Search Results (57 total)

We report on p-Λ,p-Λ̅ ,p̅ -Λ, and p̅ -Λ̅ correlation functions constructed in central Au-Au collisions at sqrt[s_NN]=200 GeV by the STAR experiment at RHIC. The proton and lambda source size is inferred from the p-Λ and p̅ -Λ̅ correlation functions. It is found to be smaller than the pion source size also measured by the STAR experiment at smaller transverse masses, in agreement with a scenario of a strong universal collective flow. The p-Λ̅ and p̅ -Λ correlation functions, which are measured for the first time, exhibit a large anticorrelation. Annihilation channels and/or a negative real part of the spin-averaged scattering length must be included in the final-state interactions calculation to reproduce the measured correlation function.

Measurements of the production of forward π⁰ mesons from p+p and d+Au collisions at sqrt[s_NN]=200  GeV are reported. The p+p yield generally agrees with next-to-leading order perturbative QCD calculations. The d+Au yield per binary collision is suppressed as η increases, decreasing to ∼30% of the p+p yield at ⟨η⟩=4.00, well below shadowing expectations. Exploratory measurements of azimuthal correlations of the forward π⁰ with charged hadrons at η≈0 show a recoil peak in p+p that is suppressed in d+Au at low pion energy. These observations are qualitatively consistent with a saturation picture of the low-x gluon structure of heavy nuclei.

We present the centrality-dependent measurement of multiplicity and pseudorapidity distributions of charged particles and photons in Au+Au collisions at sqrt[s_NN]=62.4 GeV. The charged particles and photons are measured in the pseudorapidity region 2.9≤η≤3.9 and 2.3≤η≤3.7, respectively. We have studied the scaling of particle production with the number of participating nucleons and the number of binary collisions. The photon and charged particle production in the measured pseudorapidity range has been shown to be consistent with energy-independent limiting fragmentation behavior. Photons are observed to follow a centrality-independent limiting fragmentation behavior, while for charged particles it is centrality dependent. We have carried out a comparative study of the pseudorapidity distributions of positively charged hadrons, negatively charged hadrons, photons, pions, and net protons in nucleus-nucleus collisions and pseudorapidity distributions from p+p collisions. From these comparisons, we conclude that baryons in the inclusive charged particle distribution are responsible for the observed centrality dependence of limiting fragmentation. The mesons are found to follow an energy-independent behavior of limiting fragmentation, whereas the behavior of baryons is energy dependent.

We present the directed flow (v₁) measured in Au+Au collisions at sqrt[s_NN]=62.4 GeV in the midpseudorapidity region |η|<1.3 and in the forward pseudorapidity region 2.5<|η|<4.0. The results are obtained using the three-particle cumulant method, the event plane method with mixed harmonics, and for the first time at the Relativistic Heavy Ion Collider, the standard method with the event plane reconstructed from spectator neutrons. Results from all three methods are in good agreement. Over the pseudorapidity range studied, charged particle directed flow is in the direction opposite to that of fragmentation neutrons.

We present results for two-particle transverse momentum correlations, 〈Δp_t,iΔp_t,j〉, as a function of event centrality for Au+Au collisions at sqrt[s_NN]=20, 62, 130, and 200 GeV at the BNL Relativistic Heavy Ion Collider. We observe correlations decreasing with centrality that are similar at all four incident energies. The correlations multiplied by the multiplicity density increase with incident energy, and the centrality dependence may show evidence of processes such as thermalization, jet production, or the saturation of transverse flow. The square root of the correlations divided by the event-wise average transverse momentum per event shows little or no beam energy dependence and generally agrees with previous measurements made at the CERN Super Proton Synchrotron.

Charged hadrons in 0.15<p_⊥<4  GeV/c associated with particles of p_⊥^trig>4  GeV/c are reconstructed in pp and Au+Au collisions at sqrt[s_NN]=200  GeV. The associated multiplicity and p_⊥ magnitude sum are found to increase from pp to central Au+Au collisions. The associated p_⊥ distributions, while similar in shape on the nearside, are significantly softened on the awayside in central Au+Au relative to pp and not much harder than that of inclusive hadrons. The results, consistent with jet quenching, suggest that the awayside fragments approach equilibration with the medium traversed.

