1,694 research outputs found
Mass spectra of the particle-antiparticle system with a Dirac oscillator interaction
The present view about the structure of mesons is that they are a quark-antiquark system. The mass spectrum corresponding to this system should, in principle, be given by chromodynamics, but this turns out to be a complex affair. Thus it is of some interest to consider relativistic systems of particle-antiparticle, with a simple type of interaction, which could give some insight on the spectra we can expect for mesons. This analysis is carried out when the interaction is of the Dirac oscillator type. It is shown that the Dirac equation of the antiparticle can be obtained from that of the particle by just changing the frequency omega into -omega. Following a procedure suggested by Barut, the equation for the particle-antiparticle system is derived and it is solved by a perturbation procedure. Thus, explicit expressions for the square of the mass spectra are obtained and its implications in the meson case is discussed
ATP-dependent chromatosome remodeling
Chromatin serves to package, protect and organize the complex eukaryotic genomes to assure their stable inheritance over many cell generations. At the same time, chromatin must be dynamic to allow continued use of DNA during a cell's lifetime. One important principle that endows chromatin with flexibility involves ATP-dependent `remodeling' factors, which alter DNA-histone interactions to form, disrupt or move nucleosomes. Remodeling is well documented at the nucleosomal level, but little is known about the action of remodeling factors in a more physiological chromatin environment. Recent findings suggest that some remodeling machines can reorganize even folded chromatin fibers containing the linker histone H1, extending the potential scope of remodeling reactions to the bulk of euchromatin
Single-particle transfer and nuclear supersymmetry
Transfer reactions constitute a stringent test for nuclear supersymmetry, a theory that simultaneously describes neighboring nuclei with bosonic and fermionic character. We construct and analytically evaluate one-nucleon transfer matrix elements between supersymmetric partners with the U( 6/4) case as an example, and stress the need for a careful treatment of bosonic and fermionic operators in the construction of mixed tensor operators.CONACyT 32416-E 32397-EPAGA-UNAM IN106400DGICYT PB98-111
Global priorities for conservation across multiple dimensions of mammalian diversity
Conservation priorities that are based on species distribution, endemism, and vulnerability may underrepresent biologically unique species as well as their functional roles and evolutionary histories. To ensure that priorities are biologically comprehensive, multiple dimensions of diversity must be considered. Further, understanding how the different dimensions relate to one another spatially is important for conservation prioritization, but the relationship remains poorly understood. Here, we use spatial conservation planning to (i) identify and compare priority regions for global mammal conservation across three key dimensions of biodiversity-taxonomic, phylogenetic, and traits-and (ii) determine the overlap of these regions with the locations of threatened species and existing protected areas. We show that priority areas for mammal conservation exhibit low overlap across the three dimensions, highlighting the need for an integrative approach for biodiversity conservation. Additionally, currently protected areas poorly represent the three dimensions of mammalian biodiversity. We identify areas of high conservation priority among and across the dimensions that should receive special attention for expanding the global protected area network. These high-priority areas, combined with areas of high priority for other taxonomic groups and with social, economic, and political considerations, provide a biological foundation for future conservation planning efforts
Magnetismo e Propriedades Estruturais de NanopartÃculas Magnéticas Obtidas por Processos QuÃmicos.
Neste trabalho, objetivamos sintetizar, estabilizar e caracterizar estrutural e magneticamente, nanopartÃculas à base de Fe, produzidas através dos métodos quÃmicos de redução e de precipitação. Em ambos os casos, utilizamos as técnicas de Espectroscopia Mössbauer, Difração de raios X, Microscopias Eletrônicas de Varredura e de Transmissão, além de medidas magnéticas realizadas nos modos DC e AC. NanopartÃculas preparadas por Redução QuÃmica: realizamos, à temperatura ambiente, a hidrólise-redução do FeCl3 em água deionizada, utilizando diferentes quantidades de glicerol como agente controlador da concentração de Ãons de Fe3+, juntamente com o redutor NaBH4. Investigamos os efeitos da utilização de diferentes quantidades de glicerol, no processo de sÃntese, e do processo de secagem das amostras sobre a estabilização da fase -Fe. Como resultados, obtivemos nanopartÃculas de Fe com estrutura cúbica de corpo centrado ( -Fe) encapsuladas por superfÃcies amorfas de Fe2B (a-Fe2B) à s quais nos referimos por -Fe/a-Fe2B. Verificamos o aumento na concentração fase -Fe ( ) como função do aumento na quantidade de glicerol ( ) utilizada na sÃntese, para as amostras secas a vácuo. Para as amostras úmidas notamos alta susceptibilidade a oxidação, enquanto que as amostras secas a vácuo se mantiveram estáveis por medidas de Espectroscopia Mössbauer realizadas 12 meses após a sÃntese. Para as quantidades de 2, 3 e 4 ml de glicerol, produzimos nanopartÃculas de -Fe/a-Fe2B sem a presença de óxido de Fe, adicionalmente observamos mudanças no formato das nanopartÃculas produzidas, utilizando 4 ml de glicerol, produzimos nanofolhas de -Fe/a-Fe2B e utilizando 5 ml, obtivemos nanopartÃculas de -Fe/a-Fe2B e -Fe2O3 com formato esférico irregular. NanopartÃculas preparadas por Precipitação QuÃmica: produzimos nanocompósitos de Fe2P dispersos numa matriz de carbono poroso, através do método de precipitação quÃmica. Além da produção deste nanocompósito utilizando o carvão ativado (preparado por ativação quÃmica com H3PO4), a impregnação com sais de Fe3+ em meio aquoso, e subsequentes tratamentos térmicos, sob fluxo de N2, levaram a formação da fase hexagonal não estequiométrica Fe2-XP e da fase ortorrômbica Fe-P com razão de 4:1, respectivamente. Medidas de espectroscopia
