Search Results (235 total)

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.

Background: Experimental information on the structure of low-lying levels with isospin 3/2 in systems with 13 nucleons can be used to evaluate the quality of modern theoretical predictions for the light exotic systems. This level structure is poorly known at present. Purpose: Search for T=3/2 states in ¹³C was performed in this work. Method: These states were observed in the resonance elastic scattering of radioactive beam ¹²B on protons using the thick target inverse kinematics technique. Results: Six new states in ¹³C were identified as T=3/2 states. Tentative spin-parity assignments are suggested. Comparison to the previous knowledge of the level structure of T=3/2 A=13 system and to shell-model predictions is given. Conclusion: The elastic scattering of neutron-rich beams on proton target is a convenient tool for isobaric analog states search. The observed ¹²B+p excitation function is determined by the T=3/2 states and no features associated with T=1/2 states were found. Although the level scheme of T=3/2 states in A=13 systems is still far from being complete the emerging picture shows significant disagreements with predictions of contemporary shell models.

We report measurements of a coherent coupling between surface plasmon polaritons (SPP) and quantum well excitons in a hybrid metal-semiconductor nanostructure. The hybrid structure is designed to optimize the radiative exciton-SPP interaction which is probed by low-temperature, angle-resolved, far-field reflectivity spectroscopy. As a result of the coupling, a significant shift of ∼7  meV and an increase in broadening by ∼4  meV of the quantum well exciton resonance are observed. The experiments are corroborated by a phenomenological coupled-oscillator model predicting coupling strengths as large as 50 meV in structures with optimized detunings between the coupled exciton and SPP resonances. Such a strong interaction can, e.g., be used to enhance the luminescence yield of semiconductor quantum structures or to amplify SPP waves.

The first measurements of x_F-dependent single-spin asymmetries of identified charged hadrons, π^±, K^±, and protons, from transversely polarized proton-proton collisions at 62.4 GeV at RHIC are presented. Large asymmetries are seen in the pion and kaon channels. The asymmetries in inclusive π⁺ production, A_N(π⁺), increase with x_F from 0 to ∼0.25 and A_N(π^-) decrease from 0 to ∼-0.4. Observed asymmetries for K^- unexpectedly show positive values similar to those for K⁺, increasing with x_F, whereas proton asymmetries are consistent with zero over the measured kinematic range. Comparisons of the data with predictions of QCD-based models are presented.

We present particle spectra for charged hadrons π^±, K^±, p, and p̅ from pp collisions at sqrt[s]=200  GeV measured for the first time at forward rapidities (2.95 and 3.3). The kinematics of these measurements are skewed in a way that probes the small momentum fraction in one of the protons and large fractions in the other. Large proton to pion ratios are observed at values of transverse momentum that extend up to 4  GeV/c, where protons have momenta up to 35 GeV. Next-to-leading order perturbative QCD calculations describe the production of pions and kaons well at these rapidities, but fail to account for the large proton yields and small p̅ /p ratios.

This Letter presents measurements of the elliptic flow of charged particles as a function of pseudorapidity and centrality from Cu-Cu collisions at 62.4 and 200 GeV using the PHOBOS detector at the Relativistic Heavy Ion Collider. The elliptic flow in Cu-Cu collisions is found to be significant even for the most central events. For comparison with the Au-Au results, it is found that the detailed way in which the collision geometry (eccentricity) is estimated is of critical importance when scaling out system-size effects. A new form of eccentricity, called the participant eccentricity, is introduced which yields a scaled elliptic flow in the Cu-Cu system that has the same relative magnitude and qualitative features as that in the Au-Au system.

We present a theory for size-dependent shapes of Pb nanoprecipitates in Al, introducing the concept of “magic shapes,” i.e., shapes having near-zero homogeneous elastic strains. Our quantitative atomistic calculations of edge energies show their effect on precipitate shape to be negligible, thus it appears that shapes must be due to the combined effect of strain and interface energies. By employing an algorithm for generating magic shapes, we replicate the experimental observations by selecting magic-shape precipitates with interfacial energies less than a cutoff value.

