Many-Body Currents and the Strange-Quark Content of 4he

Meson-exchange current (MEC) contributions to the parity-violating (PV) asymmetry for elastic scattering of polarized electrons from $^4$He are calculated over a range of momentum transfer using Monte Carlo methods and a variational $^4$He ground state wavefunction. The results indicate that MEC's generate a negligible contribution to the asymmetry at low-|\qv|, where a determination of the nucleon's mean square strangeness radius could be carried out at CEBAF. At larger values of momentum transfer -- beyond the first diffraction minimum -- two-body corrections from the $\rho$-$\pi$ \lq\lq strangeness charge" operator enter the asymmetry at a potentially observable level, even in the limit of vanishing strange-quark matrix elements of the nucleon. For purposes of constraining the nucleon's strangeness electric form factor, theoretical uncertainties associated with these MEC contributions do not appear to impose serious limitations.Comment: 32 TEX pages and 7 figures (not included, available from authors upon request), CEBAF Preprint #TH-94-1

Deep Inelastic Scattering at HERA

Results from the H1 and ZEUS experiments at HERA on deep inelastic scattering are reviewed. The data lead to a consistent picture of a steep rise in the F_2 structure function and in the gluon density within the proton. Important new information on the partonic structure of diffraction is emerging from H1 and ZEUS. The space-like region in which the weak and electromagnetic interactions become of equal strength is being explored for the first time. A possible excess of events at high x and Q^2 compared to the expectations of the Standard Model has been observed in both experiments.Comment: 80 pages, 42 figure

Hygromechanical properties of grenadilla wood (Dalbergia melanoxylon)

International audienceGrenadilla wood (Dalbergia melanoxylon Guill. & Perr.) is a hardwood species found in Tanzania, Mozambique, and other countries in the tropical part of Africa, especially in the Eastern-Central region. Thanks to its high density and good hygro-scopic stability, it is used in the making of various musical instruments and fine furniture. Due to the scarcity of published data on this wood species, more studies on its properties are needed to improve its processing and use, and even to search for sustainable alternative materials as its trade is increasingly limited by new regulations. This work is focused on the hygromechanical properties, which hold an important role in the applications of this wood: diffusion coefficients and adsorption-desorp-tion curve (both measured at T = 20 • C), swelling-shrinkage coefficients and full orthotropic elastic constants using an ultrasonic method. Results show that grenadilla wood possesses small water diffusion coefficients (from 1.54 ± 0.49 × 10 −7 cm 2 ∕s in T direction to 4.58 ± 0.84 × 10 −7 cm 2 ∕s in L direction), which is probably related to its high density (1250.0 ± 26.2 kg∕m 2); unique equilibrium moisture content (sorp-tion) curve with a lower fiber saturation point (0.173 ± 0.003); smaller swelling-shrinkage coefficients (0.20 ± 0.03 and 0.32 ± 0.05 in T and R directions, respectively); and elastic constants lower in the longitudinal direction (15.56 ± 1.79 GPa) and higher in the transverse ones (5.10 ± 0.46 GPa and 4.05 ± 0.35 GPa in R and T directions, respectively) than what could be expected with a standard model based on the density only. Several explanations were described here, from the effects of a high extractive content to the possibility of a high microfibril and/or fiber angle