Absorption mechanisms in photon induced two-body knockout

Calculations have been performed for the $^{16}$O($\gamma$,pn) and the $^{16}$O($\gamma$,pp) reaction in the photon-energy range $E_{\gamma}$ = 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 ($\rho, \sigma, \omega$) and intermediate $\Delta$ creation with $\pi$ and $\rho$ exchange. Whereas the $\pi$ meson is found to set the main trends, the $\rho$ meson is found not to be discardable in a theoretical description of the ($\gamma$,pn) reaction. The incorporation of an energy dependence and a decay width in the $\Delta$ propagator is observed to be essential in order to arrive at a more realistic description of ($\gamma$,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 $\pi$) exchange currents for the hypernuclear magnetic moments are calculated using the effective Lagrangian method. The seagull diagram, the mesonic diagram and the $\Sigma^0$-excitation diagram are considered. The $\Lambda$-N exchange magnetic moments for the ${}^5_{\Lambda}He$, ${}^6_{\Lambda}He$ and ${}^6_{\Lambda}Li$ 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 ${}^5_{\Lambda}He$. The $\pi$ exchange current, induced only in the $\Sigma^0$-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