We report on the first measurement of elliptic flow ν₂(p_T) of multistrange baryons Ξ^-+Ξ̅ ⁺ and Ω^-+Ω̅ ⁺ in heavy-ion collisions. In minimum-bias Au+Au collisions at sqrt[s_NN]=200  GeV, a significant amount of elliptic flow, comparable to other nonstrange baryons, is observed for multistrange baryons which are expected to be particularly sensitive to the dynamics of the partonic stage of heavy-ion collisions. The p_T dependence of ν₂ of the multistrange baryons confirms the number of constituent quark scaling previously observed for lighter hadrons. These results support the idea that a substantial fraction of the observed collective motion is developed at the early partonic stage in ultrarelativistic nuclear collisions at the Relativistic Heavy Ion Collider.

We present the first measurement of pseudorapidity distribution of photons in the region 2.3≤η≤3.7 for different centralities in Au+Au collisions at sqrt[s_NN]=62.4  GeV. We find that the photon yield scales with the number of participating nucleons at all collision centralities studied. The pseudorapidity distribution of photons, dominated by π⁰ decays, has been compared to those of charged pions, photons, and inclusive charged particles from heavy-ion and nucleon-nucleon collisions at various energies. The photon production has been shown to be consistent with the energy and centrality independent limiting fragmentation scenario.

The results from the STAR Collaboration on directed flow (v₁), elliptic flow (v₂), and the fourth harmonic (v₄) in the anisotropic azimuthal distribution of particles from Au+Au collisions at sqrt[s_NN]=200  GeV are summarized and compared with results from other experiments and theoretical models. Results for identified particles are presented and fit with a blast-wave model. Different anisotropic flow analysis methods are compared and nonflow effects are extracted from the data. For v₂, scaling with the number of constituent quarks and parton coalescence are discussed. For v₄, scaling with v₂² and quark coalescence are discussed.

We present the first large-acceptance measurement of event-wise mean transverse momentum 〈p_t〉 fluctuations for Au-Au collisions at nucleon-nucleon center-of-momentum collision energy sqrt[s_NN] = 130 GeV. The observed nonstatistical 〈p_t〉 fluctuations substantially exceed in magnitude fluctuations expected from the finite number of particles produced in a typical collision. The r.m.s. fractional width excess of the event-wise 〈p_t〉 distribution is 13.7±0.1(stat) ±1.3(syst)% relative to a statistical reference, for the 15% most-central collisions and for charged hadrons within pseudorapidity range |η|<1,2π azimuth, and 0.15≤p_t  ≤  2 GeV/c. The width excess varies smoothly but nonmonotonically with collision centrality and does not display rapid changes with centrality which might indicate the presence of critical fluctuations. The reported 〈p_t〉 fluctuation excess is qualitatively larger than those observed at lower energies and differs markedly from theoretical expectations. Contributions to 〈p_t〉 fluctuations from semihard parton scattering in the initial state and dissipation in the bulk colored medium are discussed.

The short-lived K(892)^* resonance provides an efficient tool to probe properties of the hot and dense medium produced in relativistic heavy-ion collisions. We report measurements of K^* in sqrt[s_NN]=200  GeV Au+Au and p+p collisions reconstructed via its hadronic decay channels K(892)^*0→Kπ and K(892)^*±→K_S⁰π^± using the STAR detector at the Relativistic Heavy Ion Collider at Brookhaven National Laboratory. The K^*0 mass has been studied as a function of p_T in minimum bias p+p and central Au+Au collisions. The K^*  p_T spectra for minimum bias p+p interactions and for Au+Au collisions in different centralities are presented. The K^*/K yield ratios for all centralities in Au+Au collisions are found to be significantly lower than the ratio in minimum bias p+p collisions, indicating the importance of hadronic interactions between chemical and kinetic freeze-outs. A significant nonzero K^*0 elliptic flow (v₂) is observed in Au+Au collisions and is compared to the K_S⁰ and Λ v₂. The nuclear modification factor of K^* at intermediate p_T is similar to that of K_S⁰ but different from Λ. This establishes a baryon-meson effect over a mass effect in the particle production at intermediate p_T (2<p_T≤4  GeV/c).

