Search Results (109 total)

Neutral pion transverse momentum (p_T) spectra at midrapidity (|y|≲0.35) were measured in Cu+Cu collisions at sqrt[s_NN]=22.4, 62.4, and 200 GeV. Relative to π⁰ yields in p+p collisions scaled by the number of inelastic nucleon-nucleon collisions (N_coll) the π⁰ yields for p_T≳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.

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 sqrt[s_NN]=62.4 and 200 GeV, and Cu+Cu collisions at sqrt[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.

Yields for J/ψ production in Cu+Cu collisions at sqrt[s_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/ψ 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_part∼50, corresponding to a Bjorken energy density of at least 1.5  GeV/fm³.

Measurements in Au+Au collisions at sqrt[s_NN]=200  GeV of jet correlations for a trigger hadron at intermediate transverse momentum (p_T,trig) with associated mesons or baryons at lower p_T,assoc indicate strong modification of the away-side jet. The ratio of jet-associated baryons to mesons increases with centrality and p_T,assoc. For the most central collisions, the ratio is similar to that for inclusive measurements. This trend is incompatible with in-vacuum fragmentation but could be due to jetlike contributions from correlated soft partons, which recombine upon hadronization.

Azimuthal angle (Δϕ) correlations are presented for a broad range of transverse momentum (0.4<p_T<10 GeV/c) and centrality (0–92%) selections for charged hadrons from dijets in Au+Au collisions at sqrt[s_NN]=200 GeV. With increasing p_T, the away-side Δϕ distribution evolves from a broad and relatively flat shape to a concave shape, then to a convex shape. Comparisons with p+p data suggest that the away-side distribution can be divided into a partially suppressed “head” region centered at Δϕ~π, and an enhanced “shoulder” region centered at Δϕ~π±1.1. The p_T spectrum for the associated hadrons in the head region softens toward central collisions. The spectral slope for the shoulder region is independent of centrality and trigger p_T. The properties of the near-side distributions are also modified relative to those in p+p collisions, reflected by the broadening of the jet shape in Δϕ and Δη, and an enhancement of the per-trigger yield. However, these modifications seem to be limited to p_T≲4 GeV/c, above which both the hadron pair shape and per-trigger yield become similar to p+p collisions. These observations suggest that both the away- and near-side distributions contain a jet fragmentation component which dominates for p_T≳5 GeV/c and a medium-induced component which is important for p_T≲4 GeV/c. We also quantify the role of jets at intermediate and low p_T through the yield of jet-induced pairs in comparison with binary scaled p+p pair yield. The yield of jet-induced pairs is suppressed at high pair proxy energy (sum of the p_T magnitudes of the two hadrons) and is enhanced at low pair proxy energy. The former is consistent with jet quenching; the latter is consistent with the enhancement of soft hadron pairs due to transport of lost energy to lower p_T.

The PHENIX experiment has measured the suppression of semi-inclusive single high-transverse-momentum π⁰'s in Au+Au collisions at sqrt[s_NN]=200 GeV. The present understanding of this suppression is in terms of energy loss of the parent (fragmenting) parton in a dense color-charge medium. We have performed a quantitative comparison between various parton energy-loss models and our experimental data. The statistical point-to-point uncorrelated as well as correlated systematic uncertainties are taken into account in the comparison. We detail this methodology and the resulting constraint on the model parameters, such as the initial color-charge density dN^g/dy, the medium transport coefficient 〈q-^ 〉, or the initial energy-loss parameter ε₀. We find that high-transverse-momentum π⁰ suppression in Au+Au collisions has sufficient precision to constrain these model-dependent parameters at the ±20–25% (one standard deviation) level. These constraints include only the experimental uncertainties, and further studies are needed to compute the corresponding theoretical uncertainties.

A three-dimensional correlation function obtained from midrapidity, low p_T, pion pairs in central Au+Au collisions at sqrt[s_NN]=200  GeV is studied. The extracted model-independent source function indicates a long range tail in the directions of the pion pair transverse momentum (out) and the beam (long). A proper breakup time τ₀∼9  fm/c and a mean proper emission duration Δτ∼2  fm/c, leading to sizable emission time differences (⟨|Δt_LCM|⟩≈12  fm/c), are required to allow models to be successfully matched to these tails. The model comparisons also suggest an outside-in “burning” of the emission source reminiscent of many hydrodynamical models.

We present a new analysis of J/ψ production yields in deuteron-gold collisions at sqrt[s_NN]=200 GeV using data taken from the PHENIX experiment in 2003 and previously published in S. S. Adler [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.

