1,093 research outputs found
Aspects of Nucleon Compton Scattering
We consider the spin-averaged nucleon forward Compton scattering amplitude in
heavy baryon chiral perturbation theory including all terms to order . The chiral prediction for the spin-averaged forward Compton scattering
amplitude is in good agreement with the data for photon energies MeV. We also evaluate the nucleon electric and magnetic Compton
polarizabilities to this order and discuss the uncertainties of the various
counter terms entering the chiral expansion of these quantities.Comment: 17 pp, TeX, 7 figures available from the authors, preprint CRN-93/5
Purification and characterization of an extracellular amylase from Lactobacillus plantarum strain A6
Extracellular amylase from #Lactobacillus plantarum$ A6 was purified by fractionated precipitation with ammonium sulphate and by anion exchange chromatography. The homogeneity of the purified fraction was tested by polyacrylamide gel electrophoresis and showed multiple amylase forms. A major form had an estimated molecular weight of 50 kDa. It was identified as an alpha-amylase, with an optimum pH of 5.5, an optimum temperature of 65°C and Km value of 2.38 g l-1 with soluble starch substrate. The enzyme was inhibited by N-bromosuccinimide, iodine and acetic acid. The enzyme activation energy was 30.9 kJ mol-1. (Résumé d'auteur
Role of the lutoidic tonoplast in the control of the cytosolic homeostasis within the laticiferous cells of hevea
Regularization for effective field theory with two heavy particles
A regularization for effective field theory with two propagating heavy
particles is constructed. This regularization preserves the low-energy analytic
structure, implements a low-energy power counting for the one-loop diagrams,
and preserves symmetries respected by dimensional regularization.Comment: 12 pages, 4 figures. Some typos have been corrected, a sentence has
been moved, and two formulas have been further simplifie
Ultrastructural changes during cryopreservation of plumules and embryos of coconut (Cocos nucifera L.)
This article aims to show the changes occurring during cryopreservation of embryos and plumules of coconut which are responsible of their death or survival. Embryos have been cryopreserved by preculture-dehydration for 24h, 28h, 30h, 34h, 38h and 48h on agar medium containing 600g/L of glucose combined with silica gel. The plumules were cryopreserved by encapsulation dehydration on solid medium containing 0.5 M, 0.75 M and 1M followed by dehydration with 40g of silica gel for different durations before rapid freezing. This study indicates that the damages undergone by seed samples can be divided into three types. The first stage of changes concerned the plasmolysis of cells with small vacuoles, condensation of chromatin, changing in the conformation of the DNA and the nucleus and stopping of mitosis. These types of changes are described in general in the context of a desiccation tolerance. The second degree of the changes was the retraction of the cytoplasm inside the cell, the increase in the periplasmic volume. The third degree of modification concerned the deformation of the walls, the invagination or the lysis of the plasma membrane resulting in the observation of distorted cells and and the bursting of the nucleus. These two types of modifications are irreversible and correspond to an absence of regrowth of the samples. Understanding the damage or changes that occur in cryopreserved cells is an important part of understanding how dehydration and frozen affect the viability of recalcitrant plants cells. These changes are made by dehydration and accentuated by freezing
Lattice Calculation of Heavy-Light Decay Constants with Two Flavors of Dynamical Quarks
We present results for , , , and their ratios in
the presence of two flavors of light sea quarks (). We use Wilson light
valence quarks and Wilson and static heavy valence quarks; the sea quarks are
simulated with staggered fermions. Additional quenched simulations with
nonperturbatively improved clover fermions allow us to improve our control of
the continuum extrapolation. For our central values the masses of the sea
quarks are not extrapolated to the physical , masses; that is, the
central values are "partially quenched." A calculation using "fat-link clover"
valence fermions is also discussed but is not included in our final results. We
find, for example,
MeV, , MeV, and , where in each case the first error is
statistical and the remaining three are systematic: the error within the
partially quenched approximation, the error due to the missing strange
sea quark and to partial quenching, and an estimate of the effects of chiral
logarithms at small quark mass. The last error, though quite significant in
decay constant ratios, appears to be smaller than has been recently suggested
by Kronfeld and Ryan, and Yamada. We emphasize, however, that as in other
lattice computations to date, the lattice quark masses are not very light
and chiral log effects may not be fully under control.Comment: Revised version includes an attempt to estimate the effects of chiral
logarithms at small quark mass; central values are unchanged but one more
systematic error has been added. Sections III E and V D are completely new;
some changes for clarity have also been made elsewhere. 82 pages; 32 figure
Compton Scattering and the Spin Structure of the Nucleon at Low Energies
We analyze polarized Compton scattering which provides information on the
spin-structure of the nucleon. For scattering processes with photon energies up
to 100 MeV the spin-structure dependence can be encoded into four independent
parameters-the so-called spin-polarizabilities of the
nucleon, which we calculate within the framework of the "small scale expansion"
in SU(2) baryon chiral perturbation theory. Specific application is made to
"forward" and "backward" spin- polarizabilities.Comment: 8 pages revtex file, separation between pion-pole and regular
contributions detailed + minor wording changes, results and conclusions
unchange
Determination of the chiral coupling constants c3 and c4 in new pp and np partial-wave analyses
As a first result of two new partial-wave analyses, one of the pp and another
one of the np scattering data below 500 MeV, we report a study of the
long-range chiral two-pion exchange interaction which contains the chiral
coupling constants c1, c3, and c4. By using as input a theoretical value for c1
we are able to determine in pp as well as in np scattering accurate values for
c3 and c4. The values determined from the pp data and independently from the np
data are in very good agreement, indicating the correctness of the chiral
two-pion exchange interaction. The weighted averages are c3 = -4.78(10) / GeV
and c4 = 3.96(22) / GeV, where the errors are statistical. The value of c3 is
best determined from the pp data and that of c4 from the np data.Comment: 9 pages, 1 figure. Accepted for publication in Phys. Rev.
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