23,980 research outputs found

    Higher and missing resonances in omega photoproduction

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    We study the role of the nucleon resonances (NN^*) in ω\omega photoproduction by using the quark model resonance parameters predicted by Capstick and Roberts. The employed γNN\gamma N \to N^* and NωNN^* \to \omega N amplitudes include the configuration mixing effects due to the residual quark-quark interactions. The contributions from the nucleon resonances are found to be important in the differential cross sections at large scattering angles and various spin observables. In particular, the parity asymmetry and beam-target double asymmetry at forward scattering angles are suggested for a crucial test of our predictions. The dominant contributions are found to be from N32+(1910)N\frac32^+ (1910), a missing resonance, and N32(1960)N\frac32^- (1960) which is identified as the D13(2080)D_{13}(2080) of the Particle Data Group.Comment: 8 pages, LaTeX with ws-p8-50x6-00.cls, 4 figures (5 eps files), Talk presented at the NSTAR2001 Workshop on the Physics of Excited Nucleons, Mainz, Germany, Mar. 7-10, 200

    Electromagnetic production of vector mesons at low energies

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    We have investigated exclusive photoproduction of light vector mesons (ω\omega, ρ\rho and ϕ\phi) on the nucleon at low energies. In order to explore the questions concerning the so-called missing nucleon resonances, we first establish the predictions from a model based on the Pomeron and meson exchange mechanisms. We have also explored the contributions due to the mechanisms involving ss- and uu-channel intermediate nucleon state. Some discrepancies found at the energies near threshold and large scattering angles suggest a possibility of using this reaction to identify the nucleon resonances.Comment: 9 pages, LaTeX with sprocl.sty, 5 figures (11 eps files), Talk presented at the NSTAR2000 Workshop, The Physics of Excited Nucleons, Jefferson Lab., Newport News, Feb. 16-19, 200

    Higher Derivative CP(N) Model and Quantization of the Induced Chern-Simons Term

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    We consider higher derivative CP(N) model in 2+1 dimensions with the Wess-Zumino-Witten term and the topological current density squared term. We quantize the theory by using the auxiliary gauge field formulation in the path integral method and prove that the extended model remains renormalizable in the large N limit. We find that the Maxwell-Chern-Simons theory is dynamically induced in the large N effective action at a nontrivial UV fixed point. The quantization of the Chern-Simons term is also discussed.Comment: 8 pages, no figure, a minor change in abstract, added Comments on the quantization of the Chern-Simons term whose coefficient is also corrected, and some references are added. Some typos are corrected. Added a new paragraph checking the equivalence between (3) and (5), and a related referenc

    Direct-write, focused ion beam-deposited,7 K superconducting C-Ga-O nanowire

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    We have fabricated C-Ga-O nanowires by gallium focused ion beam-induced deposition from the carbon-based precursor phenanthrene. The electrical conductivity of the nanowires is weakly temperature dependent below 300 K, and indicates a transition to a superconducting state below Tc = 7 K. We have measured the temperature dependence of the upper critical field Hc2(T), and estimate a zero temperature critical field of 8.8 T. The Tc of this material is approximately 40% higher than that of any other direct write nanowire, such as those based on C-W-Ga, expanding the possibility of fabricating direct-write nanostructures that superconduct above liquid helium temperaturesComment: Accepted for AP

    Electronic States in Diffused Quantum Wells

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    In the present study we calculate the energy values and the spatial distributions of the bound electronic states in some diffused quantum wells. The calculations are performed within the virtual crystal approximation, sp3ssp^3 s^* spin dependent empirical tight-binding model and the surface Green function matching method. A good agreement is found between our results and experimental data obtained for AlGaAs/GaAs quantum wells with thermally induced changes in the profile at the interfaces. Our calculations show that for diffusion lengths LD=20÷100L_{D}=20\div100 {\AA} the transition (C3-HH3) is not sensitive to the diffusion length, but the transitions (C1-HH1), (C1-LH1), (C2-HH2) and (C2-LH2) display large "blue shifts" as L_{D} increases. For diffusion lengths LD=0÷20L_{D}=0\div20 {\AA} the transitions (C1-HH1) and (C1-LH1) are less sensitive to the L_{D} changes than the (C3-HH3) transition. The observed dependence is explained in terms of the bound states spatial distributions.Comment: ReVTeX file, 7pp., no macros, 4 figures available on the reques

    Nucleon resonances in polarized omega photoproduction

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    The role of the nucleon resonances (NN^*) in ω\omega photoproduction is investigated by using the resonance parameters predicted by Capstick and Roberts. The contributions from the nucleon resonances are found to be significant in various spin asymmetries. In particular, we found that a crucial test of our predictions can be made by measuring the parity asymmetry and beam-target double asymmetry at forward scattering angles.Comment: 5 pages, aipproc.sty, 3 figs (4 eps files), talk at the 14th International Spin Physics Symposium (SPIN 2000), Osaka, Japan, October, 200

    Comparison of Howland and General Impedance Converter (GIC) circuit based current sources for bio-impedance measurements

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    The current source is a key component in bio-impedance measurement systems. The accuracy of the current source can be measured in terms of its output impedance together with other parameters, with certain applications demanding extremely high output impedance. This paper presents an investigation and comparison of different current source designs based on the Enhanced Howland circuit combined with a General Impedance Converter (GIC) circuit using both ideal and non-ideal operational amplifiers. Under differing load conditions two different settings of the GIC are evaluated and the results are compared to show its performance settings. Whilst the study has shown that over a wide bandwidth (i.e. 100Hz-100MHz) the output impedance is limited, operation over a more limited range offers output impedance in the Giga-ohm range, which can be considered as being infinite

    Magnetic Determination of Hc2H_{c2} under Accurate Alignment in (TMTSF)2_2ClO4_4

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    Cantilever magnetometry has been used to measure the upper critical magnetic field Hc2H_{c2} of the quasi-one dimensional molecular organic superconductor (TMTSF)2_2ClO4_4. From simultaneous resistivity and torque magnetization experiments conducted under precise field alignment, Hc2H_{c2} at low temperature is shown to reach 5T, nearly twice the Pauli paramagnetic limit imposed on spin singlet superconductors. These results constitute the first thermodynamic evidence for a large Hc2H_{c2} in this system and provide support for spin triplet pairing in this unconventional superconductorComment: Submitted July 1, 2003, Accepted December 9, 2003, Physical Review Letter
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