Search Results (384 total)

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 present a new analysis of J/ψ production yields in deuteron-gold collisions at sqrt[s_NN]=200 GeV using data taken from the PHENIX experiment in 2003 and previously published in S. S. Adler [Phys. Rev. Lett 96, 012304 (2006)]. The high statistics proton-proton J/ψ data taken in 2005 are used to improve the baseline measurement and thus construct updated cold nuclear matter modification factors (R_dAu). A suppression of J/ψ in cold nuclear matter is observed as one goes forward in rapidity (in the deuteron-going direction), corresponding to a region more sensitive to initial-state low-x gluons in the gold nucleus. The measured nuclear modification factors are compared to theoretical calculations of nuclear shadowing to which a J/ψ (or precursor) breakup cross section is added. Breakup cross sections of σ_breakup=2.8_-1.4^+1.7 (2.2_-1.5^+1.6) mb are obtained by fitting these calculations to the data using two different models of nuclear shadowing. These breakup cross-section values are consistent within large uncertainties with the 4.2±0.5 mb determined at lower collision energies. Projecting this range of cold nuclear matter effects to copper-copper and gold-gold collisions reveals that the current constraints are not sufficient to firmly quantify the additional hot nuclear matter effect.

The small-angle x-ray scattering method has been applied to study fireballs ejected into the air from molten hot spots in borosilicate glass by localized microwaves [V. Dikhtyar and E. Jerby, Phys. Rev. Lett. 96 045002 (2006)]. The fireball’s particle size distribution, density, and decay rate in atmospheric pressure were measured. The results show that the fireballs contain particles with a mean size of ∼50  nm with average number densities on the order of ∼10⁹. Hence, fireballs can be considered as a dusty plasma which consists of an ensemble of charged nanoparticles in the plasma volume. This finding is likened to the ball-lightning phenomenon explained by the formation of an oxidizing particle network liberated by lightning striking the ground [J. Abrahamson and J. Dinniss, Nature (London) 403, 519 (2000)].

Measurements of magnetic, transport properties, thermal conductivity, and magnetization under pressure as well as neutron diffraction have been made on a single crystal and powder sample of LaVO₃. The Néel temperature was found to mark a transition from the phase with both frustrated superexchange interaction and spin–orbit λL·S coupling to the phase where the Jahn-Teller orbital-lattice coupling dominates. The dramatic reduction of absolute entropy in the paramagnetic phase is explained in terms of forming a long-range coherent state due to the interference between frustrated orbits and spins.

We present transverse momentum (p_T) spectra of charged hadrons measured in deuteron-gold and nucleon-gold collisions at sqrt[s_NN]=200 GeV for four centrality classes. Nucleon-gold collisions were selected by tagging events in which a spectator nucleon was observed in one of two forward rapidity detectors. The spectra and yields were investigated as a function of the number of binary nucleon-nucleon collisions, ν, suffered by deuteron nucleons. A comparison of charged particle yields to those in p+p collisions show that yield per nucleon-nucleon collision saturates with ν for high momentum particles. We also present the charged hadron to neutral pion ratios as a function of p_T.

Azimuthal angle (Δϕ) correlations are presented for charged hadrons from dijets for 0.4<p_T<10 GeV/c in Au+Au collisions at sqrt[s_NN]=200 GeV. With increasing p_T, the away-side distribution evolves from a broad and relatively flat shape to a concave shape, then to a convex shape. Comparisons to p+p data suggest that the away-side can be divided into a partially suppressed “head” region centered at Δϕ~π and an enhanced “shoulder” region centered at Δϕ~π±1.1. The p_T spectrum for the head region softens toward central collisions, consistent with the onset of jet quenching. The spectral slope for the shoulder region is independent of centrality and trigger p_T, which offers constraints on energy transport mechanisms and suggests that it contains the medium response to energetic jets.

We have investigated the ferromagnetic (FM) domain structure of a single-crystal bilayered manganite La_2−2xSr_1+2xMn₂O₇ (x=0.32) by using low-temperature magnetic force microscopy. We observed that below 65  K, the FM domains form stable treelike patterns with out-of-plane magnetization. With increasing temperature, the FM domain patterns gradually change in the form of domain wall motion. Above 80  K, the FM domain patterns change more and more with each temperature step. The magnetization changes from the out-of-plane to an in-plane direction around 88  K. The in-plane FM domains almost completely disappear near the Curie temperature of this sample (T_C≈110  K), where the resistivity exhibits a sharp increase. We also observed large changes in the magnetic structures upon thermal cycling. We concluded that the formation of FM domains at low temperatures (T<80  K) is determined by the energy associated with surface magnetic free poles and domain walls. At high temperatures (80  K<T<T_C), the two-dimensional FM fluctuations in the basal plane may also play an important role in forming the domain structures. The evolution of the FM domain patterns with temperature coincides with the change in resistivity.

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.

