507 research outputs found

    Modeling the series of (n x 2) Si-rich reconstructions of beta-SiC(001): a prospective atomic wire?

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    We perform ab initio plane wave supercell density functional calculations on three candidate models of the (3 x 2) reconstruction of the beta-SiC(001) surface. We find that the two-adlayer asymmetric-dimer model (TAADM) is unambiguously favored for all reasonable values of Si chemical potential. We then use structures derived from the TAADM parent to model the silicon lines that are observed when the (3 x 2) reconstruction is annealed (the (n x 2) series of reconstructions), using a tight-binding method. We find that as we increase n, and so separate the lines, a structural transition occurs in which the top addimer of the line flattens. We also find that associated with the separation of the lines is a large decrease in the HOMO-LUMO gap, and that the HOMO state becomes quasi-one-dimensional. These properties are qualititatively and quantitatively different from the electronic properties of the original (3 x 2) reconstruction.Comment: 22 pages, including 6 EPS figure

    Charge Symmetry Breaking in dd->4He{\pi}0 with WASA-at-COSY

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    Charge symmetry breaking (CSB) observables are a suitable experimental tool to examine effects induced by quark masses on the nuclear level. Previous high precision data from TRIUMF and IUCF are currently used to develop a consistent description of CSB within the framework of chiral perturbation theory. In this work the experimental studies on the reaction dd->4He{\pi}0 have been extended towards higher excess energies in order to provide information on the contribution of p-waves in the final state. For this, an exclusive measurement has been carried out at a beam momentum of p=1.2 GeV/c using the WASA-at-COSY facility. The total cross section amounts to sigma(tot) = (118 +- 18(stat) +- 13(sys) +- 8(ext)) pb and first data on the differential cross section are consistent with s-wave pion production.Comment: 14 pages, 5 figure

    ABC Effect and Resonance Structure in the Double-Pionic Fusion to 3^3He

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    Exclusive and kinematically complete measurements of the double pionic fusion to 3^3He have been performed in the energy region of the so-called ABC effect, which denotes a pronounced low-mass enhancement in the ππ\pi\pi-invariant mass spectrum. The experiments were carried out with the WASA detector setup at COSY. Similar to the observations in the basic pndπ0π0pn \to d \pi^0\pi^0 reaction and in the dd4dd \to ^4Heπ0π0\pi^0\pi^0 reaction, the data reveal a correlation between the ABC effect and a resonance-like energy dependence in the total cross section. Differential cross sections are well described by the hypothesis of dd^* resonance formation during the reaction process in addition to the conventional tt-channel ΔΔ\Delta\Delta mechanism. The deduced dd^* resonance width can be understood from collision broadening due to Fermi motion of the nucleons in initial and final nuclei

    Cross section ratio and angular distributions of the reaction p + d -> 3He + eta at 48.8 MeV and 59.8 MeV excess energy

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    We present new data for angular distributions and on the cross section ratio of the p + d -> 3He + eta reaction at excess energies of Q = 48.8 MeV and Q = 59.8 MeV. The data have been obtained at the WASA-at-COSY experiment (Forschungszentrum J\"ulich) using a proton beam and a deuterium pellet target. While the shape of obtained angular distributions show only a slow variation with the energy, the new results indicate a distinct and unexpected total cross section fluctuation between Q = 20 MeV and Q = 60 MeV, which might indicate the variation of the production mechanism within this energy interval.Comment: 9 pages, 9 figure

    Neutron-Proton Scattering in the Context of the dd^*(2380) Resonance

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    New data on quasifree polarized neutron-proton scattering, in the region of the recently observed dd^* resonance structure, have been obtained by exclusive and kinematically complete high-statistics measurements with WASA at COSY. This paper details the determination of the beam polarization, checks of the quasifree character of the scattering process, on all obtained AyA_y angular distributions and on the new partial-wave analysis, which includes the new data producing a resonance pole in the 3D3^3D_3-3G3^3G_3 coupled partial waves at (2380±10i40±52380\pm10 - i40\pm5) MeV -- in accordance with the dd^* dibaryon resonance hypothesis. The effect of the new partial-wave solution on the description of total and differential cross section data as well as specific combinations of spin-correlation and spin-transfer observables available from COSY-ANKE measurements at TdT_d = 2.27 GeV is discussed

