[en] We present a new analysis of J/ψ production yields in deuteron-gold collisions at √(s_NN)=200 GeV using data taken from the PHENIX experiment in 2003 and previously published in S. S. Adler et al. [Phys. Rev. Lett 96, 012304 (2006)]. The high statistics proton-proton J/ψ data taken in 2005 are used to improve the baseline measurement and thus construct updated cold nuclear matter modification factors (R_dAu). A suppression of J/ψ in cold nuclear matter is observed as one goes forward in rapidity (in the deuteron-going direction), corresponding to a region more sensitive to initial-state low-x gluons in the gold nucleus. The measured nuclear modification factors are compared to theoretical calculations of nuclear shadowing to which a J/ψ (or precursor) breakup cross section is added. Breakup cross sections of σ_breakup=2.8_-1.4^+1.7 (2.2_-1.5^+1.6) mb are obtained by fitting these calculations to the data using two different models of nuclear shadowing. These breakup cross-section values are consistent within large uncertainties with the 4.2±0.5 mb determined at lower collision energies. Projecting this range of cold nuclear matter effects to copper-copper and gold-gold collisions reveals that the current constraints are not sufficient to firmly quantify the additional hot nuclear matter effect

[en] A comprehensive survey of event-by-event fluctuations of charged hadron multiplicity in relativistic heavy ions is presented. The survey covers Au+Au collisions at √(s_NN)=62.4 and 200 GeV, and Cu+Cu collisions at √(s_NN)=22.5,62.4, and 200 GeV. Fluctuations are measured as a function of collision centrality, transverse momentum range, and charge sign. After correcting for nondynamical fluctuations due to fluctuations in the collision geometry within a centrality bin, the remaining dynamical fluctuations expressed as the variance normalized by the mean tend to decrease with increasing centrality. The dynamical fluctuations are consistent with or below the expectation from a superposition of participant nucleon-nucleon collisions based upon p+p data, indicating that this dataset does not exhibit evidence of critical behavior in terms of the compressibility of the system. A comparison of the data with a model where hadrons are independently emitted from a number of hadron clusters suggests that the mean number of hadrons per cluster is small in heavy ion collisions

[en] We present azimuthal angle correlations of intermediate transverse momentum (1-4 GeV/c) hadrons from dijets in Cu+Cu and Au+Au collisions at √(s_NN)=62.4 and 200 GeV. The away-side dijet induced azimuthal correlation is broadened, non-Gaussian, and peaked away from Δφ=π 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

[en] We report on the event structure and double helicity asymmetry (A_LL) of jet production in longitudinally polarized p+p collisions at √(s)=200 GeV. Photons and charged particles were measured by the PHENIX experiment at midrapidity |η|<0.35 with the requirement of a high-momentum (>2 GeV/c) photon in the event. Event structure, such as multiplicity, p_T density and thrust in the PHENIX acceptance, were measured and compared with the results from the pythia event generator and the geant detector simulation. The shape of jets and the underlying event were well reproduced at this collision energy. For the measurement of jet A_LL, photons and charged particles were clustered with a seed-cone algorithm to obtain the cluster p_T sum (p_T^reco). The effect of detector response and the underlying events on p_T^reco was evaluated with the simulation. The production rate of reconstructed jets is satisfactorily reproduced with the next-to-leading-order and perturbative quantum chromodynamics jet production cross section. For 4< p_T^reco<12 GeV/c with an average beam polarization of < P>=49% we measured A_LL=-0.0014±0.0037^stat at the lowest p_T^reco bin (4-5 GeV/c) and -0.0181±0.0282^stat at the highest p_T^reco bin (10-12 GeV/c) with a beam polarization scale error of 9.4% and a p_T scale error of 10%. Jets in the measured p_T^reco range arise primarily from hard-scattered gluons with momentum fraction 0.02< x<0.3 according to pythia. The measured A_LL is compared with predictions that assume various ΔG(x) distributions based on the Gluck-Reya-Stratmann-Vogelsang parameterization. The present result imposes the limit -1.1<∫_0.02^0.3dxΔG(x,μ²=1 GeV²)<0.4 at 95% confidence level or ∫_0.02^0.3dxΔG(x,μ²=1 GeV²)<0.5 at 99% confidence level.

[en] New measurements by the PHENIX experiment at the Relativistic Heavy Ion Collider for η production at midrapidity as a function of transverse momentum (p_T) and collision centrality in √(s_NN)=200 GeV Au+Au and p+p collisions are presented. They indicate nuclear modification factors (R_AA) which are similar in both magnitude and trend to those found in earlier π⁰ measurements. Linear fits to R_AA as a function of p_T in 5-20 GeV/c show that the slope is consistent with zero within two standard deviations at all centralities, although a slow rise cannot be excluded. Having different statistical and systematic uncertainties, the π⁰ and η measurements are complementary at high p_T; thus, along with the extended p_T range of these data they can provide additional constraints for theoretical modeling and the extraction of transport properties.