Transverse momentum spectra of pions, kaons, protons, and antiprotons from Au+Au collisions at sqrt[s_NN] = 62.4 GeV have been measured by the PHOBOS experiment at the Relativistic Heavy Ion Collider at Brookhaven National Laboratory. The identification of particles relies on three different methods: low momentum particles stopping in the first detector layers; the specific energy loss (dE/dx) in the silicon spectrometer, and time-of-flight measurement. These methods cover the transverse momentum ranges 0.03–0.2, 0.2–1.0, and 0.5–3.0 GeV/c, respectively. Baryons are found to have substantially harder transverse momentum spectra than mesons. The p_T region in which the proton to pion ratio reaches unity in central Au+Au collisions at sqrt[s_NN] = 62.4 GeV fits into a smooth trend as a function of collision energy. At low transverse mass, the spectra of various species exhibit a significant deviation from transverse mass scaling. The observed particle yields at very low p_T are comparable to extrapolations from higher p_T for kaons, protons and antiprotons. By comparing our results to Au+Au collisions at sqrt[s_NN] = 200 GeV, we conclude that the net proton yield at midrapidity is proportional to the number of participant nucleons in the collision.

The reaction ¹³C(α,n) is considered to be the main source of neutrons for the s process in asymptotic giant branch stars. At low energies, the cross section is dominated by the 1/2⁺ 6.356 MeV subthreshold resonance in ¹⁷O whose contribution at stellar temperatures is uncertain by a factor of 10. In this work, we performed the most precise determination of the low-energy astrophysical S factor using the indirect asymptotic normalization (ANC) technique. The α-particle ANC for the subthreshold state has been measured using the sub-Coulomb α-transfer reaction (⁶Li,d). Using the determined ANC, we calculated S(0), which turns out to be an order of magnitude smaller than in the nuclear astrophysics compilation of reaction rates.

The PHOBOS experiment at the BNL Relativistic Heavy Ion Collider has measured the total multiplicity of primary charged particles as a function of collision centrality in Au+Au collisions at sqrt[s_NN]= 19.6, 130, and 200 GeV. An approximate independence of 〈N_ch〉/〈N_part/2〉 on the number of participating nucleons is observed, reminiscent of “wounded nucleon” scaling (N_ch∝N_part) observed in proton-nucleus collisions. Unlike p+A, the constant of proportionality does not seem to be set by the pp/p̅ p data at the same energy. Rather, there seems to be a surprising correspondence with the total multiplicity measured in e⁺e^- annihilations, as well as the rapidity shape measured over a large range. The energy dependence of the integrated multiplicity per participant pair shows that e⁺e^- and A+A data agree over a large range of center-of-mass energies (sqrt[s]>20 GeV), and pp/p̅ p data can be brought to agree approximately with the e⁺e^- data by correcting for the typical energy taken away by leading particles. This is suggestive of a mechanism for soft particle production that depends mainly on the amount of available energy. It is conjectured that the dominant distinction between A+A and p+p collisions is the multiple collisions per participant, which appears to be sufficient to substantially reduce the energy taken away by leading particles.

The charged-particle pseudorapidity density for Au+Au collisions at sqrt[s_NN]=62.4 GeV has been measured over a wide range of impact parameters and compared to results obtained at other energies. As a function of collision energy, the pseudorapidity distribution grows systematically both in height and width. The midrapidity density is found to grow approximately logarithmically between BNL Alternating Gradient Synchrotron (AGS) energies and the top BNL Relativistic Heavy Ion Collider (RHIC) energy. There is also an approximate factorization of the centrality and energy dependence of the midrapidity yields. The new results at sqrt[s_NN]=62.4 GeV confirm the previously observed phenomenon of “extended longitudinal scaling” in the pseudorapidity distributions when viewed in the rest frame of one of the colliding nuclei. It is also found that the evolution of the shape of the distribution with centrality is energy independent, when viewed in this reference frame. As a function of centrality, the total charged particle multiplicity scales linearly with the number of participant pairs as it was observed at other energies.

A coronagraph designed for high contrast imaging applications has been experimentally tested using coherent laser light and a vortex mask of topological charge m=2. Intensity contrast values of 95% were achieved in this first verification of the scheme. Improvements for approaching the theoretical value of 100% are suggested.

