Search Results (68 total)

We study quark thermal recombination as a function of energy density during the evolution of a heavy-ion collision in a numerical model that reproduces aspects of quantum chromodynamics phenomenology. We show that starting with a set of free quarks (or quarks and antiquarks) the probability to form colorless clusters of three quarks differs from that to form colorless clusters of quark-antiquark and that the former has a sharp jump at a critical energy density, whereas the latter transits smoothly from the low to the high-energy-density domains. We interpret this as a quantitative difference in the production of baryons and mesons with energy density. We use this approach to compute the proton and pion spectra in a Bjorken scenario that incorporates the evolution of these probabilities with energy density and therefore with proper time. From the spectra, we compute the proton-to-pion ratio and compare to data at the highest Relativistic Heavy Ion Collider energies. We show that for a standard choice of parameters, this ratio reaches 1, though the maximum is very sensitive to the initial evolution proper time.

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.

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.

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 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.

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.

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.

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.

The balance function is a new observable based on the principle that charge is locally conserved when particles are pair produced. Balance functions have been measured for charged particle pairs and identified charged pion pairs in Au+Au collisions at sqrt[s_NN]=130  GeV at the Relativistic Heavy Ion Collider using STAR. Balance functions for peripheral collisions have widths consistent with model predictions based on a superposition of nucleon-nucleon scattering. Widths in central collisions are smaller, consistent with trends predicted by models incorporating late hadronization.

Azimuthal correlations for large transverse momentum charged hadrons have been measured over a wide pseudorapidity range and full azimuth in Au+Au and p+p collisions at sqrt[s_NN]=200  GeV. The small-angle correlations observed in p+p collisions and at all centralities of Au+Au collisions are characteristic of hard-scattering processes previously observed in high-energy collisions. A strong back-to-back correlation exists for p+p and peripheral Au+Au. In contrast, the back-to-back correlations are reduced considerably in the most central Au+Au collisions, indicating substantial interaction as the hard-scattered partons or their fragmentation products traverse the medium.

Azimuthal anisotropy (v₂) and two-particle angular correlations of high p_T charged hadrons have been measured in Au+Au collisions at sqrt[s_NN]=130  GeV for transverse momenta up to 6  GeV/c, where hard processes are expected to contribute significantly. The two-particle angular correlations exhibit elliptic flow and a structure suggestive of fragmentation of high p_T partons. The monotonic rise of v₂(p_T) for p_T<2   GeV/c is consistent with collective hydrodynamical flow calculations. At p_T>3   GeV/c, a saturation of v₂ is observed which persists up to p_T=6   GeV/c.

We report the first observation of K^*(892)⁰→πK in relativistic heavy ion collisions. The transverse momentum spectrum of (K^*0+K^*0)/2 from central Au+Au collisions at sqrt[s_NN]=130 GeV is presented. The ratios of the K^*0 yield derived from these data to the yields of negative hadrons, charged kaons, and φ mesons have been measured in central and minimum bias collisions and compared with model predictions and comparable e⁺e^-, pp, and p̅ p results. The data indicate no dramatic reduction of K^*0 production in relativistic heavy ion collisions despite expected losses due to rescattering effects.

The STAR Collaboration reports the first observation of exclusive ρ⁰ photoproduction, AuAu→AuAuρ⁰, and ρ⁰ production accompanied by mutual nuclear Coulomb excitation, AuAu→Au^⋆Au^⋆ρ⁰, in ultraperipheral heavy-ion collisions. The ρ⁰ have low transverse momenta, consistent with coherent coupling to both nuclei. The cross sections at sqrt[s_NN]=130   GeV agree with theoretical predictions treating ρ⁰ production and Coulomb excitation as independent processes.

The NA44 experiment has measured single-particle inclusive spectra for charged pions, kaons, and protons as a function of transverse mass near midrapidity in 158A GeV/c Pb+Pb collisions. From the particle mass dependence of the observed m_T distributions, we are able to deduce a value of about 120 MeV for the temperature at thermal freeze-out. From the observed ratios of the rapidity densities, we find values of the chemical potentials for light and strange quarks and a chemical freeze-out temperature of approximately 140 MeV.

Inclusive transverse momentum distributions of charged hadrons within 0.2<p_T<6.0   GeV/c have been measured over a broad range of centrality for Au+Au collisions at sqrt[s_NN]=130  GeV. Hadron yields are suppressed at high p_T in central collisions relative to peripheral collisions and to a nucleon-nucleon reference scaled for collision geometry. Peripheral collisions are not suppressed relative to the nucleon-nucleon reference. The suppression varies continuously at intermediate centralities. The results indicate significant nuclear medium effects on high-p_T hadron production in heavy-ion collisions at high energy.