Search Results (86 total)

An analysis of inclusive pion production in proton-beryllium collisions at 6.4, 12.3, and 17.5 GeV/c proton beam momentum has been performed. The data were taken by Experiment 910 at the Alternating Gradient Synchrotron at the Brookhaven National Laboratory. The differential π⁺ and π^- production cross sections (d²σ/dpdΩ) were measured up to 400 mrad in θ_π and up to 6 GeV/c in p_π. The measured cross section was fit with a Sanford-Wang parametrization.

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 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.

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.

We report results on ρ(770)⁰→π⁺π^- production at midrapidity in p+p and peripheral Au+Au collisions at sqrt[s_NN]=200  GeV. This is the first direct measurement of ρ(770)⁰→π⁺π^- in heavy-ion collisions. The measured ρ⁰ peak in the invariant mass distribution is shifted by ∼40  MeV/c² in minimum bias p+p interactions and ∼70  MeV/c² in peripheral Au+Au collisions. The ρ⁰ mass shift is dependent on transverse momentum and multiplicity. The modification of the ρ⁰ meson mass, width, and shape due to phase space and dynamical effects are discussed.

We report the first observations of the first harmonic (directed flow, v₁) and the fourth harmonic (v₄), in the azimuthal distribution of particles with respect to the reaction plane in Au+Au collisions at the BNL Relativistic Heavy Ion Collider (RHIC). Both measurements were done taking advantage of the large elliptic flow (v₂) generated at RHIC. From the correlation of v₂ with v₁ it is determined that v₂ is positive, or in-plane. The integrated v₄ is about a factor of 10 smaller than v₂. For the sixth (v₆) and eighth (v₈) harmonics upper limits on the magnitudes are reported.

We present STAR measurements of the azimuthal anisotropy parameter v₂ and the binary-collision scaled centrality ratio R_CP for kaons and lambdas (Λ+Λ̅ ) at midrapidity in Au+Au collisions at sqrt[s_NN]=200   GeV. In combination, the v₂ and R_CP particle-type dependencies contradict expectations from partonic energy loss followed by standard fragmentation in vacuum. We establish p_T≈5   GeV/c as the value where the centrality dependent baryon enhancement ends. The K_S⁰ and Λ+Λ̅ v₂ values are consistent with expectations of constituent-quark-number scaling from models of hadron formation by parton coalescence or recombination.

Pion-kaon correlation functions are constructed from central Au+Au STAR data taken at sqrt[s_NN]=130  GeV by the STAR detector at the Relativistic Heavy Ion Collider (RHIC). The results suggest that pions and kaons are not emitted at the same average space-time point. Space-momentum correlations, i.e., transverse flow, lead to a space-time emission asymmetry of pions and kaons that is consistent with the data. This result provides new independent evidence that the system created at RHIC undergoes a collective transverse expansion.

Data from the first physics run at the Relativistic Heavy-Ion Collider at Brookhaven National Laboratory, Au+Au collisions at sqrt[s_NN]=130   GeV, have been analyzed by the STAR Collaboration using three-pion correlations with charged pions to study whether pions are emitted independently at freeze-out. We have made a high-statistics measurement of the three-pion correlation function and calculated the normalized three-particle correlator to obtain a quantitative measurement of the degree of chaoticity of the pion source. It is found that the degree of chaoticity seems to increase with increasing particle multiplicity.

Momentum spectra of charged pions over nearly full rapidity coverage from target to beam rapidity have been extracted from 0−5% most central Au+Au collisions in the beam energy range from 2A to 8A  GeV by the E895 experiment. Using a thermal parametrization to fit the transverse mass spectra, rapidity density distributions are extracted. The observed spectra are compared with predictions from the RQMD version 2.3 cascade model and also to a thermal model including longitudinal flow. The total 4π yields of the charged pions are used to infer an initial-state entropy produced in the collisions.

The yield for the multistrange Ξ^- hyperon has been measured in 6A  GeV Au+Au collisions via reconstruction of its decay products π^- and Λ, the latter also being reconstructed from its daughter tracks of π^- and p. The measurement is rather close to the threshold for Ξ^- production and therefore provides an important test of model predictions. The measured yield for Ξ^- and Λ are compared for several centralities. In central collisions the Ξ^- yield is found to be in excellent agreement with statistical and transport model predictions, suggesting that multistrange hadron production approaches chemical equilibrium in high baryon density nuclear matter.

We present the results of charged particle fluctuations measurements in Au+Au collisions at sqrt[s_NN]=130  GeV using the STAR detector. Dynamical fluctuations measurements are presented for inclusive charged particle multiplicities as well as for identified charged pions, kaons, and protons. The net charge dynamical fluctuations are found to be large and negative providing clear evidence that positive and negative charged particle production is correlated within the pseudorapidity range investigated. Correlations are smaller than expected based on model-dependent predictions for a resonance gas or a quark-gluon gas which undergoes fast hadronization and freeze-out. Qualitative agreement is found with comparable scaled p+p measurements and a heavy ion jet interaction generation model calculation based on independent particle collisions, although a small deviation from the 1∕N scaling dependence expected from this model is observed.

We report high statistics measurements of inclusive charged hadron production in Au+Au and p+p collisions at sqrt[s_NN]=200  GeV. A large, approximately constant hadron suppression is observed in central Au+Au collisions for 5<p_T<12   GeV/c. The collision energy dependence of the yields and the centrality and p_T dependence of the suppression provide stringent constraints on theoretical models of suppression. Models incorporating initial-state gluon saturation or partonic energy loss in dense matter are largely consistent with observations. We observe no evidence of p_T-dependent suppression, which may be expected from models incorporating jet attenuation in cold nuclear matter or scattering of fragmentation hadrons.

Source images are extracted from two-particle correlations constructed from strange and nonstrange hadrons produced in 6A  GeV Au+Au collisions. Very different source images result from pp vs pΛ vs π^-π^- correlations. Scaling by transverse mass can describe the apparent source size ratio for p/π^- but not for Λ/π^- or Λ/p. These observations suggest important differences in the space-time emission histories for protons, pions, and neutral strange baryons produced in the same events.

We report measurements of single-particle inclusive spectra and two-particle azimuthal distributions of charged hadrons at high transverse momentum (high p_T) in minimum bias and central d+Au collisions at sqrt[s_NN]=200  GeV. The inclusive yield is enhanced in d+Au collisions relative to binary-scaled p+p collisions, while the two-particle azimuthal distributions are very similar to those observed in p+p collisions. These results demonstrate that the strong suppression of the inclusive yield and back-to-back correlations at high p_T previously observed in central Au+Au collisions are due to final-state interactions with the dense medium generated in such collisions.