Search Results (140 total)

The STAR Collaboration at the Relativistic Heavy Ion Collider presents measurements of J/ψ→e⁺e^- at midrapidity and high transverse momentum (p_T>5 GeV/c) in p+p and central Cu+Cu collisions at sqrt[s_NN]=200 GeV. The inclusive J/ψ production cross section for Cu+Cu collisions is found to be consistent at high p_T with the binary collision-scaled cross section for p+p collisions. At a confidence level of 97%, this is in contrast to a suppression of J/ψ production observed at lower p_T. Azimuthal correlations of J/ψ with charged hadrons in p+p collisions provide an estimate of the contribution of B-hadron decays to J/ψ production of 13%±5%.

A system-reservoir nonlinear coupling model has been proposed to the situation when the system is driven externally by a random force and the associated bath is kept in thermal equilibrium, in an attempt to put forth a microscopic approach to quantum state-dependent diffusion and multiplicative noises in terms of a quantum Langevin equation in the overdamped limit (quantum Smoluchowski equation). We then obtain the analytical expression for phase induced quantum current in a periodic potential when the external noise has finite correlation time and explore the dependence of the current on various parameters related to the external noise, for example, the noise strength.

The results of midrapidity (0<y<0.8) neutral pion spectra over an extended transverse momentum range (1<p_T<12 GeV/c) in sqrt[s_NN]=200 GeV Au+Au collisions, measured by the STAR experiment, are presented. The neutral pions are reconstructed from photons measured either by the STAR Barrel Electro-Magnetic Calorimeter or by the Time Projection Chamber via tracking of conversion electron-positron pairs. Our measurements are compared to previously published π^± and π⁰ results. The nuclear modification factors R_CP and R_AA of π⁰ are also presented as a function of p_T. In the most central Au+Au collisions, the binary collision scaled π⁰ yield at high p_T is suppressed by a factor of about 5 compared to the expectation from the yield of p+p collisions. Such a large suppression is in agreement with previous observations for light quark mesons and is consistent with the scenario that partons suffer considerable energy loss in the dense medium formed in central nucleus-nucleus collisions at the Relativistic Heavy Ion Collider.

Pentaphenyl trimethyl trisiloxane is an isotropic liquid at room temperature with a dynamic glass transition at 224 K. Using x-ray reflectivity, we see surface density oscillations (layers) develop below 285 K, similar to those seen in other metallic and dielectric liquids and in computer simulations. The layering threshold is ∼0.23 times the liquid-gas critical temperature. Upon cooling further, there is a sharp increase at 224 K in the persistence of the surface layers into the bulk material, i.e., an apparently discontinuous change in static structure at the glass transition.

Forward-backward multiplicity correlation strengths have been measured with the STAR detector for Au+Au and p+p collisions at sqrt[s_NN]=200  GeV. Strong short- and long-range correlations (LRC) are seen in central Au+Au collisions. The magnitude of these correlations decrease with decreasing centrality until only short-range correlations are observed in peripheral Au+Au collisions. Both the dual parton model (DPM) and the color glass condensate (CGC) predict the existence of the long-range correlations. In the DPM, the fluctuation in the number of elementary (parton) inelastic collisions produces the LRC. In the CGC, longitudinal color flux tubes generate the LRC. The data are in qualitative agreement with the predictions of the DPM and indicate the presence of multiple parton interactions.

We report K/π fluctuations from Au+Au collisions at sqrt[s_NN]=19.6, 62.4, 130, and 200 GeV using the STAR detector at the Relativistic Heavy Ion Collider. K/π fluctuations in central collisions show little dependence on incident energy and are on the same order as those from NA49 at the Super Proton Synchrotron in central Pb+Pb collisions at sqrt[s_NN]=12.3 and 17.3 GeV. We report results for the collision centrality dependence of K/π fluctuations and results for charge-separated fluctuations. We observe that the K/π fluctuations scale with the charged particle multiplicity density.

We present a systematic analysis of two-pion interferometry in Au+Au collisions at sqrt[s_NN]=62.4 GeV and Cu+Cu collisions at sqrt[s_NN]=62.4 and 200 GeV using the STAR detector at the Relativistic Heavy Ion Collider (RHIC). The multiplicity and transverse momentum dependences of the extracted correlation lengths (radii) are studied. The scaling with charged particle multiplicity of the apparent system volume at final interaction is studied for the RHIC energy domain. The multiplicity scaling of the measured correlation radii is found to be independent of colliding system and collision energy.

