9,732 research outputs found

    The reaction πNππN\pi N \to \pi \pi N at threshold in chiral perturbation theory

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    In the framework of heavy baryon chiral perturbation theory, we give thIn the framework of heavy baryon chiral perturbation theory, we give the chiral expansion for the πNππN\pi N \to \pi \pi N threshold amplitudes D1D_1 and D2D_2 to quadratic order in the pion mass. The theoretical results agree within one standard deviation with the empirical values. We also derive a relation between the two threshold amplitudes of the reaction πNππN\pi N \to \pi \pi N and the ππ\pi \pi S--wave scattering lengths, a00a_0^0 and a02a_0^2, respectively, to order O(Mπ2){\cal O}(M_\pi^2). We show that there are uncertainties mostly related to resonance excitation which make an accurate determination of the ππ\pi \pi scattering length a00a_0^0 from the ππN\pi \pi N threshold amplitudes at present very difficult. The situation is different in the ππ\pi \pi isospin two final state. Here, the chiral series converges and one finds a02=0.031±0.007a_0^2 = -0.031 \pm 0.007 consistent with the one--loop chiral perturbation theory prediction.Comment: 30 pp, LaTeX file, uses epsf, 6 figures (appended), corrections in sections 5 and 6, conclusions unchange

    Weight Vectors of the Basic A_1^(1)-Module and the Littlewood-Richardson Rule

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    The basic representation of \A is studied. The weight vectors are represented in terms of Schur functions. A suitable base of any weight space is given. Littlewood-Richardson rule appears in the linear relations among weight vectors.Comment: February 1995, 7pages, Using AMS-Te

    Comment on "Two Phase Transitions in the Fully frustrated XY Model"

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    The conclusions of a recent paper by Olsson (Phys. Rev. Lett. 75, 2758 (1995), cond-mat/9506082) about the fully frustrated XY model in two dimensions are questioned. In particular, the evidence presented for having two separate chiral and U(1) phase transitions are critically considered.Comment: One page one table, to Appear in Physical Review Letter

    Training materials for different categories of users

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    Agricultural and Food Policy, Environmental Economics and Policy, Farm Management, Land Economics/Use, Production Economics, Teaching/Communication/Extension/Profession,

    Vortex dynamics for two-dimensional XY models

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    Two-dimensional XY models with resistively shunted junction (RSJ) dynamics and time dependent Ginzburg-Landau (TDGL) dynamics are simulated and it is verified that the vortex response is well described by the Minnhagen phenomenology for both types of dynamics. Evidence is presented supporting that the dynamical critical exponent zz in the low-temperature phase is given by the scaling prediction (expressed in terms of the Coulomb gas temperature TCGT^{CG} and the vortex renormalization given by the dielectric constant ϵ~\tilde\epsilon) z=1/ϵ~TCG22z=1/\tilde{\epsilon}T^{CG}-2\geq 2 both for RSJ and TDGL and that the nonlinear IV exponent a is given by a=z+1 in the low-temperature phase. The results are discussed and compared with the results of other recent papers and the importance of the boundary conditions is emphasized.Comment: 21 pages including 15 figures, final versio

    From scalar to string confinement

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    We outline a connection between scalar quark confinement, a phenomenologically successful concept heretofore lacking fundamental justification, and QCD. Although scalar confinement does not follow from QCD, there is an interesting and close relationship between them. We develop a simple model intermediate between scalar confinement and the QCD string for illustrative purposes. Finally, we find the bound state masses of scalar, time-component vector, and string confinement analytically through semi-classical quantization.Comment: ReVTeX, 9 pages, 5 figure

    Modeling pollinating bee visitation rates in heterogeneous landscapes from foraging theory

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    Pollination by bees is important for food production. Recent concerns about the declines of both domestic and wild bees, calls for measures to promote wild pollinator populations in farmland. However, to be able to efficiently promote and prioritize between measures that benefit pollinators, such as modified land use, agri-environment schemes, or specific conservation measures, it is important to have a tool that accurately predicts how bees use landscapes and respond to such measures. In this paper we compare an existing model for predicting pollination (the “Lonsdorf model”), with an extension of a general model for habitat use of central place foragers (the “CPF model”). The Lonsdorf model has been shown to perform relatively well in simple landscapes, but not in complex landscapes. We hypothesized that this was because it lacks a behavioral component, assuming instead that bees in essence diffuse out from the nest into the landscape. By adding a behavioral component, the CPF model in contrast assumes that bees only use those parts of the landscape that enhances their fitness, completely avoiding foraging in other parts of the landscape. Because foraging is directed towards the most rewarding foraging habitat patches as determined by quality and distance, foraging habitat will include a wide range of forage qualities close to the nest, but a much narrower range farther away. We generate predictions for both simple and complex hypothetical landscapes, to illustrate the effect of including the behavioral rule, and for real landscapes. In the real landscapes the models give similar predictions for visitation rates in simple landscapes, but more different predictions in heterogeneous landscapes. We also analyze the consequences of introducing hedgerows near a mass-flowering crop field under each model. The Lonsdorf model predicts that any habitat improvement will enhance pollination of the crop. In contrast, the CPF model predicts that the hedgerow must provide good nesting sites, and not just foraging opportunities, for it to benefit pollination of the crop, because good forage quality alone may drain bees away from the field. Our model can be used to optimize pollinator mitigation measures in real landscapes
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