Angle-resolved photoemission spectroscopy data for the bilayer manganite La_1.2Sr_1.8Mn₂O₇ show that, upon lowering the temperature below the Curie point, a coherent polaronic metallic ground state emerges very rapidly with well-defined quasiparticles which track remarkably well the electrical conductivity, consistent with macroscopic transport properties. Our data suggest that the mechanism leading to the insulator-to-metal transition in La_1.2Sr_1.8Mn₂O₇ can be regarded as a polaron coherence condensation process acting in concert with the double exchange interaction.

Kaon electroproduction from light nuclei and hydrogen, using ¹H, ²H, ³He, ⁴He, and carbon targets has been measured at the Thomas Jefferson National Accelerator Facility. The quasifree angular distributions of Λ and Σ hyperons were determined at Q²=0.35 (GeV/c)² and W=1.91 GeV. Electroproduction on hydrogen was measured at the same kinematics for reference.

Muon production at forward rapidity (1.5≤|η|≤1.8) has been measured by the PHENIX experiment over the transverse momentum range 1≤p_T≤3  GeV/c in sqrt[s]=200  GeV p+p collisions at the Relativistic Heavy Ion Collider. After statistically subtracting contributions from light hadron decays an excess remains which is attributed to the semileptonic decays of hadrons carrying heavy flavor, i.e. charm quarks or, at high p_T, bottom quarks. The resulting muon spectrum from heavy flavor decays is compared to PYTHIA and a next-to-leading-order perturbative QCD calculation. PYTHIA is used to determine the charm quark spectrum that would produce the observed muon excess. The corresponding differential cross section for charm quark production at forward rapidity is determined to be dσ_cc̅ /dy|_y=1.6=0.243±0.013(stat.)±0.105(data syst.)+0.049 / -0.087(PYTHIA syst.)  mb.

Tunneling data on La_1.28Sr_1.72Mn₂O₇ crystals confirm Coulomb interaction effects through the sqrt[E] dependence of the density of states. Importantly, the data and analysis at high energy E show a pileup of states: most of the states removed from near the Fermi level are found between ∼40 and 130  meV, from which we infer the possibility of universal behavior. The agreement of our tunneling data with recent photoemission results further confirms our analysis.

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 report the first measurement of the opening angle distribution between pairs of jets produced in high-energy collisions of transversely polarized protons. The measurement probes (Sivers) correlations between the transverse spin orientation of a proton and the transverse momentum directions of its partons. With both beams polarized, the wide pseudorapidity (-1≤η≤+2) coverage for jets permits separation of Sivers functions for the valence and sea regions. The resulting asymmetries are all consistent with zero and considerably smaller than Sivers effects observed in semi-inclusive deep inelastic scattering. We discuss theoretical attempts to reconcile the new results with the sizable transverse spin effects seen in semi-inclusive deep inelastic scattering and forward hadron production in pp collisions.

The PHENIX experiment presents results from the Relativistic Heavy Ion Collider 2005 run with polarized proton collisions at sqrt[s]=200  GeV, for inclusive π⁰ production at midrapidity. Unpolarized cross section results are given for transverse momenta p_T=0.5 to 20  GeV/c, extending the range of published data to both lower and higher p_T. The cross section is described well for p_T<1  GeV/c by an exponential in p_T, and, for p_T>2  GeV/c, by perturbative QCD. Double helicity asymmetries A_LL are presented based on a factor of 5 improvement in uncertainties as compared to previously published results, due to both an improved beam polarization of 50%, and to higher integrated luminosity. These measurements are sensitive to the gluon polarization in the proton. Using one representative model of gluon polarization it is demonstrated that the gluon spin contribution to the proton spin is significantly constrained.

Measurements of neutral pion (π⁰) production at midrapidity in sqrt[s_NN]=200 GeV Au+Au collisions as a function of transverse momentum, p_T, collision centrality, and angle with respect to reaction plane are presented. The data represent the final π⁰ results from the PHENIX experiment for the first RHIC Au+Au run at design center-of-mass energy. They include additional data obtained using the PHENIX Level-2 trigger with more than a factor of 3 increase in statistics over previously published results for p_T>6 GeV/c. We evaluate the suppression in the yield of high-p_T π⁰'s relative to pointlike scaling expectations using the nuclear modification factor R_AA. We present the p_T dependence of R_AA for nine bins in collision centrality. We separately integrate R_AA over larger p_T bins to show more precisely the centrality dependence of the high-p_T suppression. We then evaluate the dependence of the high-p_T suppression on the emission angle Δϕ of the pions with respect to event reaction plane for seven bins in collision centrality. We show that the yields of high-p_T π⁰'s vary strongly with Δϕ, consistent with prior measurements . We show that this variation persists in the most peripheral bin accessible in this analysis. For the peripheral bins we observe no suppression for neutral pions produced aligned with the reaction plane, whereas the yield of π⁰'s produced perpendicular to the reaction plane is suppressed by a factor of ~2. We analyze the combined centrality and Δϕ dependence of the π⁰ suppression in different p_T bins using different possible descriptions of parton energy loss dependence on jet path-length averages to determine whether a single geometric picture can explain the observed suppression pattern.