We present results for the measurement of ϕ meson production via its charged kaon decay channel ϕ→K⁺K^- in Au+Au collisions at sqrt[s_NN]=62.4,130, and 200 GeV, and in p+p and d+Au collisions at sqrt[s_NN]=200 GeV from the STAR experiment at the BNL Relativistic Heavy Ion Collider (RHIC). The midrapidity (|y|<0.5) ϕ meson transverse momentum (p_T) spectra in central Au+Au collisions are found to be well described by a single exponential distribution. On the other hand, the p_T spectra from p+p, d+Au, and peripheral Au+Au collisions show power-law tails at intermediate and high p_T and are described better by Levy distributions. The constant ϕ/K^- yield ratio vs beam species, collision centrality, and colliding energy is in contradiction with expectations from models having kaon coalescence as the dominant mechanism for ϕ production at RHIC. The Ω/ϕ yield ratio as a function of p_T is consistent with a model based on the recombination of thermal s quarks up to p_T~4 GeV/c, but disagrees at higher transverse momenta. The measured nuclear modification factor, R_dAu, for the ϕ meson increases above unity at intermediate p_T, similar to that for pions and protons, while R_AA is suppressed due to the energy loss effect in central Au+Au collisions. Number of constituent quark scaling of both R_cp and v₂ for the ϕ meson with respect to other hadrons in Au+Au collisions at sqrt[s_NN]=200 GeV at intermediate p_T is observed. These observations support quark coalescence as being the dominant mechanism of hadronization in the intermediate p_T region at RHIC.

We report the measurement of charged D^* mesons in inclusive jets produced in proton-proton collisions at a center-of-mass energy sqrt[s]=200  GeV with the STAR experiment at the Relativistic Heavy Ion Collider. For D^* mesons with fractional momenta 0.2<z<0.5 in inclusive jets with 11.5 GeV mean transverse energy, the production rate is found to be N(D^*++D^*-)/N(jet)=0.015±0.008(stat)±0.007(sys). This rate is consistent with perturbative QCD evaluation of gluon splitting into a pair of charm quarks and subsequent hadronization.

Identified charged-particle spectra of π^±, K^±, p, and p̅ at midrapidity (|y|<0.1) measured by the dE/dx method in the STAR (solenoidal tracker at the BNL Relativistic Heavy Ion Collider) time projection chamber are reported for pp and d+Au collisions at sqrt[s_NN]=200 GeV and for Au+Au collisions at 62.4, 130, and 200 GeV. Average transverse momenta, total particle production, particle yield ratios, strangeness, and baryon production rates are investigated as a function of the collision system and centrality. The transverse momentum spectra are found to be flatter for heavy particles than for light particles in all collision systems; the effect is more prominent for more central collisions. The extracted average transverse momentum of each particle species follows a trend determined by the total charged-particle multiplicity density. The Bjorken energy density estimate is at least several GeV/fm³ for a formation time less than 1 fm/c. A significantly larger net-baryon density and a stronger increase of the net-baryon density with centrality are found in Au+Au collisions at 62.4 GeV than at the two higher energies. Antibaryon production relative to total particle multiplicity is found to be constant over centrality, but increases with the collision energy. Strangeness production relative to total particle multiplicity is similar at the three measured RHIC energies. Relative strangeness production increases quickly with centrality in peripheral Au+Au collisions, to a value about 50% above the pp value, and remains rather constant in more central collisions. Bulk freeze-out properties are extracted from thermal equilibrium model and hydrodynamics-motivated blast-wave model fits to the data. Resonance decays are found to have little effect on the extracted kinetic freeze-out parameters because of the transverse momentum range of our measurements. The extracted chemical freeze-out temperature is constant, independent of collision system or centrality; its value is close to the predicted phase-transition temperature, suggesting that chemical freeze-out happens in the vicinity of hadronization and the chemical freeze-out temperature is universal despite the vastly different initial conditions in the collision systems. The extracted kinetic freeze-out temperature, while similar to the chemical freeze-out temperature in pp, d+Au, and peripheral Au+Au collisions, drops significantly with centrality in Au+Au collisions, whereas the extracted transverse radial flow velocity increases rapidly with centrality. There appears to be a prolonged period of particle elastic scatterings from chemical to kinetic freeze-out in central Au+Au collisions. The bulk properties extracted at chemical and kinetic freeze-out are observed to evolve smoothly over the measured energy range, collision systems, and collision centralities.

In ultraperipheral relativistic heavy-ion collisions, a photon from the electromagnetic field of one nucleus can fluctuate to a quark-antiquark pair and scatter from the other nucleus, emerging as a ρ⁰. The ρ⁰ production occurs in two well-separated (median impact parameters of 20 and 40 F for the cases considered here) nuclei, so the system forms a two-source interferometer. At low transverse momenta, the two amplitudes interfere destructively, suppressing ρ⁰ production. Since the ρ⁰ decays before the production amplitudes from the two sources can overlap, the two-pion system can only be described with an entangled nonlocal wave function, and is thus an example of the Einstein-Podolsky-Rosen paradox. We observe this suppression in 200 GeV per nucleon-pair gold-gold collisions. The interference is 87%±5%(stat.)±8%(syst.) of the expected level. This translates into a limit on decoherence due to wave function collapse or other factors of 23% at the 90% confidence level.