[en] Pair correlations between large transverse momentum neutral pion triggers (p_T=4--7 GeV/c) and charged hadron partners (p_T=3--7 GeV/c) in central (0%-20%) and midcentral (20%-60%) Au+Au collisions at √(s_NN)=200 GeV are presented as a function of trigger orientation with respect to the reaction plane. The particles are at larger momentum than where jet shape modifications have been observed, and the correlations are sensitive to the energy loss of partons traveling through hot dense matter. An out-of-plane trigger particle produces only 26±20% of the away-side pairs that are observed opposite of an in-plane trigger particle for midcentral (20%-60%) collisions. In contrast, near-side jet fragments are consistent with no suppression or dependence on trigger orientation with respect to the reaction plane. These observations are qualitatively consistent with a picture of little near-side parton energy loss either due to surface bias or fluctuations and increased away-side parton energy loss due to a long path through the medium. The away-side suppression as a function of reaction-plane angle is shown to be sensitive to both the energy loss mechanism and the space-time evolution of heavy-ion collisions.

[en] We have measured the azimuthal anisotropy of π⁰ production for 1< p_T<18 GeV/c for Au+Au collisions at √(s_NN)=200 GeV. The observed anisotropy shows a gradual decrease for 3 < or approx. p_T < or approx.l 7-10 GeV/c, but remains positive beyond 10 GeV/c. The magnitude of this anisotropy is underpredicted, up to at least ∼10 GeV/c, by current perturbative QCD (PQCD) energy-loss model calculations. An estimate of the increase in anisotropy expected from initial-geometry modification due to gluon saturation effects and fluctuations is insufficient to account for this discrepancy. Calculations that implement a path-length dependence steeper than what is implied by current PQCD energy-loss models show reasonable agreement with the data.

[en] Differential measurements of the elliptic (v₂) and hexadecapole (v₄) Fourier flow coefficients are reported for charged hadrons as a function of transverse momentum (p_T) and collision centrality or number of participant nucleons (N_part) for Au+Au collisions at √(s_NN)=200 GeV. The v_2,4 measurements at pseudorapidity |η|≤0.35, obtained with four separate reaction-plane detectors positioned in the range 1.0<|η|<3.9, show good agreement, indicating the absence of significant Δη-dependent nonflow correlations. Sizable values for v₄(p_T) are observed with a ratio v₄(p_T,N_part)/v₂²(p_T,N_part)≅0.8 for 50 < or approx. IN_part < or approx. 200, which is compatible with the combined effects of a finite viscosity and initial eccentricity fluctuations. For N_part > or approx. 200 this ratio increases up to 1.7 in the most central collisions.

[en] Measurements of electrons from the decay of open-heavy-flavor mesons have shown that the yields are suppressed in Au+Au collisions compared to expectations from binary-scaled p+p collisions. These measurements indicate that charm and bottom quarks interact with the hot dense matter produced in heavy-ion collisions much more than expected. Here we extend these studies to two-particle correlations where one particle is an electron from the decay of a heavy-flavor meson and the other is a charged hadron from either the decay of the heavy meson or from jet fragmentation. These measurements provide more detailed information about the interactions between heavy quarks and the matter, such as whether the modification of the away-side-jet shape seen in hadron-hadron correlations is present when the trigger particle is from heavy-meson decay and whether the overall level of away-side-jet suppression is consistent. We statistically subtract correlations of electrons arising from background sources from the inclusive electron-hadron correlations and obtain two-particle azimuthal correlations at √(s_NN)=200 GeV between electrons from heavy-flavor decay with charged hadrons in p+p and also first results in Au+Au collisions. We find the away-side-jet shape and yield to be modified in Au+Au collisions compared to p+p collisions.

[en] Differential measurements of elliptic flow (v₂) for Au+Au and Cu+Cu collisions at √(s_NN)=200 GeV are used to test and validate predictions from perfect fluid hydrodynamics for scaling of v₂ with eccentricity, system size, and transverse kinetic energy (KE_T). For KE_T≡m_T-m up to ∼1 GeV the scaling is compatible with hydrodynamic expansion of a thermalized fluid. For large values of KE_T mesons and baryons scale separately. Quark number scaling reveals a universal scaling of v₂ for both mesons and baryons over the full KE_T range for Au+Au. For Au+Au and Cu+Cu the scaling is more pronounced in terms of KE_T, rather than transverse momentum