Differential elliptic flow (v_2) for phi mesons and (anti)deuterons (d^bar)d is measured for Au+Au collisions at sqrt(s_NN) = 200 GeV. The v_2 for phi mesons follows the trend of lighter pi^+/- and ...K^+/- mesons, suggesting that ordinary hadrons interacting with standard hadronic cross sections are not the primary driver for elliptic flow development. The v_2 values for (d^bar)d suggest that elliptic flow is additive for composite particles. This further validation of the universal scaling of v_2 per constituent quark for baryons and mesons suggests that partonic collectivity dominates the transverse expansion dynamics.
Results from the PHENIX experiment of measurements of high-\(p_{\rm T}\) particle production presented at the Hard Probes 2004 Conference are summarized. This paper focuses on a sub-set of the ...measurements presented at the conference, namely the suppression of \(\pi^0\) production at moderate to high \(p_{\rm T}\) as a function of angle with respect to the collision reaction plane, \(\Delta\phi\), for different collision centralities. The data are presented in the form of nuclear modification factor as a function of angle with respect to the reaction plane, \(R_{AA} (\Delta \phi)\). The data are analyzed using empirical estimates of the medium-induced energy loss obtained from the \(R_{AA} (\Delta \phi)\) values. A geometric analysis is performed with the goal of understanding the simultaneous dependence of RAA on \(\Delta\phi\) and centrality. We find that the centrality and \(\Delta\phi\) dependence of the \(\pi^0\) suppression can be made approximately consistent using an admittedly over-simplistic description of the geometry of the jet propagation in the medium but only if the energy loss is effectively reduced for short parton path lengths in the medium. We find that with a more "canonical" treatment of the quenching geometry, the \(\pi^0\) suppression varies more rapidly with \(\Delta\phi\) than would be expected from the centrality dependence of the suppression.
Measurements of neutral pion production at midrapidity in {radical}(s{sub NN}) = 200 GeV Au+Au collisions as a function of transverse momentum, p{sub T}, collision centrality, and angle with respect ...to reaction plane are presented. The data represent the final {pi}{sup 0} results from the PHENIX experiment for the first RHIC Au+Au run at design center-of-mass-energy. They include additional data obtained using the PHENIX Level-2 trigger with more than a factor of three increase in statistics over previously published results for p{sub T} > 6 GeV/c. We evaluate the suppression in the yield of high-p{sub T} {pi}{sup 0}'s relative to point-like scaling expectations using the nuclear modification factor R{sub AA}. We present the p{sub T} dependence of R{sub AA} for nine bins in collision centrality. We separately integrate R{sub AA} over larger p{sub T} bins to show more precisely the centrality dependence of the high-p{sub T} suppression. We then evaluate the dependence of the high-p{sub T} suppression on the emission angle {Delta}{phi} of the pions with respect to event reaction plane for 7 bins in collision centrality. We show that the yields of high-p{sub T} {pi}{sup 0}'s vary strongly with {Delta}{phi}, consistent with prior measurements. We show that this variation persists in the most peripheral bin accessible in this analysis. For the peripheral bins we observe no suppression for neutral pions produced aligned with the reaction plane while the yield of {pi}{sup 0}'s produced perpendicular to the reaction plane is suppressed by more than a factor of 2. We analyze the combined centrality and {Delta}{phi} dependence of the {pi}{sup 0} suppression in different p{sub T} bins using different possible descriptions of parton energy loss dependence on jet path-length averages to determine whether a single geometric picture can explain the observed suppression pattern.
We present results on charged particle (
dN
ch
/
dη) and transverse energy densities (
dE
T/
dη) measured at mid-rapidity in Au
Au collisions at
S
N
N
=
200
GeV. The mean transverse energy per ...charged particle is derived. The results are presented as a function of centrality, which is defined by the number of participating nucleons (
N
p
), and compared to results obtained in Au
Au collisions at
S
N
N
=
130
GeV. A comparison with calculations from various theoretical models is performed.