We present measurements of net charge fluctuations in Au+Au collisions at sqrt[s_NN]=19.6, 62.4, 130, and 200 GeV, Cu+Cu collisions at sqrt[s_NN]=62.4 and 200 GeV, and p+p collisions at sqrt[s]=200 GeV using the dynamical net charge fluctuations measure ν_+-,dyn. We observe that the dynamical fluctuations are nonzero at all energies and exhibit a modest dependence on beam energy. A weak system size dependence is also observed. We examine the collision centrality dependence of the net charge fluctuations and find that dynamical net charge fluctuations violate 1/N_ch scaling but display approximate 1/N_part scaling. We also study the azimuthal and rapidity dependence of the net charge correlation strength and observe strong dependence on the azimuthal angular range and pseudorapidity widths integrated to measure the correlation.

Three-particle azimuthal correlation measurements with a high transverse momentum trigger particle are reported for pp, d+Au, and Au+Au collisions at sqrt[s_NN]=200  GeV by the STAR experiment. Dijet structures are observed in pp, d+Au and peripheral Au+Au collisions. An additional structure is observed in central Au+Au data, signaling conical emission of correlated charged hadrons. The conical emission angle is found to be θ=1.37±0.02(stat)_-0.07^+0.06(syst), independent of p_⊥.

We measure directed flow (v₁) for charged particles in Au+Au and Cu+Cu collisions at sqrt[s_NN]=200 and 62.4 GeV, as a function of pseudorapidity (η), transverse momentum (p_t), and collision centrality, based on data from the STAR experiment. We find that the directed flow depends on the incident energy but, contrary to all available model implementations, not on the size of the colliding system at a given centrality. We extend the validity of the limiting fragmentation concept to v₁ in different collision systems, and investigate possible explanations for the observed sign change in v₁(p_t).

We report precision measurements of the Feynman x (x_F) dependence, and first measurements of the transverse momentum (p_T) dependence, of transverse single-spin asymmetries for the production of π⁰ mesons from polarized proton collisions at sqrt[s]=200  GeV. The x_F dependence of the results is in fair agreement with perturbative QCD model calculations that identify orbital motion of quarks and gluons within the proton as the origin of the spin effects. Results for the p_T dependence at fixed x_F are not consistent with these same perturbative QCD-based calculations.

The electromagnetic transition rates for all the high spin levels of the yrast sequence of ¹⁰⁹Ag have been measured. The observed behavior of the magnetic dipole transition rates as a function of angular momentum establishes that there is a sudden change in rotation axis associated with rotational alignment of two neutrons. The projected shell model calculations give a consistent picture of the observed phenomena in ¹⁰⁹Ag.

A system-reservoir nonlinear coupling model is proposed for a quantum system when the associated bath is not in thermal equilibrium but is modulated by an external colored noise, to present a microscopic approach to quantum state-dependent diffusion and multiplicative noise in terms of a quantum Langevin description. Consequently, the Fokker-Planck equation in position space, valid in the overdamped limit, for multiplicative colored noise is constructed to explore the possibility of observing a quantum current and dependence of the current on various parameters of external noise is examined.

Neutral pion transverse momentum spectra were measured in p+C and p+Pb collisions at sqrt[s_NN]=17.4  GeV at midrapidity (2.3≲η_lab≲3.0) over the range 0.7≲p_T≲3.5  GeV/c. The spectra are compared to π⁰ spectra measured in Pb+Pb collisions at sqrt[s_NN]=17.3  GeV in the same experiment. For a wide range of Pb+Pb centralities (N_part≲300), the yield of π⁰’s with p_T≳2  GeV/c is larger than or consistent with the p+C or p+Pb yields scaled with the number of nucleon-nucleon collisions (N_coll), while for central Pb+Pb collisions with N_part≳350, the π⁰ yield is suppressed.

We report a new STAR measurement of the longitudinal double-spin asymmetry A_LL for inclusive jet production at midrapidity in polarized p+p collisions at a center-of-mass energy of sqrt[s]=200  GeV. The data, which cover jet transverse momenta 5<p_T<30  GeV/c, are substantially more precise than previous measurements. They provide significant new constraints on the gluon spin contribution to the nucleon spin through the comparison to predictions derived from one global fit to polarized deep-inelastic scattering measurements.