Longitudinal density correlations of produced matter in Au+Au collisions at sqrt[s_NN]=200 GeV have been measured from the inclusive charged particle distributions as a function of pseudorapidity window sizes. The extracted αξ parameter, related to the susceptibility of the density fluctuations in the long-wavelength limit, exhibits a nonmonotonic behavior as a function of the number of participant nucleons, N_part. A local maximum is seen at N_part~90, with corresponding energy density based on the Bjorken picture of ε_Bjτ~2.4 GeV/(fm²c) with a transverse area size of 60 fm². This behavior may suggest a critical phase boundary based on the Ginzburg-Landau framework.

Resistance and current-voltage characteristics of individual superconducting NbSe₂ nanowires are investigated. In the current-voltage curves, a stairlike structure is observed, indicating the possible formation of phase-slip centers. A close examination of the current-voltage characteristic in a selected high quality NbSe₂ nanowire with a diameter of 75  nm reveals that the characteristic voltages in the stairlike structure follow the BCS-like temperature dependence of superconducting gaps vanishing at T_C. While the phase-slip center mechanism remains to be a plausible explanation of the observed features, an alternative model involving multigap Josephson tunneling is proposed to account for the BCS-like temperature dependence. From the BCS fits, two distinct superconducting gaps are extracted. Moreover, the critical current of the 75  nm nanowire at low temperatures as well as near T_C can also be described by the Ambegaokar-Baratoff relation for multigap Josephson junctions. Our data suggest the possible observation of multiband superconductivity in NbSe₂ and are in good agreement with the predictions of recent band structure and Fermi surface calculations on NbSe₂.

We present first measurements of the ϕ-meson elliptic flow (v₂(p_T)) and high-statistics p_T distributions for different centralities from sqrt[s_NN]=200  GeV Au+Au collisions at RHIC. In minimum bias collisions the v₂ of the ϕ meson is consistent with the trend observed for mesons. The ratio of the yields of the Ω to those of the ϕ as a function of transverse momentum is consistent with a model based on the recombination of thermal s quarks up to p_T∼4  GeV/c, but disagrees at higher momenta. The nuclear modification factor (R_CP) of ϕ follows the trend observed in the K_S⁰ mesons rather than in Λ baryons, supporting baryon-meson scaling. These data are consistent with ϕ mesons in central Au+Au collisions being created via coalescence of thermalized s quarks and the formation of a hot and dense matter with partonic collectivity at RHIC.

The system created in noncentral relativistic nucleus-nucleus collisions possesses large orbital angular momentum. Because of spin-orbit coupling, particles produced in such a system could become globally polarized along the direction of the system angular momentum. We present the results of Λ and Λ̅ hyperon global polarization measurements in Au+Au collisions at sqrt[s_NN]=62.4 and 200 GeV performed with the STAR detector at the BNL Relativistic Heavy Ion Collider (RHIC). The observed global polarization of Λ and Λ̅ hyperons in the STAR acceptance is consistent with zero within the precision of the measurements. The obtained upper limit, |P_Λ,Λ̅ |≤0.02, is compared with the theoretical values discussed recently in the literature.

The 18.5  K superconductor PuCoGa₅ has many unusual properties, including those due to damage induced by self-irradiation. The superconducting transition temperature decreases sharply with time, suggesting a radiation-induced Frenkel defect concentration much larger than predicted by current radiation damage theories. Extended x-ray absorption fine-structure measurements demonstrate that while the local crystal structure in fresh material is well ordered, aged material is disordered much more strongly than expected from simple defects, consistent with strong disorder throughout the damage cascade region. These data highlight the potential impact of local lattice distortions relative to defects on the properties of irradiated materials and underscore the need for more atomic-resolution structural comparisons between radiation damage experiments and theory.

Differential elliptic flow (v₂) for ϕ mesons and (anti)deuterons (d̅ )d is measured for Au+Au collisions at sqrt[s_NN]=200  GeV. The v₂ for ϕ mesons follows the trend of lighter π^± and K^± mesons, suggesting that ordinary hadrons interacting with standard hadronic cross sections are not the primary driver for elliptic flow development. The v₂ values for (d̅ )d suggest that elliptic flow is additive for composite particles. This further validation of the universal scaling of v₂ per constituent quark for baryons and mesons suggests that partonic collectivity dominates the transverse expansion dynamics.

We have searched for strangelets in a triggered sample of 61 million central (top 4%) Au+Au collisions at sqrt[s_NN]=200 GeV near beam rapidities at the STAR solenoidal tracker detector at the BNL Relativistic Heavy Ion Collider. We have sensitivity to metastable strangelets with lifetimes of order ≥0.1 ns, in contrast to limits over ten times longer in BNL Alternating Gradient Synchrotron (AGS) studies and longer still at the CERN Super Proton Synchrotron (SPS). Upper limits of a few 10^-6 to 10^-7 per central Au+Au collision are set for strangelets with mass ≳30 GeV/c².