Search Results (292 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.

One of the remaining issues concerning the spin-triplet superconductivity of Sr₂RuO₄ is the strong limit of the in-plane upper critical field H_c2 at low temperatures. In this study, we clarified the dependence of H_c2 on the angle θ between the magnetic field and the ab plane at various temperatures, by precisely and accurately controlling the magnetic field direction. We revealed that, although the temperature dependence of H_c2 for |θ|≥5° is well explained by the orbital pair-breaking effect, H_c2(T) for |θ|<5° is clearly limited at low temperatures. We also revealed that the H_c2 limit for |θ|<5° is present not only at low temperatures but also at temperatures close to T_c. These features may provide additional hints for clarifying the origin of the H_c2 limit. Interestingly, if the anisotropic ratio in Sr₂RuO₄ is assumed to depend on temperature, the observed angular dependence of H_c2 is reproduced better at lower temperature with an effective-mass model for an anisotropic three-dimensional superconductor. We discuss the observed behavior of H_c2 based on existing theories.

Interactions of x-ray free-electron laser (XFEL) light with a single cluster target are numerically investigated by using a three-dimensional particle-in-cell code. The plasma dynamics as well as relevant atomic processes are taken into account, such as photoionization, the Auger effect, collisional ionization and relaxation, and field ionization. It is found that as the XFEL intensity increases to as high as ∼10²¹ (photons/pulse)/mm², the field ionization, which is the dominant ionization process over the other atomic processes, leads to rapid target ionization. The target damage due to the irradiation by XFEL light is numerically evaluated, which gives an estimation of the XFEL intensity so as to suppress the target damage within a tolerable range for imaging.

An approach for accelerating ions, with the use of a cluster-gas target and an ultrashort pulse laser of 150-mJ energy and 40-fs duration, is presented. Ions with energy 10–20 MeV per nucleon having a small divergence (full angle) of 3.4° are generated in the forward direction, corresponding to approximately tenfold increase in the ion energies compared to previous experiments using solid targets. It is inferred from a particle-in-cell simulation that the high energy ions are generated at the rear side of the target due to the formation of a strong dipole vortex structure in subcritical density plasmas.

β-decay spectroscopy of the proton-rich nucleus ²⁴Si was performed. The decay scheme was reconstructed from results of delayed γ-ray and proton measurements. We observed two β branches to bound states in ²⁴Al for the first time. The branching ratios were determined to be 31(4)% and 23.9(15)% for the 1₁⁺ state at 0.426 MeV and the state at 1.090 MeV, respectively. The observation of an allowed transition to the 1.090-MeV state enabled us to firmly determine its spin-parity as 1⁺. In the proton measurements performed with the ΔE-E method, we observed a new unbound level at 6.735 MeV. The branching ratios to three unbound states, including the new level, were also determined for the first time. Based on the decay scheme, the B(GT) values of ²⁴Si were deduced. The B(GT) values were smaller than those of the mirror nucleus ²⁴Ne by 22% and 10% for the 1₁⁺ and 1₂⁺ states, respectively. The mirror asymmetries of B(GT), observed in both the 1₁⁺ and the 1₂⁺ states, indicate changes in configuration in the wave function associated with the Thomas-Ehrman shift. To clarify the mechanism of this asymmetry, a comparison with shell-model calculations is also discussed. The calculations attribute the changes in configuration to the lowering of the 1s_1/2 orbital.

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 study the energy and angular distributions of two emitted neutrons from the dipole excitation of two typical, weakly bound Borromean nuclei, ¹¹Li and ⁶He, by using a three-body model. Our calculation indicates that those distributions are considerably different between the two nuclei, even though both the nuclei exhibit similar strong dineutron correlations in the ground state to each other. We point out that this different behavior primarily reflects the interaction between the neutron and the core nucleus, especially the s-wave virtual state in ¹⁰Li, rather than the interaction between the valence neutrons.

