Search Results (93 total)

We determine rapidity asymmetry in the production of charged pions, protons, and antiprotons for large transverse momentum (p_T) for d+Au collisions at sqrt[s_NN]=200 GeV. The rapidity asymmetry is defined as the ratio of particle yields at backward rapidity (Au beam direction) to those at forward rapidity (d beam direction). The identified hadrons are measured in the rapidity regions |y|<0.5 and 0.5<|y|<1.0 for the p_T range 2.5<p_T<10  GeV/c. We observe significant rapidity asymmetry for charged pion and proton+antiproton production in both the rapidity regions. The asymmetry is larger for 0.5<|y|<1.0 than for |y|<0.5 and is almost independent of particle type. The measurements are compared to various model predictions employing multiple scattering, energy loss, nuclear shadowing, saturation effects, and recombination and also to a phenomenological parton model. We find that asymmetries are sensitive to model parameters and show model preference. The rapidity dependence of π^-/π⁺ and p̅ /p ratios in peripheral d+Au and forward neutron-tagged events are used to study the contributions of valence quarks and gluons to particle production at high p_T.

We present strange particle spectra and yields measured at midrapidity in sqrt[s]=200 GeV proton-proton (p+p) collisions at the BNL Relativistic Heavy Ion Collider (RHIC). We find that the previously observed universal transverse mass (m_T≡sqrt[p_T²+m²]) scaling of hadron production in p+p collisions seems to break down at higher m_T and that there is a difference in the shape of the m_T spectrum between baryons and mesons. We observe midrapidity antibaryon to baryon ratios near unity for Λ and Ξ baryons and no dependence of the ratio on transverse momentum, indicating that our data do not yet reach the quark-jet dominated region. We show the dependence of the mean transverse momentum 〈p_T〉 on measured charged particle multiplicity and on particle mass and infer that these trends are consistent with gluon-jet dominated particle production. The data are compared with previous measurements made at the CERN Super Proton Synchrotron and Intersecting Storage Rings and in Fermilab experiments and with leading-order and next-to-leading-order string fragmentation model predictions. We infer from these comparisons that the spectral shapes and particle yields from p+p collisions at RHIC energies have large contributions from gluon jets rather than from quark jets.

We report charged particle pair correlation analyses in the space of Δϕ (azimuth) and Δη (pseudorapidity), for central Au+Au collisions at sqrt[s_NN]=200 GeV in the STAR detector. The analysis involves unlike-sign charged pairs and like-sign charged pairs, which are transformed into charge-dependent (CD) signals and charge-independent (CI) signals. We present detailed parametrizations of the data. A model featuring dense gluonic hot spots as first proposed by Van Hove predicts that the observables under investigation would have sensitivity to such a substructure should it occur, and the model also motivates selection of transverse momenta in the range 0.8<p_t<2.0 GeV/c. Both CD and CI correlations of high statistical significance are observed, and possible interpretations are discussed.

We present the scaling properties of Λ, Ξ, and Ω in midrapidity Au+Au collisions at the Brookhaven National Laboratory Relativistic Heavy Ion Collider at sqrt[s_NN]=200  GeV. The yield of multistrange baryons per participant nucleon increases from peripheral to central collisions more rapidly than that of Λ, indicating an increase of the strange-quark density of the matter produced. The strange phase-space occupancy factor γ_s approaches unity for the most central collisions. Moreover, the nuclear modification factors of p, Λ, and Ξ are consistent with each other for 2<p_T<5  GeV/c in agreement with a scenario of hadron formation from constituent quark degrees of freedom.

We report a measurement of the longitudinal double-spin asymmetry A_LL and the differential cross section for inclusive midrapidity jet production in polarized proton collisions at sqrt[s]=200  GeV. The cross section data cover transverse momenta 5<p_T<50  GeV/c and agree with next-to-leading order perturbative QCD evaluations. The A_LL data cover 5<p_T<17  GeV/c and disfavor at 98% C.L. maximal positive gluon polarization in the polarized nucleon.

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.

