Search Results (435 total)

Measurements of the azimuthal anisotropy of high-p_T neutral pion (π⁰) production in Au+Au collisions at sqrt[s_NN]=200 GeV by the PHENIX experiment are presented. The data included in this article were collected during the 2004 Relativistic Heavy Ion Collider running period and represent approximately an order of magnitude increase in the number of analyzed events relative to previously published results. Azimuthal angle distributions of π⁰ mesons detected in the PHENIX electromagnetic calorimeters are measured relative to the reaction plane determined event-by-event using the forward and backward beam-beam counters. Amplitudes of the second Fourier component (v₂) of the angular distributions are presented as a function of π⁰ transverse momentum (p_T) for different bins in collision centrality. Measured reaction plane dependent π⁰ yields are used to determine the azimuthal dependence of the π⁰ suppression as a function of p_T, R_AA(Δϕ,p_T). A jet-quenching motivated geometric analysis is presented that attempts to simultaneously describe the centrality dependence and reaction plane angle dependence of the π⁰ suppression in terms of the path lengths of hypothetical parent partons in the medium. This set of results allows for a detailed examination of the influence of geometry in the collision region and of the interplay between collective flow and jet-quenching effects along the azimuthal axis.

We present a temperature-dependent extended x-ray absorption fine structure (EXAFS)/x-ray absorption near edge structure (XANES) investigation of La_1−xSr_xCoO₃ (LSCO) over a wide doping concentration range (0≤x≤0.35). Five of the samples are nanoparticles (x=0.15, 0.20, 0.25, 0.30, and 0.35) and four are bulk powders (x=0, 0.15, 0.20, and 0.30). From the EXAFS analysis, we find that the Co-O bonds are well ordered for both bulk and nanoparticle materials and there is no clear evidence for a Jahn-Teller (JT) distortion in the LSCO system (either static or dynamic). The distortion of the Co-O bond with increasing T, parameterized by the width of the pair distribution function (PDF), σ(T), can easily be modeled using a correlated Debye model with a high correlated Debye temperature ∼800 K. There is also no evidence for a step in plots of σ² vs T. In addition, the very small nonthermal contribution to σ² for the Co-O (PDF), σ_static², sets an upper limit on the extent of any Jahn-Teller distortion at low T. These experiments are inconsistent with the existence of a significant fraction of Co sites with an intermediate spin (IS) state, for which there is a JT active e_g electron on the Co atoms. We cannot, however, exclude the possibility of a tiny fraction of sites having a JT distortion or some other (non-JT active) means of producing an IS state. The bulk samples are well ordered out to at least the third neighbors (Co-Co) while the nanoparticles show increased disorder and a reduction in coordination for Co-Co. XANES data are also presented, and, for both bulk and nanoparticle samples, there is essentially no edge shift with increasing Sr concentration. Bond-valence sums also indicate no change in effective Co valence. These results indicate that when holes are introduced via Sr doping, they have little Co(3d) character and the Co configuration remains close to 3d⁶; we argue that the holes have mostly O(2p) character and are localized more on the O sites.

The perovskite NaOsO₃ shows a Curie-Weiss metallic nature at high temperature and suddenly goes into an antiferromagnetically insulating state at 410 K on cooling. Electronic specific heat at the low-temperature limit is absent, indicating that the band gap fully opens. In situ observation in electron microscopy undetected any lattice anomalies in the vicinity of the transition temperature. It is most likely that the antiferromagnetic correlation plays an essential role in the gap opening.

The ¹⁴C(α,γ) reaction rate at temperatures below 0.3 GK depends on the properties of two near threshold resonances in ¹⁸O, the 1^- at 6.198 MeV and the 3^- at 6.404 MeV. The α+¹⁴C asymptotic normalization coefficients for these resonances were determined using the α-transfer reactions ¹⁴C(⁷Li, t) and ¹⁴C(⁶Li, d) at sub-Coulomb energies. The ¹⁴C(α,γ) reaction rate at low temperatures has been evaluated. Implications of the new reaction rate on the evolution of accreting helium white dwarfs and on the nucleosynthesis of low mass stars during the asymptotic giant branch phase are discussed.