We present angle-resolved photoelectron spectroscopy data probing the electronic structure of the Nd-substituted high-T_c cuprate La_1.48Nd_0.4Sr_0.12CuO₄. Data have been acquired at low and high photon energies, hν=55 and 500 eV, respectively. The two extracted Fermi surfaces show significant differences. The differences can be attributed to either the change in probing depth suggesting dissimilarity of the intrinsic electronic structure between surface and bulk regions, or a considerable c-axis dispersion signaling a strong interlayer coupling. At both photon energies, considerable spectral weight is observed at all points along the Fermi surface and the intensity distribution as well as Fermi-surface shape observed at low as well as high photon energy is markedly different from what has been previously reported for La_1.28Nd_0.6Sr_0.12CuO₄ by Zhou [Science 286, 268 (1999)].

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.

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 have performed soft x-ray photoemission studies of Ca-intercalated graphite superconductor CaC₆(T_c=11.2 K). The valence-band spectrum shows six main structures, with a peak at the Fermi level (E_F), which correspond to those of calculated DOS. The intensity of the E_F peak is resonantly enhanced at the Ca 2p-3d threshold, providing spectroscopic evidence for the existence of Ca 3d electrons at E_F, which is confirmed experimentally for the first time. The Ca 2p core-level spectrum has a very large asymmetric line shape, which is further possible suggestion of the existence of Ca 3d derived conduction electrons at Ca sites. These results strongly support the picture where Ca 3d derived interlayer band plays a crucial role for the superconductivity of this material with relatively high T_c.

We report on the first spectroscopic study of the N=22 nucleus ³²Ne at the newly completed RIKEN Radioactive Ion Beam Factory. A single γ-ray line with an energy of 722(9) keV was observed in both inelastic scattering of a 226  MeV/u ³²Ne beam on a carbon target and proton removal from ³³Na at 245  MeV/u. This transition is assigned to the deexcitation of the first J^π=2⁺ state in ³²Ne to the 0⁺ ground state. Interpreted through comparison with state-of-the-art shell-model calculations, the low excitation energy demonstrates that the “island of inversion” extends to at least N=22 for the Ne isotopes.

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.

A quantum-thermal annealing method using a cluster-flip algorithm is studied in the two-dimensional spin-glass model. The temperature (T) and the transverse field (Γ) are decreased simultaneously with the same rate along a linear path on the T-Γ plane. We found that the additional pulse of the transverse field to the frozen local spins produces a good approximate solution with a low computational cost.

Proton inelastic scattering on the neutron-rich nucleus ³²Mg has been studied at 46.5 MeV/nucleon in inverse kinematics. Populated states were identified by measuring de-excitation γ rays, in which five new states were found by γ-γ coincidence analyses. By analyzing the angular differential cross sections via coupled-channel calculations, their spins and parities were constrained and the amplitudes for each transition were extracted. The spin and parity of the 2321-keV state was assigned as 4₁⁺. The ratio between the energies of the 2₁⁺ and 4₁⁺ states indicates that ³²Mg is a transitional nucleus rather than an axially deformed rigid rotor. The collectivities in the nucleus ³²Mg with N=20 are discussed based on the results obtained in the present experiment.

The neutron capture reaction on ¹⁴C leading to the ¹⁵C ground state, which plays an important role in various nucleosynthesis processes, has been studied using the Coulomb breakup of ¹⁵C on a Pb target at 68 MeV/nucleon. The breakup cross section has been converted into the energy-dependent neutron capture cross section using the principle of detailed balance. The energy spectrum shows typical p-wave neutron capture characteristics, which is explained by the fact that the ground state of ¹⁵C possesses a strong single-particle s-wave component and a moderate-sized neutron halo structure. The capture cross section for the ¹⁴C(n,γ)¹⁵C reaction derived from the present experiment has been found to be consistent with the most recent data, directly measured using a ¹⁴C target. This result assures the validity of the Coulomb breakup method in deriving the neutron capture cross section for neutron-rich nuclei.

The electronic excitation energy transfer between excitons in silicon nanocrystal assemblies and iodine molecules in an organic solution is studied. From the time-resolved photoluminescence the rate of the energy transfer is found to increase with approaching a wavelength region, where the photoluminescence spectrum of nanocrystals overlaps the absorption spectrum of iodine molecules, and with increasing the radiative recombination rate of nanocrystals. The energy-transfer rate is also found to depend on the concentration of iodine molecules. This dependence is well explained by Förster-type dipole-dipole interaction mechanism in which the diffusion of the assemblies and molecules is taken into consideration.

