ( $\gamma*}_{L} \rightarrow\rho_{T}$ ), and T sub(1-1) ( $\gamma*}_{-T} \rightarrow\rho_{T}$ ) to T sub(00) ( $\gamma*}_{L}\rightarrow\rho_{L}$ ) are extracted from the data. For the unnatural-parity-exchange amplitude U sub(11), the ratio |U sub(11)/T sub(00)| is obtained. The Q super(2) and t' dependences of these ratios are presented and compared with perturbative QCD predictions.
Transverse momentum spectra of electrons (p{sub T}{sup e}) from semileptonic weak decays of heavy-flavor mesons in the range of 0.3 < p{sub T}{sup e} < 9.0 GeV/c have been measured at midrapidity ...(|y| < 0.35) by the PHENIX experiment at the Relativistic Heavy Ion Collider in p+p and Au+Au collisions at {radical}s{sub NN} = 200 GeV. In addition, the azimuthal anisotropy parameter v{sub 2} has been measured for 0.3 < p{sub T}{sup e} < 5.0 GeV/c in Au+Au collisions. The substantial modification in the p{sub T}{sup e} spectra in Au+Au compared with p+p collisions as well as the nonzero v{sub 2} indicate substantial interactions and flow of heavy quarks in traversing the produced medium. Comparisons of these observables with detailed theoretical calculations can be used to identify the nature of these interactions and to quantify their extent.
The momentum distribution of electrons from semileptonic decays of charm and bottom quarks for midrapidity |y|<0.35 in p+p collisions at sq root(s)=200 GeV is measured by the PHENIX experiment at the ...Relativistic Heavy Ion Collider over the transverse momentum range 2<p{sub T}<7 GeV/c. The ratio of the yield of electrons from bottom to that from charm is presented. The ratio is determined using partial D/D->e{sup +}-K{sup +}-X (K unidentified) reconstruction. It is found that the yield of electrons from bottom becomes significant above 4 GeV/c in p{sub T}. A fixed-order-plus-next-to-leading-log perturbative quantum chromodynamics calculation agrees with the data within the theoretical and experimental uncertainties. The extracted total bottom production cross section at this energy is sigma{sub bb}=3.2{sub -1.1}{sup +1.2}(stat){sub -1.3}{sup +1.4}(syst)mub.
The double helicity asymmetry in neutral pion production for p{sub T}=1 to 12 GeV/c was measured with the PHENIX experiment to access the gluon-spin contribution, DELTAG, to the proton spin. Measured ...asymmetries are consistent with zero, and at a theory scale of mu{sup 2}=4 GeV{sup 2} a next to leading order QCD analysis gives DELTAG{sup 0.02,0.3}=0.2, with a constraint of -0.7<DELTAG{sup 0.02,0.3}<0.5 at DELTAchi{sup 2}=9 (approx3sigma) for the sampled gluon momentum fraction (x) range, 0.02 to 0.3. The results are obtained using predictions for the measured asymmetries generated from four representative fits to polarized deep inelastic scattering data. We also consider the dependence of the DELTAG constraint on the choice of the theoretical scale, a dominant uncertainty in these predictions.
Back-to-back hadron pair yields in d+Au and p+p collisions at sqrt(s_NN)=200 GeV were measured with the PHENIX detector at the Relativistic Heavy Ion Collider. Rapidity separated hadron pairs were ...detected with the trigger hadron at pseudorapidity |eta|<0.35 and the associated hadron at forward rapidity (deuteron direction, 3.0
For Au+Au collisions at 200 GeV, we measure neutral pion production with good statistics for transverse momentum, p{sub T}, up to 20 GeV/c. A fivefold suppression is found, which is essentially ...constant for 5<p{sub T}<20 GeV/c. Experimental uncertainties are small enough to constrain any model-dependent parametrization for the transport coefficient of the medium, e.g., <q-circumflex> in the parton quenching model. The spectral shape is similar for all collision classes, and the suppression does not saturate in Au+Au collisions.
Neutral pion transverse momentum (p{sub T}) spectra at midrapidity (|y| < or approx. 0.35) were measured in Cu+Cu collisions at {radical}(s{sub NN})=22.4, 62.4, and 200 GeV. Relative to {pi}{sup 0} ...yields in p+p collisions scaled by the number of inelastic nucleon-nucleon collisions (N{sub coll}) the {pi}{sup 0} yields for p{sub T}(> or approx. 2 GeV/c in central Cu+Cu collisions are suppressed at 62.4 and 200 GeV whereas an enhancement is observed at 22.4 GeV. A comparison with a jet-quenching model suggests that final state parton energy loss dominates in central Cu+Cu collisions at 62.4 and 200 GeV, while the enhancement at 22.4 GeV is consistent with nuclear modifications in the initial state alone.
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