2,592 research outputs found
Fermionic bound states in Minkowski-space: Light-cone singularities and structure
The Bethe-Salpeter equation for two-body bound system with spin
constituent is addressed directly in the Minkowski space. In order to
accomplish this aim we use the Nakanishi integral representation of the
Bethe-Salpeter amplitude and exploit the formal tool represented by the exact
projection onto the null-plane. This formal step allows one i) to deal with
end-point singularities one meets and ii) to find stable results, up to
strongly relativistic regimes, that settles in strongly bound systems. We apply
this technique to obtain the numerical dependence of the binding energies upon
the coupling constants and the light-front amplitudes for a fermion-fermion
state with interaction kernels, in ladder approximation, corresponding to
scalar-, pseudoscalar- and vector boson exchanges, respectively. After
completing the numerical survey of the previous cases, we extend our approach
to a quark-antiquark system in state, taking both constituent-fermion and
exchanged boson masses, from lattice calculations. Interestingly, the
calculated light-front amplitudes for such a mock pion show peculiar signatures
of the spin degrees of freedom.Comment: 22 pages, 7 figures, bst file include
Experimental Investigations into the Mechanical Behaviour of the Breccias Around the Proposed Gibraltar Strait Tunnel
The proposed Gibraltar Strait tunnel will cross two zones with breccia consisting of a chaotic mixture of blocks and stones embedded in a clay matrix. The breccia is saturated, has a high porosity and exhibits poor mechanical properties in the range between hard soils and weak rocks. The overburden and high in situ pore pressures in combination with the low strength of the breccia may lead to heavy squeezing. The crossing of the breccia zones thus represents one of the key challenges in the construction of the tunnel. In order to improve our understanding of the mechanical behaviour of the breccias, a series of triaxial compressions tests were carried out. Standard rock mechanics test equipment was not adequate for this purpose, because it does not provide pore pressure control, which is important in the case of saturated porous materials. Pore pressure control is routine in soil mechanics tests, but standard soil mechanics equipment allows only for relatively low nominal loads and pressures. In addition, the low hydraulic conductivity of the breccias demands extremely low loading rates and a long test duration. For these reasons, we re-designed several components of the test apparatus to investigate the mechanical behaviour of the breccia by means of consolidated drained and undrained tests. The tests provided important results concerning the strength, volumetric behaviour, consolidation state and hydraulic conductivity of the breccias. The present paper describes the test equipment and procedures, provides an overview of the test results and discusses features of the mechanical behaviour of the breccias which make them qualitatively different from other weak rocks such as kakirites—a typical squeezing rock in alpine tunnelling. The paper also demonstrates the practical importance of the experimental findings for tunnelling in general. More specifically, it investigates the short-term ground response to tunnel excavation from the perspective of elasto-plastic behaviour with the Mohr-Coulomb yield criterion. The computational results indicate that the breccias will probably experience very large deformations already around the advancing tunnel heading, which can be reduced considerably, however, by advance drainage. The analyses additionally show that plastic dilatancy is favourable with respect to the short-term response, thus highlighting the importance of the constitutive model when it comes to theoretical prediction
Crystal structures and proton dynamics in potassium and cesium hydrogen bistrifluoroacetate salts with strong symmetric hydrogen bonds
The crystal structures of potassium and cesium bistrifluoroacetates were
determined at room temperature and at 20 K and 14 K, respectively, with the
single crystal neutron diffraction technique. The crystals belong to the I2/a
and A2/a monoclinic space groups, respectively, and there is no visible phase
transition. For both crystals, the trifluoroacetate entities form dimers linked
by very short hydrogen bonds lying across a centre of inversion. Any proton
disorder or double minimum potential can be rejected. The inelastic neutron