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.

To investigate the universality of magnetic turbulence in space plasmas, we analyze seven time periods in the free solar wind under different plasma conditions. Three instruments on Cluster spacecraft operating in different frequency ranges give us the possibility to resolve spectra up to 300 Hz. We show that the spectra form a quasiuniversal spectrum following the Kolmogorov’s law ∼k^-5/3 at MHD scales, a ∼k^-2.8 power law at ion scales, and an exponential ∼exp⁡[-sqrt[kρ_e]] at scales kρ_e∼[0.1,1], where ρ_e is the electron gyroradius. This is the first observation of an exponential magnetic spectrum in space plasmas that may indicate the onset of dissipation. We distinguish for the first time between the role of different spatial kinetic plasma scales and show that the electron Larmor radius plays the role of a dissipation scale in space plasma turbulence.

The first-order structural phase transition at T_S∼240 K in the extended kagome antiferromagnet TmBaCo₄O₇ has been studied by neutron powder diffraction. In order to comprehend the microscopic origin of the transition, a detailed symmetry analysis is performed, based on crystallographic parameters obtained by Rietveld analysis of the neutron data. The results are consistent with the P31c→Pna2₁ symmetry lowering and support a displacive nature of the phase transition. The complex tilting pattern of CoO₄ tetrahedra in both triangular and kagome sublattices is described based on symmetry-adapted pseudovector distortion modes of the parent P6₃mc hexagonal structure. Our analysis reveals that the unusual topology of the crystal structure does not allow CoO₄ tetrahedra to rotate as rigid units, resulting in their inevitable distortions, whatever the combination of rotational modes considered. A possible analogy between polyhedral distortions and spin frustration in this system is discussed.

Bose-Einstein correlations of charged kaons are used to probe Au+Au collisions at sqrt[s_NN]=200  GeV and are compared to charged pion probes, which have a larger hadronic scattering cross section. Three-dimensional Gaussian source radii are extracted, along with a one-dimensional kaon emission source function. The centrality dependences of the three Gaussian radii are well described by a single linear function of N_part^1/3 with a zero intercept. Imaging analysis shows a deviation from a Gaussian tail at r≳10  fm, although the bulk emission at lower radius is well described by a Gaussian. The presence of a non-Gaussian tail in the kaon source reaffirms that the particle emission region in a heavy-ion collision is extended, and that similar measurements with pions are not solely due to the decay of long-lived resonances.

We present inclusive charged hadron elliptic flow (v₂) measured over the pseudorapidity range |η|<0.35 in Au+Au collisions at sqrt[s_NN]=200 GeV. Results for v₂ are presented over a broad range of transverse momentum (p_T=0.2-8.0 GeV/c) and centrality (0–60%). To study nonflow effects that are correlations other than collective flow, as well as the fluctuations of v₂, we compare two different analysis methods: (1) the event-plane method from two independent subdetectors at forward (|η|=3.1-3.9) and beam (|η|>6.5) pseudorapidities and (2) the two-particle cumulant method extracted using correlations between particles detected at midrapidity. The two event-plane results are consistent within systematic uncertainties over the measured p_T and in centrality 0–40%. There is at most a 20% difference in the v₂ between the two event-plane methods in peripheral (40–60%) collisions. The comparisons between the two-particle cumulant results and the standard event-plane measurements are discussed.

