Search Results (66 total)

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.

Antiparticle to particle ratios for identified protons, kaons, and pions at sqrt[s_NN]=62.4 and 200 GeV in Cu+Cu collisions are presented as a function of centrality for the midrapidity region of 0.2<η<1.4. No strong dependence on centrality is observed. For the 〈p̅ 〉/〈p〉 ratio at 〈p_T〉≈0.51 GeV/c, we observe an average value of 0.50±0.003_(stat)±0.04_(syst) and 0.77±0.008_(stat)±0.05_(syst) for the 10% most central collisions of 62.4 and 200 GeV Cu+Cu, respectively. The values for all three particle species measured at sqrt[s_NN]=200 GeV are in agreement within systematic uncertainties with that seen in both heavier and lighter systems measured at the same RHIC energy. This indicates that system size does not appear to play a strong role in determining the midrapidity chemical freeze-out properties affecting the antiparticle to particle ratios of the three most abundant particle species produced in these collisions.

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.

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.

This Letter presents measurements of the elliptic flow of charged particles as a function of pseudorapidity and centrality from Cu-Cu collisions at 62.4 and 200 GeV using the PHOBOS detector at the Relativistic Heavy Ion Collider. The elliptic flow in Cu-Cu collisions is found to be significant even for the most central events. For comparison with the Au-Au results, it is found that the detailed way in which the collision geometry (eccentricity) is estimated is of critical importance when scaling out system-size effects. A new form of eccentricity, called the participant eccentricity, is introduced which yields a scaled elliptic flow in the Cu-Cu system that has the same relative magnitude and qualitative features as that in the Au-Au system.

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.

We present results on two-particle angular correlations in proton-proton collisions at center-of-mass energies of 200 and 410 GeV. The PHOBOS experiment at the BNL Relativistic Heavy Ion Collider has a uniquely large coverage for charged particles, giving the opportunity to explore the correlations at both short- and long-range scales. At both energies, a complex two-dimensional correlation structure in Δη and Δϕ is observed. In the context of an independent cluster model of short-range correlations, the cluster size and its decay width are extracted from the two-particle pseudorapidity correlation function and compared with previous measurements in proton-proton and proton-antiproton collisions, as well as PYTHIA and HIJING predictions.

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.

Transverse momentum spectra of pions, kaons, protons, and antiprotons from Au+Au collisions at sqrt[s_NN] = 62.4 GeV have been measured by the PHOBOS experiment at the Relativistic Heavy Ion Collider at Brookhaven National Laboratory. The identification of particles relies on three different methods: low momentum particles stopping in the first detector layers; the specific energy loss (dE/dx) in the silicon spectrometer, and time-of-flight measurement. These methods cover the transverse momentum ranges 0.03–0.2, 0.2–1.0, and 0.5–3.0 GeV/c, respectively. Baryons are found to have substantially harder transverse momentum spectra than mesons. The p_T region in which the proton to pion ratio reaches unity in central Au+Au collisions at sqrt[s_NN] = 62.4 GeV fits into a smooth trend as a function of collision energy. At low transverse mass, the spectra of various species exhibit a significant deviation from transverse mass scaling. The observed particle yields at very low p_T are comparable to extrapolations from higher p_T for kaons, protons and antiprotons. By comparing our results to Au+Au collisions at sqrt[s_NN] = 200 GeV, we conclude that the net proton yield at midrapidity is proportional to the number of participant nucleons in the collision.

The momentum distribution of electrons from decays of heavy flavor (charm and bottom) for midrapidity |y|<0.35 in p+p collisions at sqrt[s]=200  GeV has been measured by the PHENIX experiment at the BNL Relativistic Heavy Ion Collider 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 perturbative QCD calculation agrees with the data within the theoretical and experimental uncertainties, with the data/theory ratio of 1.71±0.02^stat±0.18^sys for 0.3<p_T<9  GeV/c. The total charm production cross section at this energy has also been deduced to be σ_cc̅ =567±57^stat±193^sys  μb.

The PHOBOS experiment at the BNL Relativistic Heavy Ion Collider has measured the total multiplicity of primary charged particles as a function of collision centrality in Au+Au collisions at sqrt[s_NN]= 19.6, 130, and 200 GeV. An approximate independence of 〈N_ch〉/〈N_part/2〉 on the number of participating nucleons is observed, reminiscent of “wounded nucleon” scaling (N_ch∝N_part) observed in proton-nucleus collisions. Unlike p+A, the constant of proportionality does not seem to be set by the pp/p̅ p data at the same energy. Rather, there seems to be a surprising correspondence with the total multiplicity measured in e⁺e^- annihilations, as well as the rapidity shape measured over a large range. The energy dependence of the integrated multiplicity per participant pair shows that e⁺e^- and A+A data agree over a large range of center-of-mass energies (sqrt[s]>20 GeV), and pp/p̅ p data can be brought to agree approximately with the e⁺e^- data by correcting for the typical energy taken away by leading particles. This is suggestive of a mechanism for soft particle production that depends mainly on the amount of available energy. It is conjectured that the dominant distinction between A+A and p+p collisions is the multiple collisions per participant, which appears to be sufficient to substantially reduce the energy taken away by leading particles.