We present a systematic analysis of two-pion interferometry in Au+Au collisions at sqrt[s_NN]=200  GeV using the STAR detector at Relativistic Heavy Ion Collider. We extract the Hanbury-Brown and Twiss radii and study their multiplicity, transverse momentum, and azimuthal angle dependence. The Gaussianness of the correlation function is studied. Estimates of the geometrical and dynamical structure of the freeze-out source are extracted by fits with blast-wave parametrizations. The expansion of the source and its relation with the initial energy density distribution is studied.

Correlations in the hadron distributions produced in relativistic Au+Au collisions are studied in the discrete wavelet expansion method. The analysis is performed in the space of pseudorapidity (|η|≤1) and azimuth(full 2π) in bins of transverse momentum (p_t) from 0.14≤p_t≤2.1  GeV/c. In peripheral Au+Au collisions a correlation structure ascribed to minijet fragmentation is observed. It evolves with collision centrality and p_t in a way not seen before, which suggests strong dissipation of minijet fragmentation in the longitudinally expanding medium.

Midrapidity open charm spectra from direct reconstruction of D⁰(D⁰̅ )→K^∓π^± in d+Au collisions and indirect electron-positron measurements via charm semileptonic decays in p+p and d+Au collisions at sqrt[s_NN]=200  GeV are reported. The D⁰(D⁰̅ ) spectrum covers a transverse momentum (p_T) range of 0.1<p_T<3  GeV/c, whereas the electron spectra cover a range of 1<p_T<4  GeV/c. The electron spectra show approximate binary collision scaling between p+p and d+Au collisions. From these two independent analyses, the differential cross section per nucleon-nucleon binary interaction at midrapidity for open charm production from d+Au collisions at BNL RHIC is dσ_cc̅ ^NN/dy=0.30±0.04(stat)±0.09(syst)   mb. The results are compared to theoretical calculations. Implications for charmonium results in A+A collisions are discussed.

The pseudorapidity asymmetry and centrality dependence of charged hadron spectra in d+Au collisions at sqrt[s_NN]=200  GeV are presented. The charged particle density at midrapidity, its pseudorapidity asymmetry, and centrality dependence are reasonably reproduced by a multiphase transport model, by HIJING, and by the latest calculations in a saturation model. Ratios of transverse momentum spectra between backward and forward pseudorapidity are above unity for p_T below 5  GeV∕c. The ratio of central to peripheral spectra in d+Au collisions shows enhancement at 2<p_T<6  GeV∕c, with a larger effect at backward rapidity than forward rapidity. Our measurements are in qualitative agreement with gluon saturation and in contrast to calculations based on incoherent multiple partonic scatterings.

Results on high transverse momentum charged particle emission with respect to the reaction plane are presented for Au+Au collisions at sqrt[s_NN]=200  GeV. Two- and four-particle correlations results are presented as well as a comparison of azimuthal correlations in Au+Au collisions to those in p+p at the same energy. The elliptic anisotropy v₂ is found to reach its maximum at p_t∼3  GeV/c, then decrease slowly and remain significant up to p_t≈7–10  GeV/c. Stronger suppression is found in the back-to-back high-p_t particle correlations for particles emitted out of plane compared to those emitted in plane. The centrality dependence of v₂ at intermediate p_t is compared to simple models based on jet quenching.

Transverse energy (E_T) distributions have been measured for Au+Au collisions at sqrt[s_NN]=200  GeV by the STAR Collaboration at RHIC. E_T is constructed from its hadronic and electromagnetic components, which have been measured separately. E_T production for the most central collisions is well described by several theoretical models whose common feature is large energy density achieved early in the fireball evolution. The magnitude and centrality dependence of E_T per charged particle agrees well with measurements at lower collision energy, indicating that the growth in E_T for larger collision energy results from the growth in particle production. The electromagnetic fraction of the total E_T is consistent with a final state dominated by mesons and independent of centrality.

We report inclusive photon measurements about midrapidity (∣y∣<0.5) from ¹⁹⁷Au+¹⁹⁷Au collisions at sqrt[s_NN]=130  GeV at RHIC. Photon pair conversions were reconstructed from electron and positron tracks measured with the Time Projection Chamber (TPC) of the STAR experiment. With this method, an energy resolution of ΔE∕E≈2% at 0.5  GeV has been achieved. Reconstructed photons have also been used to measure the transverse momentum (p_t) spectra of π⁰ mesons about midrapidity (∣y∣<1) via the π⁰→γγ decay channel. The fractional contribution of the π⁰→γγ decay to the inclusive photon spectrum decreases by 20%±5% between p_t=1.65  GeV∕c and p_t=2.4  GeV∕c in the most central events, indicating that relative to π⁰→γγ decay the contribution of other photon sources is substantially increasing.

