22,118 research outputs found

    Three-body spin-orbit forces from chiral two-pion exchange

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    Using chiral perturbation theory, we calculate the density-dependent spin-orbit coupling generated by the two-pion exchange three-nucleon interaction involving virtual Δ\Delta-isobar excitation. From the corresponding three-loop Hartree and Fock diagrams we obtain an isoscalar spin-orbit strength Fso(kf)F_{\rm so}(k_f) which amounts at nuclear matter saturation density to about half of the empirical value of 9090 MeVfm5^5. The associated isovector spin-orbit strength Gso(kf)G_{\rm so}(k_f) comes out about a factor of 20 smaller. Interestingly, this three-body spin-orbit coupling is not a relativistic effect but independent of the nucleon mass MM. Furthermore, we calculate the three-body spin-orbit coupling generated by two-pion exchange on the basis of the most general chiral ππNN\pi\pi NN-contact interaction. We find similar (numerical) results for the isoscalar and isovector spin-orbit strengths Fso(kf)F_{\rm so}(k_f) and Gso(kf)G_{\rm so}(k_f) with a strong dominance of the p-wave part of the ππNN\pi\pi NN-contact interaction and the Hartree contribution.Comment: 8 pages, 4figure, published in : Physical Review C68, 054001 (2003

    Nuclear pairing from chiral pion-nucleon dynamics

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    We use a recently improved version of the chiral nucleon-nucleon potential at next-to-next-to-leading order to calculate the 1S0^1S_0 pairing gap in isospin-symmetric nuclear matter. The pairing potential consists of the long-range one- and two-pion exchange terms and two short-distance NN-contact couplings. We find that the inclusion of the two-pion exchange at next-to-next-to-leading order reduces substantially the cut-off dependence of the 1S0^1S_0 pairing gap determined by solving a regularised BCS equation. Our results are close to those obtained with the universal low-momentum nucleon-nucleon potential VlowkV_{\rm low-k} or the phenomenological Gogny D1S force.Comment: 9 pages, 3 eps figures, submitted to PR

    Making electromagnetic wavelets

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    Electromagnetic wavelets are constructed using scalar wavelets as superpotentials, together with an appropriate polarization. It is shown that oblate spheroidal antennas, which are ideal for their production and reception, can be made by deforming and merging two branch cuts. This determines a unique field on the interior of the spheroid which gives the boundary conditions for the surface charge-current density necessary to radiate the wavelets. These sources are computed, including the impulse response of the antenna.Comment: 29 pages, 4 figures; minor corrections and addition

    Global stability analysis of birhythmicity in a self-sustained oscillator

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    We analyze global stability properties of birhythmicity in a self-sustained system with random excitations. The model is a multi-limit cycles variation of the van der Pol oscillatorintroduced to analyze enzymatic substrate reactions in brain waves. We show that the two frequencies are strongly influenced by the nonlinear coefficients α\alpha and β\beta. With a random excitation, such as a Gaussian white noise, the attractor's global stability is measured by the mean escape time τ\tau from one limit-cycle. An effective activation energy barrier is obtained by the slope of the linear part of the variation of the escape time τ\tau versus the inverse noise-intensity 1/D. We find that the trapping barriers of the two frequencies can be very different, thus leaving the system on the same attractor for an overwhelming time. However, we also find that the system is nearly symmetric in a narrow range of the parameters.Comment: 17 pages, 8 figures, to appear on Choas, 201

    Raman spectroscopy with ultrashort coherent excitation. Narrowing of spectral lines beyond the dephasing linewidth

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    Spectroscopists are constantly faced with the task of improved spectral resolution. Two points are of major interest: (i) The precise frequency of the quantized transition and (ii) the detection of new neighboring transitions. Besides experimental factors the ultimate spectral resolution is determined by the inherent linewidth of the transition. Optical spectroscopists have to deal with different line-broadening processes; for instance with the Doppler effect or with collision broadening in gases, with dephasing processes in condensed systems and with the population relaxation which results in the natural linewidth. In recent years, different novel techniques have been devised which provide spectral resolution beyond the transition linewidth. For instance, Doppler broadening can be eliminated by saturation spectroscopy or by two counter-propagating beams for two-photon transitions/I/. Even measurements beyond the natural linewidth have been performed taking biased signals from the fluorescent decay /2-5/. Techniques have been proposed where the difference between the decay rates of the two states rather than their sum determines the linewidth /6,7/, and narrowing of the natural linewidth by decaying-pulse excitation has been discussed /8/. Very recently, we have demonstrated substantial line narrowing of Raman type transitions in condensed phases /9-11/. The lines were broadened by vibrational dephasing. New information was obtained in congestedspectral regions

    Determination of the η\eta-η\eta^\prime mixing angle

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    We extract η\eta-η\eta^\prime mixing angle and the ratios of decay constants of light pseudoscalar mesons π0\pi^0, η\eta and η\eta^\prime using recently available BaBar measurements on η\eta-photon and η\eta^\prime-photon transition form factors and more accurate experimental data for the masses and two-photon decay widths of the light pseduoscalar mesons.Comment: 5 pages, revtex, no figures, accepted for publication as a Brief Report in Physical Review

    Applications of computer-graphics animation for motion-perception research

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    The advantages and limitations of using computer animated stimuli in studying motion perception are presented and discussed. Most current programs of motion perception research could not be pursued without the use of computer graphics animation. Computer generated displays afford latitudes of freedom and control that are almost impossible to attain through conventional methods. There are, however, limitations to this presentational medium. At present, computer generated displays present simplified approximations of the dynamics in natural events. Very little is known about how the differences between natural events and computer simulations influence perceptual processing. In practice, the differences are assumed to be irrelevant to the questions under study, and that findings with computer generated stimuli will generalize to natural events
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