379 research outputs found
Remarks on sum rules in the heavy quark limit of QCD
We underline a problem existing in the heavy quark limit of QCD concerning
the rates of semileptonic B decays into P-wave mesons, where (wide states) or (narrow states). The leading order
sum rules of Bjorken and Uraltsev suggest , in contradiction with experiment. The same trend follows also from a sum
rule for the subleading curent matrix element correction .
The problem is made explicit in relativistic quarks models \`a la Bakamjian and
Thomas, that give a transparent physical interpretation of the latter as due,
not to a force, but to the Wigner rotation of the light quark spin.
We point out moreover that the selection rule for decay constants of states, , predicts, assuming the model of factorization,
the opposite hierarchy .Comment: Contribution to the International Europhysics Conference on HEP,
Budapest, July 2001 (presented by L. Oliver); 5 page
Nuclear models on a lattice
We present the first results of a quantum field approach to nuclear models
obtained by lattice techniques. Renormalization effects for fermion mass and
coupling constant in case of scalar and pseudoscalar interaction lagrangian
densities are discussed.Comment: 4 pages - 7 figures ; Invited talk to QCD 05: 12th International QCD
Conference, 4-9 Jul 2005, Montpellier, France ; To appear in Nucl. Phys. B
(Proc. Suppl.
The Infrared Behaviour of the Pure Yang-Mills Green Functions
We study the infrared behaviour of the pure Yang-Mills correlators using
relations that are well defined in the non-perturbative domain. These are the
Slavnov-Taylor identity for three-gluon vertex and the Schwinger-Dyson equation
for ghost propagator in the Landau gauge. We also use several inputs from
lattice simulations. We show that lattice data are in serious conflict with a
widely spread analytical relation between the gluon and ghost infrared critical
exponents. We conjecture that this is explained by a singular behaviour of the
ghost-ghost-gluon vertex function in the infrared. We show that, anyhow, this
discrepancy is not due to some lattice artefact since lattice Green functions
satisfy the ghost propagator Schwinger-Dyson equation. We also report on a
puzzle concerning the infrared gluon propagator: lattice data seem to favor a
constant non vanishing zero momentum gluon propagator, while the Slavnov-Taylor
identity (complemented with some regularity hypothesis of scalar functions)
implies that it should diverge.Comment: 25 pages, 7 figures; replaced version with some references adde and
an enlarged discussion of the non-renormalization theorem; second replacement
with improved figures and added reference
Asymptotic behavior of the ghost propagator in SU3 lattice gauge theory
We study the asymptotic behavior of the ghost propagator in the quenched
SU(3) lattice gauge theory with Wilson action. The study is performed on
lattices with a physical volume fixed around 1.6 fm and different lattice
spacings: 0.100 fm, 0.070 fm and 0.055 fm. We implement an efficient algorithm
for computing the Faddeev-Popov operator on the lattice. We are able to
extrapolate the lattice data for the ghost propagator towards the continuum and
to show that the extrapolated data on each lattice can be described up to
four-loop perturbation theory from 2.0 GeV to 6.0 GeV. The three-loop values
are consistent with those extracted from previous perturbative studies of the
gluon propagator. However the effective \Lambda_{\ms} scale which reproduces
the data does depend strongly upon the order of perturbation theory and on the
renormalization scheme used in the parametrization. We show how the truncation
of the perturbative series can account for the magnitude of the dependency in
this energy range. The contribution of non-perturbative corrections will be
discussed elsewhere.Comment: 26 pages, 7 figure
Élaboration et caractérisation des sources solides de diffusion sur silicium par la méthode sol-gel pour la réalisation de l’émetteur d’une cellule solaire
La réalisation de l„émetteur d„une cellule solaire au silicium est une étape très importante et critique dans le procédé de fabrication des photopiles en silicium. En effet, la concentration en surface du phosphore ou du bore et la profondeur de la jonction déterminent le rendement de conversion photovoltaïque des cellules solaires au silicium. Plusieurs techniques sont utilisées pour la réalisation des émetteurs n+p et n+pp+ parmi lesquelles la diffusion en phase vapeur, l„implantation ionique et la diffusion à partir de sources solides. Ce travail sera consacré à la réalisation d„émetteurs par diffusion de sources solides que nous avons élaboré par la méthode sol gel associé au spin coating sur silicium polix monocristallins et polycristallins. Les solutions de dopage ont été préparées par la méthode sol gel en utilisant le methyltriethoxysilane « MTEOS » ou le tétraethoxysilane « TEOS » et l„acide phosphorique H4PO3 comme précurseurs par émulsion d„acide phosphorique dans l„isopropanol pour la jonction n+, et l‟acide borique H3BO3 ou l‟aluminium pour la jonction p+. Cette expérience nous a permis d‟étudier les propriétés électriques des émetteurs n+p et n+pp+ par la méthode des quatre pointes, les résultats des mesures effectuées ont montré des valeurs de la résistance carrée R comparables aux résultats de la littérature et confirme la diffusion du phosphore et du bore à partir des sources solides de diffusion préparées par sol gel.Mots-clés : réalisation d‟émetteurs, sources dopantes solides, sol-gel, diffusion phosphore ou bore