The charged-particle pseudorapidity density for Au+Au collisions at sqrt[s_NN]=62.4 GeV has been measured over a wide range of impact parameters and compared to results obtained at other energies. As a function of collision energy, the pseudorapidity distribution grows systematically both in height and width. The midrapidity density is found to grow approximately logarithmically between BNL Alternating Gradient Synchrotron (AGS) energies and the top BNL Relativistic Heavy Ion Collider (RHIC) energy. There is also an approximate factorization of the centrality and energy dependence of the midrapidity yields. The new results at sqrt[s_NN]=62.4 GeV confirm the previously observed phenomenon of “extended longitudinal scaling” in the pseudorapidity distributions when viewed in the rest frame of one of the colliding nuclei. It is also found that the evolution of the shape of the distribution with centrality is energy independent, when viewed in this reference frame. As a function of centrality, the total charged particle multiplicity scales linearly with the number of participant pairs as it was observed at other energies.

Forward-backward correlations of charged-particle multiplicities in symmetric bins in pseudorapidity are studied to gain insight into the underlying correlation structure of particle production in Au+Au collisions. The PHOBOS detector is used to measure integrated multiplicities in bins centered at η, defined within |η|<3, and covering intervals Δη. The variance σ_C² of a suitably defined forward-backward asymmetry variable C is calculated as a function of η,Δη, and centrality. It is found to be sensitive to short-range correlations, and the concept of “clustering” is used to interpret comparisons to phenomenological models.

We report on measurements of directed flow as a function of pseudorapidity in Au+Au collisions at energies of sqrt[s_NN]=19.6, 62.4, 130 and 200 GeV as measured by the PHOBOS detector at the BNL Relativistic Heavy Ion Collider. These results are particularly valuable because of the extensive, continuous pseudorapidity coverage of the PHOBOS detector. There is no significant indication of structure near midrapidity and the data surprisingly exhibit extended longitudinal scaling similar to that seen for elliptic flow and charged particle pseudorapidity density.

We present transverse momentum distributions of charged hadrons produced in Cu+Cu collisions at sqrt[s_NN]=62.4 and 200 GeV. The spectra are measured for transverse momenta of 0.25<p_T<5.0  GeV/c at sqrt[s_NN]=62.4  GeV and 0.25<p_T<7.0  GeV/c at sqrt[s_NN]=200  GeV, in a pseudorapidity range of 0.2<η<1.4. The nuclear modification factor R_AA is calculated relative to p+p data at both collision energies as a function of collision centrality. At a given collision energy and fractional cross section, R_AA is observed to be systematically larger in Cu+Cu collisions compared to Au+Au. However, for the same number of participating nucleons, R_AA is essentially the same in both systems over the measured range of p_T, in spite of the significantly different geometries of the Cu+Cu and Au+Au systems.

Two-particle correlations of identical charged pion pairs from Au+Au collisions at sqrt[s_NN]=62.4 and 200 GeV were measured by the PHOBOS experiment at BNL Relativistic Heavy Ion Collider (RHIC). Data for the 15% most central events were analyzed with Bertsch-Pratt and Yano-Koonin-Podgoretskii parametrizations using pairs with rapidities of 0.4<y_ππ<1.3 and transverse momenta 0.1<k_T<1.4 GeV/c. The Bertsch-Pratt radii R_o and R_ℓ decrease as a function of pair transverse momentum. R_o and R_s are independent of collision energy, while R_ℓ shows a slight increase. The source rapidity y_YKP scales roughly with the pair rapidity y_ππ, indicating strong dynamical correlations.

This Rapid Communication describes the measurement of elliptic flow for charged particles in Au+Au collisions at sqrt[s_NN]=200 GeV using the PHOBOS detector at the Relativistic Heavy Ion Collider. The measured azimuthal anisotropy is presented over a wide range of pseudorapidity for three broad collision centrality classes for the first time at this energy. Two distinct methods of extracting the flow signal were used to reduce systematic uncertainties. The elliptic flow falls sharply with increasing |η| at 200 GeV for all the centralities studied, as observed for minimum-bias collisions at sqrt[s_NN]=130 GeV.

The measured pseudorapidity distributions of primary charged particles over a wide pseudorapidity range of |η|≤5.4 and integrated charged particle multiplicities in d+Au collisions at sqrt[s_NN]=200  GeV are presented as a function of collision centrality. The longitudinal features of d+Au collisions at sqrt[s_NN]=200  GeV are found to be very similar to those seen in p+A collisions at lower energies. The total multiplicity of charged particles is found to scale with the total number of participants according to N_ch^dAu=1 / 2〈N_part〉N_ch^pp, and the energy dependence of the density of charged particles produced in the fragmentation region exhibits extended longitudinal scaling.

This Letter describes the measurement of the energy dependence of elliptic flow for charged particles in Au+Au collisions using the PHOBOS detector at the Relativistic Heavy Ion Collider. Data taken at collision energies of sqrt[s_NN]=19.6, 62.4, 130, and 200 GeV are shown over a wide range in pseudorapidity. These results, when plotted as a function of η^′=|η|-y_beam, scale with approximate linearity throughout η^′, implying no sharp changes in the dynamics of particle production as a function of pseudorapidity or increasing beam energy.