Recent results on low mass dilepton measurements from the PHENIX experiment are reported. Invariant mass spectra of \(\phi\rightarrow e^ + e^-\) are measured for the first time in Au-Au collisions at ...\(\sqrt{s_{NN}} = 200\) GeV in Run2. In d-Au collisions, the yields and MT slopes of both \(\phi\rightarrow e^ + e^-\) and \(\phi \rightarrow K^ + K^-\) are measured. Both results are consistent with each other within errors. In the future, a Hadron Blind Detector will be installed in PHENIX which will enhance our capabilities of rejecting external photon conversions and Dalitz pairs, that will result in a significant reduction of the large combinatorial background.
Deuteron-gold (d+Au) collisions at the Relativistic Heavy Ion Collider provide ideal platforms for testing QCD theories in dense nuclear matter at high energy. In particular, models suggesting strong ...saturation effects for partons carrying small nucleon momentum fraction (x) predict modifications to jet production at forward rapidity (deuteron-going direction) in d+Au collisions. We report on two-particle azimuthal angle correlations between charged hadrons at forward/backward (deuteron/gold going direction) rapidity and charged hadrons at midrapidity in d+Au and p+p collisions at sqrt(ssub NN)=200 GeV. Jet structures observed in the correlations are quantified in terms of the conditional yield and angular width of away-side partners. The kinematic region studied here samples partons in the gold nucleus with x~0.1 to ~0.01. Within this range, we find no x dependence of the jet structure in d+Au collisions.
The momentum distribution of electrons from decays of heavy flavor (charm and beauty) for midrapidity |y| < 0.35 in p+p collisions at sqrt(s) = 200 GeV has been measured by the PHENIX experiment at ...the Relativistic Heavy Ion Collider (RHIC) over the transverse momentum range 0.3 < p_T < 9 GeV/c. Two independent methods have been used to determine the heavy flavor yields, and the results are in good agreement with each other. A fixed-order-plus-next-to-leading-log pQCD calculation agrees with the data within the theoretical and experimental uncertainties, with the data/theory ratio of 1.72 +/- 0.02^stat +/- 0.19^sys for 0.3 < p_T < 9 GeV/c. The total charm production cross section at this energy has also been deduced to be sigma_(c c^bar) = 567 +/- 57^stat +/- 224^sys micro barns.
J/psi production in d+Au and p+p collisions at sqrt(s_NN) = 200 GeV has been measured by the PHENIX experiment at rapidities -2.2 < y < +2.4. The cross sections and nuclear dependence of J/\psi ...production versus rapidity, transverse momentum, and centrality are obtained and compared to lower energy p+A results and to theoretical models. The observed nuclear dependence in d+Au collisions is found to be modest, suggesting that the absorption in the final state is weak and the shadowing of the gluon distributions is small and consistent with DGLAP-based parameterizations that fit deep-inelastic scattering and Drell-Yan data at lower energies.
The PHENIX experiment at RHIC has observed a large enhancement of baryon and anti-baryon production at ~ 2-5 GeV/c, compared to expectations from jet fragmentation. While a number of theoretical ...interpretations of the data are available, there is not yet a definitive answer to the “baryon puzzle”. We investigate the centrality dependence of -meson production at mid-rapidity in Au + Au collisions with $sqrt {^{s}NN}=200 GeV$. Comparison with the proton and anti-proton spectra reveal similar shapes, as expected for soft production described by hydrodynamics. However, the absolute yields show a different centrality dependence. The nuclear modification factors for $\phi$ are similar to those of pions, rather than (anti)protons that have similar mass. At intermediate , baryon/meson effects seem to be more important than the mass effects, in support of recombination models.
The invariant differential cross section for inclusive electron production in p+p collisions at sqrt(s) = 200 GeV has been measured by the PHENIX experiment at the Relativistic Heavy Ion Collider ...over the transverse momentum range 0.4 <= p_T <= 5.0 GeV/c at midrapidity (eta <= 0.35). The contribution to the inclusive electron spectrum from semileptonic decays of hadrons carrying heavy flavor, i.e. charm quarks or, at high p_T, bottom quarks, is determined via three independent methods. The resulting electron spectrum from heavy flavor decays is compared to recent leading and next-to-leading order perturbative QCD calculations. The total cross section of charm quark-antiquark pair production is determined as sigma_(c c^bar) = 0.92 +/- 0.15 (stat.) +- 0.54 (sys.) mb.
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