We present transverse momentum (p_T) spectra of charged hadrons measured in deuteron-gold and nucleon-gold collisions at sqrt[s_NN]=200 GeV for four centrality classes. Nucleon-gold collisions were selected by tagging events in which a spectator nucleon was observed in one of two forward rapidity detectors. The spectra and yields were investigated as a function of the number of binary nucleon-nucleon collisions, ν, suffered by deuteron nucleons. A comparison of charged particle yields to those in p+p collisions show that yield per nucleon-nucleon collision saturates with ν for high momentum particles. We also present the charged hadron to neutral pion ratios as a function of p_T.

Azimuthal angle (Δϕ) correlations are presented for charged hadrons from dijets for 0.4<p_T<10 GeV/c in Au+Au collisions at sqrt[s_NN]=200 GeV. With increasing p_T, the away-side distribution evolves from a broad and relatively flat shape to a concave shape, then to a convex shape. Comparisons to p+p data suggest that the away-side can be divided into a partially suppressed “head” region centered at Δϕ~π and an enhanced “shoulder” region centered at Δϕ~π±1.1. The p_T spectrum for the head region softens toward central collisions, consistent with the onset of jet quenching. The spectral slope for the shoulder region is independent of centrality and trigger p_T, which offers constraints on energy transport mechanisms and suggests that it contains the medium response to energetic jets.

The PHENIX experiment presents results from the Relativistic Heavy Ion Collider 2005 run with polarized proton collisions at sqrt[s]=200  GeV, for inclusive π⁰ production at midrapidity. Unpolarized cross section results are given for transverse momenta p_T=0.5 to 20  GeV/c, extending the range of published data to both lower and higher p_T. The cross section is described well for p_T<1  GeV/c by an exponential in p_T, and, for p_T>2  GeV/c, by perturbative QCD. Double helicity asymmetries A_LL are presented based on a factor of 5 improvement in uncertainties as compared to previously published results, due to both an improved beam polarization of 50%, and to higher integrated luminosity. These measurements are sensitive to the gluon polarization in the proton. Using one representative model of gluon polarization it is demonstrated that the gluon spin contribution to the proton spin is significantly constrained.

π⁺ and π^- elastic and inelastic scattering to the 2₁⁺(4.44 MeV) state on ¹²C at 995 MeV/c were measured over an angular range for elastic-scattering from 5.4^° to 28.2^° and for inelastic scattering from 15.2^° to 22.8^°. Both of the elastic-scattering data sets were well reproduced by first-order factorized momentum-space optical potential calculations with free π-N elementary amplitudes and three different ground state densities, which were deduced from the charge density and microscopic model calculations, the cluster model and the shell model. We also extracted σ_tot,σ_el, and σ_R phenomenologically and compared them with a Fermi averaging model. The inelastic cross sections of π⁺-¹²C and π^--¹²C were compared with the DWIA calculations, one using the transition density(0⁺→2₁⁺) deduced by the cluster model and the other using the transition density deduced by the shell model.

The pressure-induced structural transition of YH₃ has been investigated by x-ray diffraction experiments under hydrostatic pressure at room temperature. The diffraction patterns of the intermediate state between the hexagonal and cubic phases are interpreted in terms of long-period polytypes with the periodic arrangements of hexagonal-type and fcc-type stacking layers of the Y metals. The hydrogen-hydrogen repulsive interactions and hydrogen-metal bonding likely play a dominant role in the gradual hexagonal-fcc lattice conversion via the long-period polytypic structures.

Differential elliptic flow (v₂) for ϕ mesons and (anti)deuterons (d̅ )d is measured for Au+Au collisions at sqrt[s_NN]=200  GeV. The v₂ for ϕ mesons follows the trend of lighter π^± and K^± mesons, suggesting that ordinary hadrons interacting with standard hadronic cross sections are not the primary driver for elliptic flow development. The v₂ values for (d̅ )d suggest that elliptic flow is additive for composite particles. This further validation of the universal scaling of v₂ per constituent quark for baryons and mesons suggests that partonic collectivity dominates the transverse expansion dynamics.

Raman and visible absorption spectra of YH₃ were measured at high pressures, in order to clarify the structural and electronic phase transitions. The Raman results revealed the presence of an intermediate phase at 9–24  GPa between the low-pressure hexagonal and high-pressure fcc phases. The Y-framework structure and the position of interstitial H atoms are changed on the transition to intermediate phase. The results of the absorption measurements demonstrate that the optical band gap starts to close in response to the phase transition.

J/ψ production in p+p collisions at sqrt[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_T<9  GeV/c. The size of the present data set allows a detailed measurement of both the p_T and the rapidity distributions and is sufficient to constrain production models. The total cross section times the branching ratio is B_llσ_pp^J/ψ=178±3^stat±53^sys±18^norm  nb.

We present azimuthal angle correlations of intermediate transverse momentum (1–4  GeV/c) hadrons from dijets in Cu+Cu and Au+Au collisions at sqrt[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.