We report on the rapidity and centrality dependence of proton and antiproton transverse mass distributions from ¹⁹⁷Au+¹⁹⁷Au collisions at sqrt[s_NN]=130  GeV as measured by the STAR experiment at the Relativistic Heavy Ion Collider (RHIC). Our results are from the rapidity and transverse momentum range of ∣y∣<0.5 and 0.35<p_t<1.00  GeV∕c. For both protons and antiprotons, transverse mass distributions become more convex from peripheral to central collisions demonstrating characteristics of collective expansion. The measured rapidity distributions and the mean transverse momenta versus rapidity are flat within ∣y∣<0.5. Comparisons of our data with results from model calculations indicate that in order to obtain a consistent picture of the proton (antiproton) yields and transverse mass distributions the possibility of prehadronic collective expansion may have to be taken into account.

We present STAR measurements of charged hadron production as a function of centrality in Au+Au collisions at sqrt[s_NN]=130  GeV. The measurements cover a phase space region of 0.2<p_T<6.0  GeV∕c in transverse momentum and −1<η<1 in pseudorapidity. Inclusive transverse momentum distributions of charged hadrons in the pseudorapidity region 0.5<∣η∣<1 are reported and compared to our previously published results for ∣η∣<0.5. No significant difference is seen for inclusive p_T distributions of charged hadrons in these two pseudorapidity bins. We measured dN∕dη distributions and truncated mean p_T in a region of p_T>p_T^cut, and studied the results in the framework of participant and binary scaling. No clear evidence is observed for participant scaling of charged hadron yield in the measured p_T region. The relative importance of hard scattering processes is investigated through binary scaling fraction of particle production.

We present data on e⁺e⁻ pair production accompanied by nuclear breakup in ultraperipheral gold-gold collisions at a center of mass energy of 200  GeV per nucleon pair. The nuclear breakup requirement selects events at small impact parameters, where higher-order diagrams for pair production should be enhanced. We compare the data with two calculations: one based on the equivalent photon approximation, and the other using lowest-order quantum electrodynamics (QED). The data distributions agree with both calculations, except that the pair transverse momentum spectrum disagrees with the equivalent photon approach. We set limits on higher-order contributions to the cross section.

We present the results of a systematic study of the shape of the pion distribution in coordinate space at freeze-out in Au+Au collisions at BNL RHIC using two-pion Hanbury Brown–Twiss (HBT) interferometry. Oscillations of the extracted HBT radii versus emission angle indicate sources elongated perpendicular to the reaction plane. The results indicate that the pressure and expansion time of the collision system are not sufficient to completely quench its initial shape.

The transverse mass spectra and midrapidity yields for Ξs and Ωs are presented. For the 10% most central collisions, the Ξ̅ ⁺/h^- ratio increases from the Super Proton Synchrotron to the Relativistic Heavy Ion Collider energies while the Ξ^-/h^- stays approximately constant. A hydrodynamically inspired model fit to the Ξ spectra, which assumes a thermalized source, seems to indicate that these multistrange particles experience a significant transverse flow effect, but are emitted when the system is hotter and the flow is smaller than values obtained from a combined fit to π, K, p, and Λs.

Measurements of the production of forward high-energy π⁰ mesons from transversely polarized proton collisions at sqrt[s]=200  GeV are reported. The cross section is generally consistent with next-to-leading order perturbative QCD calculations. The analyzing power is small at x_F below about 0.3, and becomes positive and large at higher x_F, similar to the trend in data at sqrt[s]≤20  GeV. The analyzing power is in qualitative agreement with perturbative QCD model expectations. This is the first significant spin result seen for particles produced with p_T>1  GeV/c at a polarized proton collider.

Transverse mass and rapidity distributions for charged pions, charged kaons, protons, and antiprotons are reported for sqrt[s_NN]=200  GeV pp and Au+Au collisions at Relativistic Heary Ion Collider (RHIC). Chemical and kinetic equilibrium model fits to our data reveal strong radial flow and long duration from chemical to kinetic freeze-out in central Au+Au collisions. The chemical freeze-out temperature appears to be independent of initial conditions at RHIC energies.