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Mössbauer em baixas temperaturas revelam que parte da fração (~ 28 %) deste material se encontra no estado paramagnético, sugerindo que parte da fase não estequiométrica Fe2-XP é constituÃda por partÃculas muito pequenas. Observamos ainda uma transição de fase metamagnética em torno de 150 K, associada à nanopartÃculas pertencentes à fase Fe2-XP, por estar bem abaixo da temperatura de ordenamento do composto Fe2P (~ 230 K) e por ser dependente da frequência e dos campos externos AC e DC aplicados. Além disso, as nanopartÃculas de Fe2-XP possuem um caráter magneto-duro em baixas temperaturas, com campo coercivo . Considerando estas interessantes propriedades magnéticas e hiperfinas e a área de superfÃcie elevada da matriz de carvão ativado, que não é fortemente reduzida após a impregnação com os compostos contendo Fe, podemos apontar aplicações tecnológicas promissoras para o nanocompósito produzido
Banking Efficiency in Chile: a Profit Frontier Approach
This paper characterizes the evolution of the efficiency X in the Chilean banking industry over 1987 to 2007, based on a profit frontier approach. Our results suggest that over this period the Chilean banking sector has attained just a 15% over its maximum profits. This inefficiency basically arises from a technical source rather than an assignative one, and mainly affects domestic and small banks. Nevertheless, the level of efficiency X of the industry as a whole has dramatically improved since the late 1990’s, which is consistent with important economic, technological and regulatory transformations.
Observaciones de planetas y pequeños planetas realizadas en 1983, en el cÃrculo meridiano Repsold de Cerro Calán
Se presentan los resultados de 34 observaciones diferenciales , en ascensión recta de los planetas Urano y Neptuno y de los pequeños planetas Ceres, Juno y Vesta, realizadas con el CÃrculo Meridiano Repsold de Cerro Calán. El error medio de cada observación es ε_α cos δ ≃ ± 0Ë¢.016 y se dan los valores de la ascensión recta observada y los (O-C) para cada una de las observaciones en el sentido instrumento - American Ephemeris.Asociación Argentina de AstronomÃ
Estudio farmacológico y fitoquÃmico de dos especies de salandra nativas de México
A pharmacological and phytochemical sreening study of two species of Solandra native of Mexico is reported due to their indigenous use in traditional medicine. Water extracts of flowers of S. nÃtida and barks of S. guerrerense have marked anticholinergic effectis inhibiting acetiylicholine induced contraction of the smooth muscle of isolated ileum of guinea pig and depressing EEG activity in rats after intraperitoneal injection. By gas chromatography and NMR spectrometry the prodomine is abundant in S. nÃtida flowers, vhile Hyoscyamine is the mayin alkaloid in S. guerrerense bark. The presence of these tropane alkaloids in Solandra extracts and popular use of the plants for ceremonial and medicinal procedures is discussed.Se realizó un screening farmacológico y fitoquÃrnico de dos especies de Solandra que se usan en México con propósitos medicinales y rituales. Los extractos de flores de S. nÃtida y de cortezas de S. guerrerense contienen escopolamina e hiosciamina respectivamente como alcaloides predominantes. El efecto anticolinérgico de los extractos acuosos de estas plantas se estudió en el Ãleon aislado del cobayo y su efecto sedante del sistema nervioso central en la rata implantada con electrodos y sometida a estÃmulos dolorosos. El uso como psicotrópico que se hace de estas especies se explica como una consecuencia de la intoxicación que producen los extractos administrados por vÃa oral. Se discute el papel de la acetilcolina y los alcaloides del tropano en la acción anestésica local atribuida a estas plantas
Bayesian additive regression trees for probabilistic programming
Bayesian additive regression trees (BART) is a non-parametric method to
approximate functions. It is a black-box method based on the sum of many trees
where priors are used to regularize inference, mainly by restricting trees'
learning capacity so that no individual tree is able to explain the data, but
rather the sum of trees. We discuss BART in the context of probabilistic
programming languages (PPL), i.e., we present BART as a primitive that can be
used as a component of a probabilistic model rather than as a standalone model.
Specifically, we introduce the Python library PyMC-BART, which works by
extending PyMC, a library for probabilistic programming. We showcase a few
examples of models that can be built using PyMC-BART, discuss recommendations
for the selection of hyperparameters, and finally, we close with limitations of
our implementation and future directions for improvement.Comment: 22 pages, 17 figure
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