266 research outputs found
Absorption mechanisms in photon induced two-body knockout
Calculations have been performed for the O(,pn) and the
O(,pp) reaction in the photon-energy range = 60-300
MeV. Besides the contribution from the more common photoabsorption on the
pionic degrees of freedom, we have investigated the influence of heavier meson
exchange () and intermediate creation with
and exchange. Whereas the meson is found to set the main trends,
the meson is found not to be discardable in a theoretical description of
the (,pn) reaction. The incorporation of an energy dependence and a
decay width in the propagator is observed to be essential in order to
arrive at a more realistic description of (,NN) reactions at higher
photon energies.Comment: 10 pages, 5 figures in seperate postscript file, Submitted to Phys.
Lett. B. - INW9306I
On the influence of model physics on simulations of Arctic and Antarctic sea ice
Two hindcast (1983–2007) simulations are performed with the global, ocean-sea ice models NEMO-LIM2 and NEMO-LIM3 driven by atmospheric reanalyses and climatologies. The two simulations differ only in their sea ice component, while all other elements of experimental design (resolution, initial conditions, atmospheric forcing) are kept identical. The main differences in the sea ice models lie in the formulation of the subgrid-scale ice thickness distribution, of the thermodynamic processes, of the sea ice salinity and of the sea ice rheology. To assess the differences in model skill over the period of investigation, we develop a set of metrics for both hemispheres, comparing the main sea ice variables (concentration, thickness and drift) to available observations and focusing on both mean state and seasonal to interannual variability. Based upon these metrics, we discuss the physical processes potentially responsible for the differences in model skill. In particular, we suggest that (i) a detailed representation of the ice thickness distribution increases the seasonal to interannual variability of ice extent, with spectacular improvement for the simulation of the recent observed summer Arctic sea ice retreats, (ii) the elastic-viscous-plastic rheology enhances the response of ice to wind stress, compared to the classical viscous-plastic approach, (iii) the grid formulation and the air-sea ice drag coefficient affect the simulated ice export through Fram Strait and the ice accumulation along the Canadian Archipelago, and (iv) both models show less skill in the Southern Ocean, probably due to the low quality of the reanalyses in this region and to the absence of important small-scale oceanic processes at the models' resolution (~1°)
Systematic renormalization scheme in light-front dynamics with Fock space truncation
Within the framework of the covariant formulation of light-front dynamics, we
develop a general non-perturbative renormalization scheme based on the Fock
decomposition of the state vector and its truncation. The counterterms and bare
parameters needed to renormalize the theory depend on the Fock sectors. We
present a general strategy in order to calculate these quantities, as well as
state vectors of physical systems, in a truncated Fock space. The explicit
dependence of our formalism on the orientation of the light front plane is
essential in order to analyze the structure of the counterterms. We apply our
formalism to the two-body (one fermion and one boson) truncation in the Yukawa
model and in QED, and to the three-body truncation in a scalar model. In QED,
we recover analytically, without any perturbative expansion, the
renormalization of the electric charge, according to the requirements of the
Ward identity.Comment: 32 pages, 14 figures, submitted in Phys. Rev.
Modifications of the Rho Meson from the Virtual Pion Cloud in Hot and Dense Matter
The modification of the rho-meson self-energy due to the coupling to
in-medium pions is calculated consistently at finite baryon density and
temperature, keeping the full 3-momentum dependence in a gauge invariant way.
As a function of nucleon density, the rho-meson spectral function is strongly
enhanced in the invariant mass region M < 650 MeV, while the maximum, i.e. the
pole mass, is slightly shifted upwards. As a function of temperature, for fixed
nucleon density, the imaginary part of the self-energy increases further due to
Bose-enhancement. At the same time the mass shift from the real part becomes
very large. As a consequence of these medium effects, the dilepton rate in the
low-mass region M < 650 MeV increases strongly, while the peak at M = 770 MeV
disappears.Comment: 18 pages, 9 figures; resonance contributions adde
Nonperturbative calculation of the anomalous magnetic moment in the Yukawa model within truncated Fock space
Within the covariant formulation of light-front dynamics, we calculate the
state vector of a physical fermion in the Yukawa model. The state vector is
decomposed in Fock sectors and we consider the first three ones: the single
constituent fermion, the constituent fermion coupled to one scalar boson, and
the constituent fermion coupled to two scalar bosons. This last three-body
sector generates nontrivial and nonperturbative contributions to the state
vector, which are calculated numerically. Field-theoretical divergences are
regularized using Pauli-Villars fermion and boson fields. Physical observables
can be unambiguously deduced using a systematic renormalization scheme we have
developed previously. As a first application, we consider the anomalous
magnetic moment of the physical fermion.Comment: 24 pages, 16 figure
Exchange Currents for Hypernuclear Magnetic Moments
The meson(K and ) exchange currents for the hypernuclear magnetic
moments are calculated using the effective Lagrangian method. The seagull
diagram, the mesonic diagram and the -excitation diagram are
considered. The -N exchange magnetic moments for the
, and are calculated
employing the harmonic oscillator shell model. It is found that the two-body
correction is about -9% of the single particle value for .
The exchange current, induced only in the -excitation diagram,
is found to give dominant contribution for the isovector magnetic moments of
hypernuclei with A=6.Comment: 11pp, LaTeX, 7 EPS figures, uses epsf.st
Non-abelian color dielectric - towards the effective model of the low energy QCD
Lattice motivated triplet color scalar field theory is analyzed. We consider
non-minimal as well as covariant derivative coupling with SU(2) gauge fields.
Field configurations generated by external electric sources are presented.
Moreover non-Abelian magnetic monopoles are found. Dependence on the spatial
coordinates in the obtained solutions is identical as in the usual Abelian
case. We show also that after a decomposition of the fields a modified
Faddeev-Niemi action can be obtained. It contains explicit O(3) symmetry
breaking term parameterized by the condensate of an isoscalar field. Due to
that Goldstone bosons observed in the original Faddeev-Niemi model are removed.Comment: 19 page
Relativistic description of electron scattering on the deuteron
Within a quasipotential framework a relativistic analysis is presented of the
deuteron current. Assuming that the singularities from the nucleon propagators
are important, a so-called equal time approximation of the current is
constructed. This is applied to both elastic and inelastic electron scattering.
As dynamical model the relativistic one boson exchange model is used.
Reasonable agreement is found with a previous relativistic calculation of the
elastic electromagnetic form factors of the deuteron. For the unpolarized
inelastic electron scattering effects of final state interactions and
relativistic corrections to the structure functions are considered in the
impulse approximation. Two specific kinematic situations are studied as
examples.Comment: (19 pages in revtex + 15 figures not included, available upon
request.) report THU-93-10
Rift Valley fever outbreak, Mauritania, 1998: seroepidemiologic, virologic, entomologic, and zoologic investigations.
A Rift Valley fever outbreak occurred in Mauritania in 1998. Seroepidemiologic and virologic investigation showed active circulation of the Rift Valley fever virus, with 13 strains isolated, and 16% (range 1.5%-38%) immunoglobulin (Ig) M-positivity in sera from 90 humans and 343 animals (sheep, goats, camels, cattle, and donkeys). One human case was fatal
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