We present the first spin alignment measurements for the K^*0(892) and ϕ(1020) vector mesons produced at midrapidity with transverse momenta up to 5 GeV/c at sqrt[s_NN]=200 GeV at RHIC. The diagonal spin-density matrix elements with respect to the reaction plane in Au+Au collisions are ρ₀₀=0.32±0.04 (stat) ± 0.09 (syst) for the K^*0 (0.8<p_T<5.0 GeV/c) and ρ₀₀=0.34±0.02 (stat) ± 0.03 (syst) for the ϕ (0.4<p_T<5.0 GeV/c) and are constant with transverse momentum and collision centrality. The data are consistent with the unpolarized expectation of 1/3 and thus no evidence is found for the transfer of the orbital angular momentum of the colliding system to the vector-meson spins. Spin alignments for K^*0 and ϕ in Au+Au collisions were also measured with respect to the particle's production plane. The ϕ result, ρ₀₀=0.41±0.02 (stat) ± 0.04 (syst), is consistent with that in p+p collisions, ρ₀₀=0.39±0.03 (stat) ± 0.06 (syst), also measured in this work. The measurements thus constrain the possible size of polarization phenomena in the production dynamics of vector mesons.

We present STAR results on the elliptic flow v₂ of charged hadrons, strange and multistrange particles from sqrt[s_NN]=200 GeV Au+Au collisions at the BNL Relativistic Heavy Ion Collider (RHIC). The detailed study of the centrality dependence of v₂ over a broad transverse momentum range is presented. Comparisons of different analysis methods are made in order to estimate systematic uncertainties. To discuss the nonflow effect, we have performed the first analysis of v₂ with the Lee-Yang zero method for K_S⁰ and Λ. In the relatively low p_T region, p_T≤2 GeV/c, a scaling with m_T-m is observed for identified hadrons in each centrality bin studied. However, we do not observe v₂(p_T) scaled by the participant eccentricity to be independent of centrality. At higher p_T,2≤p_T≤6 GeV/c,v₂ scales with quark number for all hadrons studied. For the multistrange hadron Ω, which does not suffer appreciable hadronic interactions, the values of v₂ are consistent with both m_T-m scaling at low p_T and number-of-quark scaling at intermediate p_T. As a function of collision centrality, an increase of p_T-integrated v₂ scaled by the participant eccentricity has been observed, indicating a stronger collective flow in more central Au+Au collisions.

We report on the observed differences in production rates of strange and multistrange baryons in Au+Au collisions at sqrt[s_NN]=200 GeV compared to p+p interactions at the same energy. The strange baryon yields in Au+Au collisions, when scaled down by the number of participating nucleons, are enhanced relative to those measured in p+p reactions. The enhancement observed increases with the strangeness content of the baryon, and it increases for all strange baryons with collision centrality. The enhancement is qualitatively similar to that observed at the lower collision energy sqrt[s_NN]=17.3 GeV. The previous observations are for the bulk production, while at intermediate p_T,1<p_T<4 GeV/c, the strange baryons even exceed binary scaling from p+p yields.

Photoproduction reactions occur when the electromagnetic field of a relativistic heavy ion interacts with another heavy ion. The STAR Collaboration presents a measurement of ρ⁰ and direct π⁺π^- photoproduction in ultraperipheral relativistic heavy ion collisions at sqrt[s_NN]=200 GeV. We observe both exclusive photoproduction and photoproduction accompanied by mutual Coulomb excitation. We find a coherent cross section of σ(AuAu→Au^*Au^*ρ⁰)=530±19(stat.)±57(syst.) mb, in accord with theoretical calculations based on a Glauber approach, but considerably below the predictions of a color dipole model. The ρ⁰ transverse momentum spectrum (p_T²) is fit by a double exponential curve including both coherent and incoherent coupling to the target nucleus; we find σ_inc/σ_coh=0.29±0.03(stat.)±0.08(syst.). The ratio of direct π⁺π^- to ρ⁰ production is comparable to that observed in γp collisions at HERA and appears to be independent of photon energy. Finally, the measured ρ⁰ spin helicity matrix elements agree within errors with the expected s-channel helicity conservation.

We report the measurement of Λ and Λ̅ yields and inverse slope parameters in d+Au collisions at sqrt[s_NN]=200 GeV at forward and backward rapidities (y=±2.75), using data from the STAR forward time projection chambers. The contributions of different processes to baryon transport and particle production are probed exploiting the inherent asymmetry of the d+Au system. Comparisons to model calculations show that baryon transport on the deuteron side is consistent with multiple collisions of the deuteron nucleons with gold participants. On the gold side, HIJING-based models without a hadronic rescattering phase do not describe the measured particle yields, while models that include target remnants or hadronic rescattering do. The multichain model can provide a good description of the net baryon density in d+Au collisions at energies currently available at the BNL Relativistic Heavy Ion Collider, and the derived parameters of the model agree with those from nuclear collisions at lower energies.

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.