Forward-backward correlations of charged-particle multiplicities in symmetric bins in pseudorapidity are studied to gain insight into the underlying correlation structure of particle production in Au+Au collisions. The PHOBOS detector is used to measure integrated multiplicities in bins centered at η, defined within |η|<3, and covering intervals Δη. The variance σ_C² of a suitably defined forward-backward asymmetry variable C is calculated as a function of η,Δη, and centrality. It is found to be sensitive to short-range correlations, and the concept of “clustering” is used to interpret comparisons to phenomenological models.

A subwavelength array of holes in a metal film residing on a silicon photonic crystal exhibits very sharp resonant absorption modes and extraordinary thermal emission. We developed a rigorous electromagnetic theory of subwavelength emitters based on the scattering matrix approach. Simulations exhibit a resonant absorption and emission when the photonic crystal is present, in good agreement with measurements. Diffraction resonances are found within the silicon photonic crystal. These combine with the extraordinary transmission through subwavelength hole arrays to generate resonant absorption in subwavelength arrays of metallodielectric photonic crystals. There is enormous enhancement of fields within the holes for resonant emissive modes.

We report on measurements of directed flow as a function of pseudorapidity in Au+Au collisions at energies of sqrt[s_NN]=19.6, 62.4, 130 and 200 GeV as measured by the PHOBOS detector at the BNL Relativistic Heavy Ion Collider. These results are particularly valuable because of the extensive, continuous pseudorapidity coverage of the PHOBOS detector. There is no significant indication of structure near midrapidity and the data surprisingly exhibit extended longitudinal scaling similar to that seen for elliptic flow and charged particle pseudorapidity density.

We report a liquid-crystal pattern-formation experiment in which we break the lateral (translational) symmetry of a nematic medium with a laser-induced thermal gradient. The work is motivated by an improved measurement (reported here) of the temperature dependence of the electroconvection threshold voltage in planar-nematic 4-methoxybenzylidene-4-butylaniline. In contrast with other broken-symmetry-pattern studies that report a uniform drift, we observe a strip of counterpropagating rolls that collide at a sink point, and a strong temporally periodic amplitude modulation within a width of 3–4 rolls about the sink point. The time dependence of the amplitude at a fixed position is periodic but displays a nonsinusoidal relaxation-oscillation profile. After reporting experimental results based on spacetime contours and wave number profiles, along with a measurement of the change in the drift frequency with applied voltage at a fixed control parameter, we propose some potential guidelines for a theoretical model based on saddle-point solutions for Eckhaus-unstable states and coupled complex Ginzburg-Landau equations.

Two-particle correlations of identical charged pion pairs from Au+Au collisions at sqrt[s_NN]=62.4 and 200 GeV were measured by the PHOBOS experiment at BNL Relativistic Heavy Ion Collider (RHIC). Data for the 15% most central events were analyzed with Bertsch-Pratt and Yano-Koonin-Podgoretskii parametrizations using pairs with rapidities of 0.4<y_ππ<1.3 and transverse momenta 0.1<k_T<1.4 GeV/c. The Bertsch-Pratt radii R_o and R_ℓ decrease as a function of pair transverse momentum. R_o and R_s are independent of collision energy, while R_ℓ shows a slight increase. The source rapidity y_YKP scales roughly with the pair rapidity y_ππ, indicating strong dynamical correlations.

We show that antiferromagnetic nanoparticles of α-Fe₂O₃ (hematite) under wet conditions can attach into chains along a common [001] axis. Electron microscopy shows that such chains typically consist of two to five particles. X-ray and neutron diffraction studies show that both structural and magnetic correlations exist across the interfaces along the [001] direction. This gives direct evidence for exchange coupling between particles. Exchange coupling between nanoparticles can suppress superparamagnetic relaxation and it may play a role for attachment along preferred directions. The relations between exchange coupling, magnetic properties, and oriented attachment are discussed.

This Rapid Communication describes the measurement of elliptic flow for charged particles in Au+Au collisions at sqrt[s_NN]=200 GeV using the PHOBOS detector at the Relativistic Heavy Ion Collider. The measured azimuthal anisotropy is presented over a wide range of pseudorapidity for three broad collision centrality classes for the first time at this energy. Two distinct methods of extracting the flow signal were used to reduce systematic uncertainties. The elliptic flow falls sharply with increasing |η| at 200 GeV for all the centralities studied, as observed for minimum-bias collisions at sqrt[s_NN]=130 GeV.