    Evidence for a New Resonance from Polarized Neutron-Proton Scattering

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    Exclusive and kinematically complete high-statistics measurements of quasifree polarized np\vec{n}p scattering have been performed in the energy region of the narrow resonance structure dd^* with I(JP)=0(3+)I(J^P) = 0(3^+), MM \approx 2380 MeV/c2c^2 and Γ\Gamma \approx 70 MeV observed recently in the double-pionic fusion channels pndπ0π0pn \to d\pi^0\pi^0 and pndπ+πpn \to d\pi^+\pi^-. The experiment was carried out with the WASA detector setup at COSY having a polarized deuteron beam impinged on the hydrogen pellet target and utilizing the quasifree process dpnp+pspectator\vec{d}p \to np + p_{spectator}. That way the npnp analyzing power AyA_y was measured over a large angular range. The obtained AyA_y angular distributions deviate systematically from the current SAID SP07 NN partial-wave solution. Incorporating the new AyA_y data into the SAID analysis produces a pole in the 3D33G3^3D_3 - ^3G_3 waves as expected from the dd^* resonance hypothesis

    Feasibility studies of time-like proton electromagnetic form factors at PANDA at FAIR

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    Simulation results for future measurements of electromagnetic proton form factors at \PANDA (FAIR) within the PandaRoot software framework are reported. The statistical precision with which the proton form factors can be determined is estimated. The signal channel pˉpe+e\bar p p \to e^+ e^- is studied on the basis of two different but consistent procedures. The suppression of the main background channel, i.e.\textit{i.e.} pˉpπ+π\bar p p \to \pi^+ \pi^-, is studied. Furthermore, the background versus signal efficiency, statistical and systematical uncertainties on the extracted proton form factors are evaluated using two different procedures. The results are consistent with those of a previous simulation study using an older, simplified framework. However, a slightly better precision is achieved in the PandaRoot study in a large range of momentum transfer, assuming the nominal beam conditions and detector performance

    QTL for phytosterol and sinapate ester content in Brassica napus L. collocate with the two erucic acid genes

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    Improving oil and protein quality for food and feed purposes is an important goal in rapeseed (Brassica napus L.) breeding programs. Rapeseed contains phytosterols, used to enrich food products, and sinapate esters, which are limiting the utilization of rapeseed proteins in the feed industry. Increasing the phytosterol content of oil and lowering sinapate ester content of meal could increase the value of the oilseed rape crop. The objective of the present study was to identify quantitative trait loci (QTL) for phytosterol and sinapate ester content in a winter rapeseed population of 148 doubled haploid lines, previously found to have a large variation for these two traits. This population also segregated for the two erucic acid genes. A close negative correlation was found between erucic acid and phytosterol content (Spearman’s rank correlation, rs = −0.80**). For total phytosterol content, three QTL were detected, explaining 60% of the genetic variance. The two QTL with the strongest additive effects were mapped on linkage groups N8 and N13 within the confidence intervals of the two erucic acid genes. For sinapate ester content four QTL were detected, explaining 53% of the genetic variance. Again, a close negative correlation was found between erucic acid and sinapate ester content (rs = −0.66**) and the QTL with the strongest additive effects mapped on linkage groups N8 and N13 within the confidence intervals of the two erucic acid genes. The results suggests, that there is a pleiotropic effect of the two erucic acid genes on phytosterol and sinapate ester content; the effect of the alleles for low erucic acid content is to increase phytosterol and sinapate ester content. Possible reasons for this are discussed based on known biosynthetic pathways

    Observation of ηcωω\eta_c\to\omega\omega in J/ψγωωJ/\psi\to\gamma\omega\omega

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    Using a sample of (1310.6±7.0)×106(1310.6\pm7.0)\times10^6 J/ψJ/\psi events recorded with the BESIII detector at the symmetric electron positron collider BEPCII, we report the observation of the decay of the (11S0)(1^1 S_0) charmonium state ηc\eta_c into a pair of ω\omega mesons in the process J/ψγωωJ/\psi\to\gamma\omega\omega. The branching fraction is measured for the first time to be B(ηcωω)=(2.88±0.10±0.46±0.68)×103\mathcal{B}(\eta_c\to\omega\omega)= (2.88\pm0.10\pm0.46\pm0.68)\times10^{-3}, where the first uncertainty is statistical, the second systematic and the third is from the uncertainty of B(J/ψγηc)\mathcal{B}(J/\psi\to\gamma\eta_c). The mass and width of the ηc\eta_c are determined as M=(2985.9±0.7±2.1)M=(2985.9\pm0.7\pm2.1)\,MeV/c2c^2 and Γ=(33.8±1.6±4.1)\Gamma=(33.8\pm1.6\pm4.1)\,MeV.Comment: 13 pages, 6 figure
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