We present the first statistically meaningful results from two-K_s⁰ interferometry in heavy-ion collisions at sqrt[s_NN]=200 GeV. A model that takes the effect of the strong interaction into account has been used to fit the measured correlation function. The effects of single and coupled channels were explored. At the mean transverse mass 〈m_T〉=1.07 GeV, we obtain the values R=4.09±0.46(stat)±0.31(sys) fm and λ=0.92±0.23(stat)±0.13(sys), where R and λ are the invariant radius and chaoticity parameters, respectively. The results are qualitatively consistent with m_T systematics established with pions in a scenario characterized by a strong collective flow.

The STAR Collaboration at the Relativistic Heavy Ion Collider reports measurements of azimuthal correlations of high transverse momentum (p_T) charged hadrons in Au+Au collisions at higher p_T than reported previously. As p_T is increased, a narrow, back-to-back peak emerges above the decreasing background, providing a clear dijet signal for all collision centralities studied. Using these correlations, we perform a systematic study of dijet production and suppression in nuclear collisions, providing new constraints on the mechanisms underlying partonic energy loss in dense matter.

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.

Transverse momentum spectra of π^±, p, and p̅ up to 12  GeV/c at midrapidity in centrality selected Au+Au collisions at sqrt[s_NN]=200  GeV are presented. In central Au+Au collisions, both π^± and p(p̅ ) show significant suppression with respect to binary scaling at p_T≳4  GeV/c. Protons and antiprotons are less suppressed than π^±, in the range 1.5≲p_T≲6  GeV/c. The π^-/π⁺ and p̅ /p ratios show at most a weak p_T dependence and no significant centrality dependence. The p/π ratios in central Au+Au collisions approach the values in p+p and d+Au collisions at p_T≳5  GeV/c. The results at high p_T indicate that the partonic sources of π^±, p, and p̅ have similar energy loss when traversing the nuclear medium.

We report the measurements of Σ(1385) and Λ(1520) production in p+p and Au+Au collisions at sqrt[s_NN]=200  GeV from the STAR Collaboration. The yields and the p_T spectra are presented and discussed in terms of chemical and thermal freeze-out conditions and compared to model predictions. Thermal and microscopic models do not adequately describe the yields of all the resonances produced in central Au+Au collisions. Our results indicate that there may be a time span between chemical and thermal freeze-out during which elastic hadronic interactions occur.

We report measurements of transverse momentum p_t spectra for ten event multiplicity classes of p-p collisions at sqrt[s]=200  GeV. By analyzing the multiplicity dependence we find that the spectrum shape can be decomposed into a part with amplitude proportional to multiplicity and described by a Lévy distribution on transverse mass m_t, and a part with amplitude proportional to multiplicity squared and described by a Gaussian distribution on transverse rapidity y_t. The functional forms of the two parts are nearly independent of event multiplicity. The two parts can be identified with the soft and hard components of a two-component model of p-p collisions. This analysis then provides the first isolation of the hard component of the p_t spectrum as a distribution of simple form on y_t.

Measurements of two-particle correlations on angular difference variables η₁-η₂ (pseudorapidity) and ϕ₁-ϕ₂ (azimuth) are presented for all primary charged hadrons with transverse momentum 0.15≤p_t≤2 GeV/c and |η|≤1.3 from Au-Au collisions at sqrt[s_NN]=130 GeV. Large-amplitude correlations are observed over a broad range in relative angles where distinct structures appear on the same-side and away-side (i.e., relative azimuth less than π/2 or greater than π/2). The principal correlation structures include that associated with elliptic flow plus a strong, same-side peak. It is hypothesized that the latter results from correlated hadrons associated with semi-hard parton scattering in the early stage of the heavy-ion collision which produces a jet-like correlation peak at small relative angles. The width of the jet-like peak on η₁-η₂ increases by a factor 2.3 from peripheral to central collisions, suggesting strong coupling of semi-hard scattered partons to a longitudinally-expanding medium. The new methods of jet analysis introduced here provide access to scattered partons at low transverse momentum well below the kinematic range where perturbative quantum chromodynamics and standard fragmentation models are applicable.

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.