We report the observation at the Relativistic Heavy Ion Collider of suppression of back-to-back correlations in the direct photon+jet channel in Au+Au relative to p+p collisions. Two-particle correlations of direct photon triggers with associated hadrons are obtained by statistical subtraction of the decay photon-hadron (γ-h) background. The initial momentum of the away-side parton is tightly constrained, because the parton-photon pair exactly balance in momentum at leading order in perturbative quantum chromodynamics, making such correlations a powerful probe of the in-medium parton energy loss. The away-side nuclear suppression factor, I_AA, in central Au+Au collisions, is 0.32±0.12^stat±0.09^syst for hadrons of 3<p_T^h<5 in coincidence with photons of 5<p_T^γ<15 GeV/c. The suppression is comparable to that observed for high-p_T single hadrons and dihadrons. The direct photon associated yields in p+p collisions scale approximately with the momentum balance, z_T≡p_T^h/p_T^γ, as expected for a measurement of the away-side parton fragmentation function. We compare to Au+Au collisions for which the momentum balance dependence of the nuclear modification should be sensitive to the path-length dependence of parton energy loss.

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.

The momentum distribution of electrons from semileptonic decays of charm and bottom quarks for midrapidity |y|<0.35 in p+p collisions at sqrt[s]=200  GeV is measured by the PHENIX experiment at the Relativistic Heavy Ion Collider over the transverse momentum range 2<p_T<7  GeV/c. The ratio of the yield of electrons from bottom to that from charm is presented. The ratio is determined using partial D/D̅ →e^±K^∓X (K unidentified) reconstruction. It is found that the yield of electrons from bottom becomes significant above 4  GeV/c in p_T. A fixed-order-plus-next-to-leading-log perturbative quantum chromodynamics calculation agrees with the data within the theoretical and experimental uncertainties. The extracted total bottom production cross section at this energy is σ_bb̅ =3.2_-1.1^+1.2(stat)_-1.3^+1.4(syst)μb.

We report the magnetic and electrical characteristics of polycrystalline FeTiO₃ synthesized at high pressure that is isostructural with acentric LiNbO₃ (LBO). Piezoresponse force microscopy, optical second harmonic generation, and magnetometry demonstrate ferroelectricity at and below room temperature and weak ferromagnetism below ∼120  K. These results validate symmetry-based criteria and first-principles calculations of the coexistence of ferroelectricity and weak ferromagnetism in a series of transition metal titanates crystallizing in the LBO structure.

We have studied the frustrated system YBaCo₄O_7.0 generally described as an alternating stacking of kagome and triangular layers of magnetic ions on a trigonal lattice, by single-crystal neutron diffraction experiments above the Néel ordering transition. Experimental data reveal pronounced magnetic diffuse scattering, which is successfully modeled by direct Monte Carlo simulations. Long-range magnetic correlations are found along the c axis, due to the presence of corner-sharing bipyramids, creating quasi–one-dimensional order at finite temperature. In contrast, in the kagome layers (ab plane), the spin-spin correlation function, displaying a short-range 120° configuration, decays rapidly as typically found in spin liquids. YBaCo₄O₇ experimentally realizes a new class of two-dimensional frustrated systems where the strong out-of-plane coupling does not lift the in-plane degeneracy, but instead acts as an external “field.”

In contrast to ferromagnetic (FM) three-dimensional manganites, ⁵⁵Mn NMR spectra obtained for the FM phase of the colossal magnetoresistance bilayer manganite La_1.2Sr_1.8Mn₂O₇ show a broad distribution of hyperfine fields at Mn sites. The hyperfine distribution reflects variations in the electronic structure at the local level. ⁵⁵Mn spin-lattice relaxation rates have a surprisingly weak dependence both on temperature and on applied magnetic field. Significant departures of the relaxation rate from Korringa temperature dependence below 40 K provide evidence for nonFermi liquid behavior in this quasi-two-dimensional metal. At temperatures approaching T_C from below, in the range where colossal magnetoresistance appears, further anomalous and field-dependent behavior is found in the relaxation rate temperature dependence. The results provide evidence for changes in the electronic structure with temperature in this poorly metallic system. At low temperatures the changes are possibly linked to orbital ordering effects. In addition, statistical fluctuations in dopant concentration may play some role in inducing local variations in the electronic structure. Above 90 K the emergence of polarons is likely to be responsible for the observed decrease in the relaxation rate.

