47,799 research outputs found
The importance of habitat quality for marine reserve fishery linkages
We model marine reserve - fishery linkages to evaluate the potential contribution of habitat-quality improvements inside a marine reserve to fish productivity and fishery catches. Data from Mombasa Marine National Park, Kenya, and the adjacent fishery are used. Marine reserves increase total fish biomass directly by providing refuge from exploitation and indirectly by improving fish habitat in the reserve. As natural mortality of the fish stock decreases in response to habitat enhancement in the reserve, catches increase by up to 2.6 tonnes (t).km(-2).year(-1) and total fish biomass by up to 36 t.km(-2). However, if habitat-quality improvement reduces the propensity of fish to move out of the reserve, catches may fall by up to 0.9 t.km(-2).year(-1). Our results indicate that habitat protection in reserves can underpin fish productivity and, depending on its effects on fish movements, augment catches
Dyson-Schwinger Equations - aspects of the pion
The contemporary use of Dyson-Schwinger equations in hadronic physics is
exemplified via applications to the calculation of pseudoscalar meson masses,
and inclusive deep inelastic scattering with a determination of the pion's
valence-quark distribution function.Comment: 4 pages. Contribution to the Proceedings of ``DPF 2000,'' the Meeting
of the Division of Particles and Fields of the American Physical Society,
August 9-12, 2000, Department of Physics, the Ohio State University,
Columbus, Ohi
The effect of Mach number on unstable disturbances in shock/boundary-layer interactions
The effect of Mach number on the growth of unstable disturbances in a boundary layer undergoing a strong interaction with an impinging oblique shock wave is studied by direct numerical simulation and linear stability theory (LST). To reduce the number of independent parameters, test cases are arranged so that both the interaction location Reynolds number (based on the distance from the plate leading edge to the shock impingement location for a corresponding inviscid flow) and the separation bubble length Reynolds number are held fixed. Small-amplitude disturbances are introduced via both white-noise and harmonic forcing and, after verification that the disturbances are convective in nature, linear growth rates are extracted from the simulations for comparison with parallel flow LST and solutions of the parabolized stability equations (PSE). At Mach 2.0, the oblique modes are dominant and consistent results are obtained from simulation and theory. At Mach 4.5 and Mach 6.85, the linear Navier-Stokes results show large reductions in disturbance energy at the point where the shock impinges on the top of the separated shear layer. The most unstable second mode has only weak growth over the bubble region, which instead shows significant growth of streamwise structures. The two higher Mach number cases are not well predicted by parallel flow LST, which gives frequencies and spanwise wave numbers that are significantly different from the simulations. The PSE approach leads to good qualitative predictions of the dominant frequency and wavenumber at Mach 2.0 and 4.5, but suffers from reduced accuracy in the region immediately after the shock impingement. Three-dimensional Navier-Stokes simulations are used to demonstrate that at finite amplitudes the flow structures undergo a nonlinear breakdown to turbulence. This breakdown is enhanced when the oblique-mode disturbances are supplemented with unstable Mack modes
The Calculation of Vacuum Properties from the Global Color Symmetry Model
A modified method for calculating the non-perturbative quark vacuum
condensates from the global color symmetry model is derived. Within this
approach it is shown that the vacuum condensates are free of ultraviolet
divergence which is different from previous studies. As a special, the
two-quark condensate and the mixed quark-gluon condensate are calculated. A
comparision with the results of the other nonperturbative QCD approaches is
given.Comment: 17 page
K -> pi pi and a light scalar meson
We explore the Delta-I= 1/2 rule and epsilon'/epsilon in K -> pi pi
transitions using a Dyson-Schwinger equation model. Exploiting the feature that
QCD penguin operators direct K^0_S transitions through 0^{++} intermediate
states, we find an explanation of the enhancement of I=0 K -> pi pi transitions
in the contribution of a light sigma-meson. This mechanism also affects
epsilon'/epsilon.Comment: 7 pages, REVTE
Calculation of the Chiral Lagrangian Coefficients
We present a systematic way to combine the global color model and the
instanton liquid model to calculate the chiral
Lagrangian coefficients. Our numerical results are in agreement well with the
experimental values.Comment: 7 pages, To appear in Chin.Phys.Lett, Year 200
pi-pi scattering in a QCD based model field theory
A model field theory, in which the interaction between quarks is mediated by
dressed vector boson exchange, is used to analyse the pionic sector of QCD. It
is shown that this model, which incorporates dynamical chiral symmetry
breaking, asymptotic freedom and quark confinement, allows one to calculate
, , and the partial wave amplitudes in -
scattering and obtain good agreement with the experimental data, with the
latter being well described up to energies \mbox{ MeV}.Comment: 23 Pages, 4 figures in PostScript format, PHY-7512-TH-93, REVTEX
Available via anonymous ftp in /pub: login anonymou get pipi93.tex Fig1.ps
Fig2.ps Fig3.ps Fig4.p
Pseudovector components of the pion, pi^0 -> gamma gamma, and F_pi(q^2)
As a consequence of dynamical chiral symmetry breaking the pion
Bethe-Salpeter amplitude necessarily contains terms proportional to gamma_5
gamma.P and gamma_5 gamma.k, where k is the relative and P the total momentum
of the constituents. These terms are essential for the preservation of low
energy theorems, such as the Gell-Mann--Oakes-Renner relation and those
describing anomalous decays of the pion, and to obtaining an electromagnetic
pion form factor that falls as 1/q^2 for large q^2, up to calculable
ln(q^2)-corrections. In a simple model, which correlates low- and high-energy
pion observables, we find q^2 F_pi(q^2) ~ 0.12 - 0.19 GeV^2 for q^2 >~10 GeV^2.Comment: 15 pages, 2 figures, REVTE
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