The PHENIX experiment presents results from the RHIC 2006 run with polarized p+p collisions at sqrt[s]=62.4  GeV, for inclusive π⁰ production at midrapidity. Unpolarized cross section results are measured for transverse momenta p_T=0.5 to 7  GeV/c. Next-to-leading order perturbative quantum chromodynamics calculations are compared with the data, and while the calculations are consistent with the measurements, next-to-leading logarithmic corrections improve the agreement. Double helicity asymmetries A_LL are presented for p_T=1 to 4  GeV/c and probe the higher range of Bjorken x of the gluon (x_g) with better statistical precision than our previous measurements at sqrt[s]=200  GeV. These measurements are sensitive to the gluon polarization in the proton for 0.06<x_g<0.4.

The structure of neutron-rich isotopes ⁶⁰Cr and ⁶²Cr was studied via proton inelastic scattering in inverse kinematics. The deformation lengths (δ) for ⁶⁰Cr and ⁶²Cr were extracted as 1.12(16) and 1.36(14) fm, respectively, providing evidence for enhanced collectivity in these nuclei. An excited state at 1180(10) keV in ⁶²Cr was identified for the first time. We adopted 4⁺ as its spin and parity, leading to the rapid increase of the E_x(4⁺)/E_x(2⁺) ratio, which indicates the development of large deformation in ⁶²Cr near N=40. Importance of the admixture of the gd-shell component above N=40 is also discussed by comparing with a modern shell model calculation.

We have studied the one-neutron removal reactions of ¹⁸C at 81 MeV/nucleon and ¹⁹C at 68 MeV/nucleon on a liquid hydrogen target. Transverse-momentum distributions of the core fragments ¹⁷C and ¹⁸C, and their partial cross sections, are measured using coincidences with the γ rays. These are compared to theoretical calculations based on the continuum-discretized coupled-channels (CDCC) method. We have found that the transverse-momentum distributions of ¹⁷C are well described by the CDCC calculations, and the agreement provides evidence for the spin-parity assignments of J^π=1/2⁺ for the 0.21-MeV state and J^π=5/2⁺ for the 0.33-MeV state in ¹⁷C. The J^π=(2,3)⁺ assignment for the 4.0-MeV state in ¹⁸C is obtained for the first time based on the momentum distribution of ¹⁸C. The measured partial cross sections are consistent with the CDCC calculations combined with shell-model spectroscopic factors. The present study demonstrates that the transverse-momentum distribution obtained in a one-neutron removal reaction on a proton target is as useful as the conventional longitudinal-momentum distribution measured with the heavier Be or C targets for determining the orbital angular momentum of a removed neutron.

We have studied the production of the projectile-like fragments ⁸Be and ⁹B produced in interactions of 200 to 400 MeV/nucleon carbon ions with water, using emulsion detectors. In this Brief Report we present the first published production cross section of the projectile-like fragment ⁹B in the energy region above 100 MeV/nucleon. The measured production cross sections of these nuclides were compared to calculations using a semiempirical model. We found that the measured cross sections deviate from the calculated values by a factor up to about six. This information is of importance for benchmarking and improving heavy ion nuclear reaction models.

For Au+Au collisions at 200 GeV, we measure neutral pion production with good statistics for transverse momentum, p_T, up to 20  GeV/c. A fivefold suppression is found, which is essentially constant for 5<p_T<20  GeV/c. Experimental uncertainties are small enough to constrain any model-dependent parametrization for the transport coefficient of the medium, e.g., ⟨q-^ ⟩ in the parton quenching model. The spectral shape is similar for all collision classes, and the suppression does not saturate in Au+Au collisions.

We develop an efficient Monte Carlo algorithm, which accelerates slow Monte Carlo dynamics in quasi-one-dimensional Ising spin systems. The loop algorithm of the quantum Monte Carlo method is applied to the classical spin models with highly anisotropic exchange interactions. Both correlation time and real CPU time are reduced drastically. The algorithm is demonstrated in the layered triangular-lattice antiferromagnetic Ising model. We have obtained the relation between the transition temperature and the exchange interaction parameters, which modifies the result of the chain-mean-field theory.