Three-loop beta function and non-perturbative in asymmetric momentum scheme
We determine the three-loop coefficient of the beta function in the
asymmetric momentum subtraction scheme in Landau gauge. This scheme is
convenient for lattice studies of \alpha_s, the running coupling constant of
QCD. We present high statistics lattice results for \alpha_s in the SU(3)
Yang-Mills theory without quark, compare with the three-loop running and
extract the value of the corresponding \Lambda_\msbar parameter. We estimate
the systematic error coming from four-loop terms. We obtain the result:
\Lambda_\msbar = 295 (5) (15) {a^{-1}(\beta=6.0) \over 1.97 GeV} MeV.Comment: 9 pages, LaTeX with two figures. The final analysis has been
revisited to include an estimation of the systematic error coming from
higher-order term
Evaluation agronomique de variétés de canne à sucre en début de campagne de récolte à Ferké au Nord de la Côte d’Ivoire : vers un allègement du schéma de sélection
Dans le but de contribuer à l’allègement du schéma de sélection de la canne à sucre, 16 variétés ont été évaluées, sur la base du rendement en sucre extractible, de la tolérance au charbon (U. scitaminea), à l’échaudure des feuilles (maladies endémiques) et au foreur de tiges (E. saccharina), dans 4 essais, conduits sur 5 ans, en début de saison de récolte à Sucaf-Ferké 2 (nord de la Côte d’Ivoire). Les variétés commerciales SP701143, SP711406, MEX73523, N19, R575, SP75184 et SP791230 ont présenté, respectivement, des TSE/ha de 12,2 ; 15,3 ; 11,6 ; 11,7 ; 10,6 ; 11,5 et 12,1 significativement supérieures à celles de NCo376 (variété la plus cultivée jusqu’en 2008). Par ailleurs, celles-ci ont été moins sensibles au charbon et au foreur de tiges que NCo376 (taux de charbon < 5000 fouets/ha et ENA % < 3 %). Les variétés commerciales se sont avérées plus adaptées aux conditions de culture du site que les présélectionnées, avec des taux d’adaptation de 33 %, 100 %, 67 % et 75 %. L’étude suggère en outre, la possibilité de réduire la 2e étape de sélection de 2 ans et de conduire le criblage en une seule étape de 3 ans. Ce qui ramènerait la durée du cycle de sélection à 6 ans, contre 11, voire 15 ans auparavant.Mots clés : Variétés de canne à sucre, adaptation, charbon, foreur, Côte d’Ivoir
A low-energy effective Yang-Mills theory for quark and gluon confinement
We derive a gauge-invariant low-energy effective model of the Yang-Mills
theory. We find that the effective gluon propagator belongs to the
Gribov-Stingl type and agrees with it when a mass term which breaks nilpotency
of the BRST symmetry is included. We show that the effective model with gluon
propagator of the Gribov-Stingl type exhibits both quark and gluon confinement:
the Wilson loop average has the area law and the Schwinger function violates
reflection positivity. However, we argue that both quark and gluon confinement
can be obtained even in the absence of such a mass term.Comment: 5 pages, no figures; accepted for publication in Physical Review D
(Rapid Communication
Assessment of various strategies for the preservation of clonal genetic resources in oil palm (Elaeis guineensis Jacq.)
Three different approaches for the preservation of oil palm (Elaeis guineensis Jacq.) clonal genetic resources and their impacts on the induction of the « mantled » somaclonal variation were assessed. In vitro long term preservation of somatic embryos stock-cultures was studied : after a 5 year cultivation period, 75 % of clonal lines were still normal. Between 8 and 13 years of embryo cultures, half of the considered clonal lines were found to be « mantled ». Finally, 40 % were found to be normal over 15 years of in vitro conservation. Clonal conformity of ramets resulting from the re-cloning of somaplants depended, on one hand, on the floral status of the mother plant at the time of sampling and, on the other hand, on its origin. Re-cloning of abnormal regenerants led, in all cases, to 100 % abnormal offspring. The age of the ramet used as mother palm at the time of sampling was found to be critical for true-to-type regeneration. There is a high risk of obtaining variant regenerant palms if the clonal mother palm is sampled at nursery stage. Field observations carried out on palms originating from somatic embryos cryopreserved at -196 °C showed floral conformity rates comparable to those obtained from standard not-cryopreserved clonal palms, for 6 out of the 8 clonal lines studied. From the 2 remaining clonal lines, a few regenerant palms originating from standard batch were found to be « mantled », whereas those resulting from cryopreserved embryos were all normal. The assumption of changes in levels of genomic DNA methylation during preservation was discussed, together with the capacity of our cryopreservation protocol to select embryogenic cells which were only suited to trueto-type regeneration
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