The measured pseudorapidity distributions of primary charged particles over a wide pseudorapidity range of |η|≤5.4 and integrated charged particle multiplicities in d+Au collisions at sqrt[s_NN]=200  GeV are presented as a function of collision centrality. The longitudinal features of d+Au collisions at sqrt[s_NN]=200  GeV are found to be very similar to those seen in p+A collisions at lower energies. The total multiplicity of charged particles is found to scale with the total number of participants according to N_ch^dAu=1 / 2〈N_part〉N_ch^pp, and the energy dependence of the density of charged particles produced in the fragmentation region exhibits extended longitudinal scaling.

This Letter describes the measurement of the energy dependence of elliptic flow for charged particles in Au+Au collisions using the PHOBOS detector at the Relativistic Heavy Ion Collider. Data taken at collision energies of sqrt[s_NN]=19.6, 62.4, 130, and 200 GeV are shown over a wide range in pseudorapidity. These results, when plotted as a function of η^′=|η|-y_beam, scale with approximate linearity throughout η^′, implying no sharp changes in the dynamics of particle production as a function of pseudorapidity or increasing beam energy.

We have measured transverse momentum distributions of charged hadrons produced in Au+Au collisions at sqrt[s_NN]=62.4  GeV. The spectra are presented for transverse momenta 0.25<p_T<4.5  GeV/c, in a pseudorapidity range of 0.2<η<1.4. The nuclear modification factor R_AA is calculated relative to p+p data at the same collision energy as a function of collision centrality. For 2<p_T<4.5  GeV/c, R_AA is found to be significantly larger than in Au+Au collisions at sqrt[s_NN]= 130 and 200 GeV. In contrast to the large change in R_AA, we observe a very similar centrality evolution of the p_T spectra at sqrt[s_NN]=62.4 and 200 GeV. The dynamical origin of this surprising factorization of energy and centrality dependence of particle production in heavy-ion collisions remains to be understood.

The ratios of the yields of primary charged antiparticles to particles have been obtained for pions, kaons, and protons near midrapidity for p+p collisions at sqrt[s_NN]=200  GeV. Ratios of 〈π^-/π⁺〉=1.000±0.012 (stat.) ±  0.019 (syst.), 〈K^-/K⁺〉=0.93±0.05 (stat.) ±  0.03 (syst.), and 〈p̅ /p〉=0.85±0.04 (stat.) ±  0.03 (syst.) have been measured. The reported values represent the ratio of the yields averaged over the rapidity range of 0.1<y_π<1.3 and 0<y_K,p<0.8, and for transverse momenta of 0.1<p_T^π,K<1.0  GeV/c and 0.3<p_T^p<1.0  GeV/c. Within the uncertainties, all three ratios are consistent with the values measured in d+Au collisions at the same energy. The data are compared to results from other collision systems and energies.

We have measured the transverse momentum distributions of charged hadrons in d+Au collisions at sqrt[s_NN]=200  GeV in the range of 0.5<p_T<4.0  GeV∕c. The total range of pseudorapidity, η, is 0.2<η<1.4, where positive η is in the deuteron direction. The data has been divided into three regions of pseudorapidity, covering 0.2<η<0.6, 0.6<η<1.0, and 1.0<η<1.4, and has been compared to charged hadron spectra from p+p̅ collisions at the same energy. There is a significant change in the spectral shape as a function of pseudorapidity. As η increases we see a decrease in the nuclear modification factor R_dAu.

We present results on charged particle production at very low transverse momenta in the 15% most central Au+Au collisions at sqrt[s_NN]=200  GeV obtained with the PHOBOS detector at the Relativistic Heavy Ion Collider. The invariant yields were measured at midrapidity in the transverse momentum ranges from 30  to  50  MeV∕c for charged pions, 90  to  130  MeV∕c for charged kaons and 140  to  210  MeV∕c for protons and antiprotons. No significant enhancement in low transverse momentum particle production is observed as compared to extrapolations of identified particle spectra measured at an intermediate p_T range. The spectra tend to flatten at low p_T, consistent with the expectations of transverse expansion of the system.