scattering spectral profiles in the OH stretching region between 500 and 1000
cm^{-1} previously published [Fillaux and Tomkinson, Chem. Phys. 158 (1991)
113] are reanalyzed. The best fitting potential has the major characteristics
already reported for potassium hydrogen maleate [Fillaux et al. Chem. Phys. 244
(1999) 387]. It is composed of a narrow well containing the ground state and a
shallow upper part corresponding to dissociation of the hydrogen bond.Comment: 31 pages, 7 figure
Magnetic Properties of Weakly Doped Antiferromagnets
We study the spin excitations and the transverse susceptibility of a
two-dimensional antiferromagnet doped with a small concentration of holes in
the t-J model. The motion of holes generates a renormalization of the magnetic
properties. The Green's functions are calculated in the self-consistent Born
approximation. It is shown that the long-wavelength spin waves are
significantly softened and the shorter-wavelength spin waves become strongly
damped as the doping increases. The spin wave velocity is reduced by the
coherent motion of holes, and not increased as has been claimed elsewhere. The
transverse susceptibility is found to increase considerably with doping, also
as a result of coherent hole motion. Our results are in agreement with
experimental data for the doped copper oxide superconductors.Comment: 20 page
Bianchi I Quantum cosmology in the Bergmann-Wagoner theory
The Wheeler-DeWitt equation is considered in the context of generalized
scalar-tensor theories of gravitation for Bianchi type I cosmology. Exact
solutions are found for two selfinteracting potentials and arbitary coupling
function. The WKB wavefunctions are obtained and a family of solutions
satisfying the Hawking-Page regularity conditions of wormholes are found.Comment: 12 pages, Latex fil
Caracterização e regime hídrico de uma área com vegetação florestal no Jardim Botânico do Rio de Janeiro.
O entorno do Jardim Botânico do Rio de Janeiro (JBRJ) é caracterizado por uma densa ocupação populacional que, consequentemente, modificou determinadas características da região, a exemplo do aumento da impermeabilização do solo afetando diretamente o ciclo hidrológico local. Juntamente aos problemas de drenagem existentes no bairro, observam-se eventos de alagamentos e inundações cada vez mais frequentes. O presente estudo tem como objetivo caracterizar o regime hídrico de uma amostra de solo do Jardim Botânico para que, futuramente, seja possível quantificar o balanço hídrico da região e determinar a influência dos tipos de solos e da vegetação no ciclo hidrológico. Para isso, foi utilizado o programa HYDRUS-1D e informações de uma amostra de solo localizada no Roseiral do JBRJ. As simulações foram realizadas em uma série histórica de 20 anos, no período compreendido entre 2000 e 2020. Os resultados permitiram estimar os componentes do balanço hídrico da amostra que envolvem a evapotranspiração real da vegetação. A pesquisa demonstrou grande relevância para a compreensão da influência do JBRJ no bairro em questão. Em estudos futuros, espera-se estimar de forma mais precisa os dados de entrada na simulação, que certamente irão conferir resultados mais satisfatórios
Colheita, beneficiamento e armazenamento de sementes de pimenta longa (Piper hispidinervium).
A identificação da pimenta longa (Piper hispidinervum) no estado do Acre, por meio do Programa de Triagem de Plantas Aromáticas da Amazônia vem despertando, nos últimos seis anos, grande interesse por parte dos produtores e empresas nacionais de internacionais, processadoras de óleos essenciais ricos em safrol. O objetivo desta publicação é fornecer informações sobre a produção de sementes, bem como sua conservação, para que possam manter um bom padrão de qualidade.bitstream/CPAF-AC/3641/1/it20.pd
Recomendações de boas práticas de armazenamento de milho em espiga para agricultura familiar.
bitstream/item/46071/1/circ-161.pd
Impacto da seleção de espigas de milho na infestação por carunchos durante o armazenamento em propriedades familiares do estado de Minas Gerais.
Generating --cosmologies with perfect fluid in dilaton gravity
We present a method for generating exact diagonal -cosmological
solutions in dilaton gravity coupled to a radiation perfect fluid and with a
cosmological potential of a special type. The method is based on the symmetry
group of the system of -field equations. Several new classes of explicit
exact inhomogeneous perfect fluid scalar-tensor cosmologies are presented.Comment: 10 pages, LaTe
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