The double helicity asymmetry in neutral pion production for p_T=1 to 12  GeV/c was measured with the PHENIX experiment to access the gluon-spin contribution, ΔG, to the proton spin. Measured asymmetries are consistent with zero, and at a theory scale of μ²=4  GeV² a next to leading order QCD analysis gives ΔG^[0.02,0.3]=0.2, with a constraint of -0.7<ΔG^[0.02,0.3]<0.5 at Δχ²=9 (∼3σ) for the sampled gluon momentum fraction (x) range, 0.02 to 0.3. The results are obtained using predictions for the measured asymmetries generated from four representative fits to polarized deep inelastic scattering data. We also consider the dependence of the ΔG constraint on the choice of the theoretical scale, a dominant uncertainty in these predictions.

The diamagnetic to paramagnetic spin state transition in LaCoO₃ (LCO) that occurs in the temperature range 30–120 K is generally attributed to the small energy gap between the Co³⁺ t_2g and e_g states. Evidence for this thermally activated transition has been interpreted as leading to either the intermediate spin state, t_2g⁵e_g¹(S=1), or, alternatively, to the high-spin state, t_2g⁴e_g²(S=2) of the Co³⁺ ion, with the issue proving highly controversial. In an effort to obtain a consistent description of the temperature dependence of the magnetic and thermal properties of this system, we have made measurements of both the magnetization in applied fields of up to 33 T and the specific heat at 0 and 9 T on a single crystal of LCO. In addition, EPR measurements were made on the same sample using high-field EPR spectrometers. The spin-Hamiltonian parameters are consistent with the previous pulsed-field EPR work and support the atomic-like energy level description of the Co ion. The low-lying first-excited state is part of the ⁵T_2g (⁵D) set and is a triplet state with effective spin S_eff=1. The magnetization results are analyzed using a mean-field model allowing for antiferromagnetic correlations between the spins. The model is used to estimate the spin contribution to the specific heat.

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.

Years of intensive work on perovskite manganites has led to a detailed understanding of the phenomena that emerge from competition between the electronic and lattice degrees of freedom in these correlated electron systems. It is well understood that the related cobaltites provide an additional spin-state degree of freedom. Here, we use the magnetic properties of a particular cobaltite, Pr_1−xSr_xCoO₃, to demonstrate the vital role played by a further ingredient often negligible in manganites; magnetocrystalline anisotropy. Pr_1−xSr_xCoO₃ exhibits an anomalous “double magnetic transition” that cannot be ascribed to a spin-state transition or the usual charge/orbital/antiferromagnetic ordering and has thus far evaded explanation. We show that this is actually due to a coupled structural/magnetocrystalline anisotropy transition driven, in this case, by Pr-O hybridization. The results point to the existence of a distinct class of phenomena in the cobaltites due to the unique interplay between structure and magnetic anisotropy.

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.

Low temperature Mn K-edge x-ray magnetic circular dichroism and x-ray diffraction measurements were carried out to investigate the stability of the ferromagnetic ground state in manganite La_0.75Ca_0.25MnO₃ under nearly uniform compression using diamond anvil cells. The magnetic dichroism signal gradually decreases with pressure and disappears at 23 GPa, and meanwhile a uniaxial compression of MnO₆ octahedra along the b axis is observed to continuously increase with pressure and become anomalously large at 23.5 GPa. These changes are attributed to a ferromagnetic-antiferromagnetic transition that is associated with orbital ordering at high pressure.

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.

The nature of the competing ferromagnetic and incommensurate spin correlations in the spin-glass phase of La_0.85Sr_0.15CoO₃ has been investigated by various neutron scattering techniques. Spin-polarized scattering indicates that the observed incommensurate peaks are dominantly magnetic in nature. Magnetic field experiments show that a field applied perpendicular to the short-range ordering wave vector destroys the incommensurate correlations and induces long-range ferromagnetic order. However, even for fields up to 7 T, short-range ferromagnetic correlations still coexist with the long-range ordered regions.