The PHENIX experiment at the BNL Relativistic Heavy Ion Collider (RHIC)has measured J/ψ production for rapidities -2.2<y<2.2 in Au+Au collisions at sqrt[s_NN]=200  GeV. The J/ψ invariant yield and nuclear modification factor R_AA as a function of centrality,transverse momentum, and rapidity are reported. A suppression of J/ψ relative to binary collision scaling of proton-protonreaction yields is observed. Models which describe the lower energy J/ψ data at the CERN Super Proton Synchrotron invoking only J/ψ 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 PHENIX experiment at RHIC has measured the invariant cross section for ω-meson production at midrapidity in the transverse momentum range 2.5<p_T<9.25 GeV/c in p+p and d+Au collisions at sqrt[s_NN]=200 GeV. Measurements in two decay channels (ω→π⁰π⁺π^- and ω→π⁰γ) yield consistent results, and the reconstructed ω mass agrees with the accepted value within the p_T range of the measurements. The ω/π⁰ ratio is found to be 0.85±0.05^stat±0.09^sys in p+p and 0.94±0.08^stat±0.12^sys in d+Au collisions, independent of p_T. The nuclear modification factor R_dA^ω is 1.03±0.12^stat±0.21^sys and 0.83±0.21^stat±0.17^sys in minimum bias and central (0–20%) d+Au collisions, respectively.

The dependence of transverse momentum spectra of neutral pions and η mesons with p_T<16  GeV/c and p_T<12  GeV/c, respectively, on the centrality of the collision has been measured at midrapidity by the PHENIX experiment at the BNL Relativistic Heavy Ion Collider (RHIC) in d+Au collisions at sqrt[s_NN]=200  GeV. The measured yields are compared to those in p+p collisions at the same sqrt[s_NN] scaled by the number of underlying nucleon-nucleon collisions in d+Au. At all centralities, the yield ratios show no suppression, in contrast to the strong suppression seen for central Au+Au collisions at RHIC. Only a weak p_T and centrality dependence can be observed.

The PHENIX experiment at the BNL Relativistic Heavy Ion Collider (RHIC) has measured electrons with 0.3<p_T<9  GeV/c at midrapidity (|y|<0.35) from heavy-flavor (charm and bottom) decays in Au+Au collisions at sqrt[s_NN]=200  GeV. The nuclear modification factor R_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 v₂ with respect to the reaction plane is observed for 0.5<p_T<5  GeV/c indicating substantial heavy-flavor elliptic flow. Both R_AA and v₂ show a p_T dependence different from those of neutral pions. A comparison to transport models which simultaneously describe R_AA(p_T) and v₂(p_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 (v₂) for Au+Au and Cu+Cu collisions at sqrt[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.

Inclusive transverse momentum spectra of η mesons in the range p_T≈2–12 GeV/c have been measured at midrapidity (|η|<0.35) by the PHENIX experiment at RHIC in p+p,d+Au, and Au+Au collisions at sqrt[s_NN]=200 GeV. The η mesons are reconstructed through their η→γ γ channel for the three colliding systems as well as through the η→π⁰π⁺π^- decay mode in p+p and d+Au collisions. The nuclear modification factor in d+Au collisions, R_dAu(p_T)≈1.0–1.1, suggests at most only modest p_T broadening (“Cronin enhancement”). In central Au+Au reactions, the η yields are significantly suppressed, with R_AuAu(p_T)≈0.2. The ratio of η to π⁰ yields is approximately constant as a function of p_T for the three colliding systems in agreement with the high-p_T world average of R_η/π⁰≈0.5 in hadron-hadron, hadron-nucleus, and nucleus-nucleus collisions for a wide range of center-of-mass energies (sqrt[s_NN]≈3–1800 GeV) as well as, for high scaled momentum x_p, in e⁺e^- annihilations at sqrt[s]=91.2 GeV. These results are consistent with a scenario where high-p_T η production in nuclear collisions at the Relativistic Heavy Ion Collider is largely unaffected by initial-state effects but where light-quark mesons (π⁰,η) are equally suppressed due to final-state interactions of the parent partons in the dense medium produced in Au+Au reactions.

The H-dibaryon resonance was sought by a ΛΛ invariant mass spectrum that was obtained by the ¹²C (K^-,K⁺ΛΛX) reactions. We observed a bump near the ΛΛ threshold, as reported by the previous experiment (KEK E224), with better statistics. Data were compared with results of a cascade model calculation including ΛΛ final state interactions consistent with the newly measured binding energy of _ΛΛ⁶He. No significant enhancements above levels of the model predictions were observed. The resulting upper limit for the production cross section of the H with a mass range between the ΛΛ and ΞN threshold is found to be 2.1 ± 0.6 (stat.) ± 0.1 (syst.) μb/sr at a 90% confidence level.