17,497 research outputs found
Causal Structure and Birefringence in Nonlinear Electrodynamics
We investigate the causal structure of general nonlinear electrodynamics and
determine which Lagrangians generate an effective metric conformal to
Minkowski. We also proof that there is only one analytic nonlinear
electrodynamics presenting no birefringence.Comment: 11 pages, no figure
Electromagnetic structure and weak decay of pseudoscalar mesons in a light-front QCD-inspired model
We study the scaling of the meson mass splitting and the
pseudoscalar weak decay constants with the mass of the meson, as seen in the
available experimental data. We use an effective light-front QCD-inspired
dynamical model regulated at short-distances to describe the valence component
of the pseudoscalar mesons. The experimentally known values of the mass
splittings, decay constants (from global lattice-QCD averages) and the pion
charge form factor up to 4 [GeV/c] are reasonably described by the modelComment: 27 Pages, 7 eps figures,use revtex
Nematódeos resistentes a anti-helmíntico em rebanhos de ovinos e caprinos do estado do Ceará, Brasil.
Resumo: O controle do parasitismo por nematódeos gastrintestinais é feito basicamente com a utilização de anti-helmínticos. Falhas no controle são o primeiro sinal do aparecimento de resistência anti-helmíntica. A real situação da prevalência da resistência anti-helmíntica, em fazendas comerciais de criação de ovinos e caprinos no Brasil, é desconhecida. Esse experimento teve como objetivo, estimar a ocorrência de resistência ao oxfendazol, levamisol e ivermectina em propriedades comerciais de criação de ovinos e caprinos, na região do médio e baixo Jaguaribe, através do teste de redução na contagem de ovos nas fezes acompanhados de coproculturas. O trabalho foi realizado em 25 criações, sendo 16 de ovinos, 7 de caprinos e uma de ovinos e caprinos. Os dados obtidos foram analisados pelo programa estatístico RESO (1989). A prevalência de nematódeos resistentes ao oxfendazol, levamisol e ivermectina em ovinos foi de 88%, 41% e 59%, e em caprinos de 87,5%, 75% e 37,5%, respectivamente. Observou-se que o gênero Haemonchus foi o mais prevalente na população resistente a todos os anti-helmínticos, tanto em ovinos quanto em caprinos, seguido de Trichostrongylus e Oesophagostomum. [Nematodes resistant to anthelmintics in sheep and goat flock in the State of Ceará, Brazil]. ABSTRACT - The control of gastrointestinal nematodes parasitism is made basically with anthelmintics. Control failure is the first sign of anthelmintic resistance development. The actual situation of anthelmintic resistance prevalence in commercial farms of sheep and goats in Brazil is unknown. The aim of this work was to estimate the occurrence of oxfendazole, levamisole and ivermectin resistance in sheep and goats from medio and baixo Jaguaribe region by faecal egg count reduction followed by coprocultures. The work involved 25 farms (16 sheep farms, 7 goat farms and one of sheep and goats). Data were analysed by RESO (1989). The prevalence of resistant nematodes to oxfendazole, levamisole and ivermectin was respectively 88%, 41% and 59% in sheep and 87.5%, 75% and 37.5%, in goats. Haemonchus was the most prevalent genus in the resistant population to all anthelmintics in sheep and goats, followed by Trichostrongylus and Oesophagostomum
Avaliação de linhagem e cultivares de feijoeiro comum, grupo preto, no período de inverno, em Uberlândia-MG.
Em esforço conjunto, a Universidade Federal de Uberlândia e a Embrapa, conduziram ensaios de competição de linhagens e cultivares no município de Uberlândia, MG com objetivo de avaliar o comportamento agronômico, dentre estes a produtividade, de cultivares de feijoeiro comum, do grupo preto, na época de inverno, em 2006 e 2007
Evolution from BCS to BKT superfluidity in one-dimensional optical lattices
We analyze the finite temperature phase diagram of fermion mixtures in
one-dimensional optical lattices as a function of interaction strength. At low
temperatures, the system evolves from an anisotropic three-dimensional
Bardeen-Cooper-Schrieffer (BCS) superfluid to an effectively two-dimensional
Berezinskii-Kosterlitz-Thouless (BKT) superfluid as the interaction strength
increases. We calculate the critical temperature as a function of interaction
strength, and identify the region where the dimensional crossover occurs for a
specified optical lattice potential. Finally, we show that the dominant vortex
excitations near the critical temperature evolve from multiplane elliptical
vortex loops in the three-dimensional regime to planar vortex-antivortex pairs
in the two-dimensional regime, and we propose a detection scheme for these
excitations.Comment: 4 pages with 2 figure
Bopp-Podolsky black holes and the no-hair theorem
Bopp-Podolsky electrodynamics is generalized to curved space-times. The
equations of motion are written for the case of static spherically symmetric
black holes and their exterior solutions are analyzed using Bekenstein's
method. It is shown the solutions split-up into two parts, namely a
non-homogeneous (asymptotically massless) regime and a homogeneous
(asymptotically massive) sector which is null outside the event horizon. In
addition, in the simplest approach to Bopp-Podolsky black holes, the
non-homogeneous solutions are found to be Maxwell's solutions leading to a
Reissner-Nordstr\"om black hole. It is also demonstrated that the only exterior
solution consistent with the weak and null energy conditions is the Maxwell's
one. Thus, in light of energy conditions, it is concluded that only Maxwell
modes propagate outside the horizon and, therefore, the no-hair theorem is
satisfied in the case of Bopp-Podolsky fields in spherically symmetric
space-times.Comment: 9 pages, updated to match published versio
Cultivares e linhagens de feijoeiro comum, do grupo carioca, no período de inverno, em Uberlândia-MG.
A Universidade Federal de Uberlândia e a Embrapa, conduziram ensaios de competição de linhagens e cultivares em Uberlândia, MG, com o objetivo de avaliar o comportamento agronômico, dentre estes a produtividade, de genótipos de feijoeiro comum, do grupo comercial carioca, na época de inverno, em 2006 e 2007
Weak decay constant of pseudscalar meson in a QCD-inspired model
We show that a linear scaling between the weak decay constants of
pseudoscalar and the vector mesons masses is supported by the available
experimental data. The decay constant scale as (f_m
is decay constant and M_V vector meson ground state mass). This simple form is
justified within a renormalized light-front QCD-inpired model for
quark-antiquark bound states.Comment: 4 pages, use revtex style. To appear "Brazilian Journal of Physics
(2003)
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