53,690 research outputs found
Meson decay in a corrected model
Extensively applied to both light and heavy meson decay and standing as one
of the most successful strong decay models is the model, in which
pair production is the dominant mechanism. The pair production can
be obtained from the non-relativistic limit of a microscopic interaction
Hamiltonian involving Dirac quark fields. The evaluation of the decay amplitude
can be performed by a diagrammatic technique for drawing quark lines. In this
paper we use an alternative approach which consists in a mapping technique, the
Fock-Tani formalism, in order to obtain an effective Hamiltonian starting from
same microscopic interaction. An additional effect is manifest in this
formalism associated to the extended nature of mesons: bound-state corrections.
A corrected is obtained and applied, as an example, to
and decays.Comment: 3 figures. To appear in Physical Review
Parabolic dunes in north-eastern Brazil
In this work we present measurements of vegetation cover over parabolic dunes
with different degree of activation along the north-eastern Brazilian coast. We
are able to extend the local values of the vegetation cover density to the
whole dune by correlating measurements with the gray-scale levels of a high
resolution satellite image of the dune field. The empirical vegetation
distribution is finally used to validate the results of a recent continuous
model of dune motion coupling sand erosion and vegetation growth.Comment: 18 pages, 14 figures, aubmitted to Geomorpholog
The class of n-entire operators
We introduce a classification of simple, regular, closed symmetric operators
with deficiency indices (1,1) according to a geometric criterion that extends
the classical notions of entire operators and entire operators in the
generalized sense due to M. G. Krein. We show that these classes of operators
have several distinctive properties, some of them related to the spectra of
their canonical selfadjoint extensions. In particular, we provide necessary and
sufficient conditions on the spectra of two canonical selfadjoint extensions of
an operator for it to belong to one of our classes. Our discussion is based on
some recent results in the theory of de Branges spaces.Comment: 33 pages. Typos corrected. Changes in the wording of Section 2.
References added. Examples added. arXiv admin note: text overlap with
arXiv:1104.476
The Levi-Civita spacetime
We consider two exact solutions of Einstein's field equations corresponding
to a cylinder of dust with net zero angular momentum. In one of the cases, the
dust distribution is homogeneous, whereas in the other, the angular velocity of
dust particles is constant [1]. For both solutions we studied the junction
conditions to the exterior static vacuum Levi-Civita spacetime. From this study
we find an upper limit for the energy density per unit length of the
source equal for the first case and for the second
one. Thus the homogeneous cluster provides another example [2] where the range
of is extended beyond the limit value previously found in
the literature [3,4]. Using the Cartan Scalars technics we show that the
Levi-Civita spacetime gets an extra symmetry for or
. We also find that the cluster of homogeneous dust has a superior
limit for its radius, depending on the constant volumetric energy density
Aggregate stability in soils cultivated with eucalyptus.
The objective of this work was to evaluate the aggregate stability of tropical soils under eucalyptus plantation and native vegetation, and assess the relationships between aggregate stability and some soil chemical and physical properties. Argisols, Cambisol, Latosols and Plinthosol within three eucalyptus-cultivated regions, in the states of Espírito Santo, Rio Grande do Sul and Minas Gerais, Brazil, were studied. For each region, soils under native vegetation were compared to those under minimum tillage with eucalyptus cultivation. The aggregate stability was measured using the high‑energy moisture characteristic (HEMC) technique, i.e., the moisture release curve at very low suctions. This method compares the resistance of aggregates to slaking on a relative scale from zero to one. Thus, the aggregate stability from different soils and management practices can be directly compared. The aggregate stability ratio was greater than 50% for all soils, which shows that the aggregate stability index is high, both in eucalyptus and native vegetation areas. This suggests that soil management adopted for eucalyptus cultivation does not substantially modify this property. In these soils, the aggregate stability ratio does not show a good relationship with clay or soil organic matter contents. However, soil organic matter shows a positive relationship with clay content and cation exchange capacity
Determining the Weak Phase From Charged Decays
A quadrangle relation is shown to be satisfied by the amplitudes for , and . By comparison with the
corresponding relation satisfied by decay amplitudes, it is shown that
the relative phases of all the amplitudes can be determined up to discrete
ambiguities. Making use of an SU(3) relation between amplitudes contributing to
the above decays and those contributing to , it is
then shown that one can determine the weak phase , where is the Cabibbo-Kobayashi-Maskawa
matrix describing the charge-changing weak interactions between the quarks
and .Comment: 16 pages, latex, 7 uuencoded figure
Evapotranspiração de referência obtida pelo Tanque Classe A e por Penmam Monteith (FAO): influência na determinação do Kc da goiabeira em Petrolina, PE.
Esse trabalho teve por objetivo comparar a estimativa da evapotranspiração de referência (ETo), obtida pelos métodos de PenmamMonteith (PM-FAO) e do tanque classe A, na região de Petrolina - PE. As estimativas de ETo foram utilizadas para a determinação do coeficiente de cultura (Kc) da goiabeira cv. Paluma, irrigada por micropaspersão, durante o terceiro ciclo de produção, entre junho e dezembro de 2001. Os valores de ETo estimados pelo tanque classe A foram superiores aos estimados pelo método PM-FAO, com médias de 5,22 e 4,68 mm.dia1, respectivamente. Conseqüentemente, os valores de Kc foram maiores para a ETo estimada pelo método de PM FAO
Applications of M.G. Krein's Theory of Regular Symmetric Operators to Sampling Theory
The classical Kramer sampling theorem establishes general conditions that
allow the reconstruction of functions by mean of orthogonal sampling formulae.
One major task in sampling theory is to find concrete, non trivial realizations
of this theorem. In this paper we provide a new approach to this subject on the
basis of the M. G. Krein's theory of representation of simple regular symmetric
operators having deficiency indices (1,1). We show that the resulting sampling
formulae have the form of Lagrange interpolation series. We also characterize
the space of functions reconstructible by our sampling formulae. Our
construction allows a rigorous treatment of certain ideas proposed recently in
quantum gravity.Comment: 15 pages; v2: minor changes in abstract, addition of PACS numbers,
changes in some keywords, some few changes in the introduction, correction of
the proof of the last theorem, and addition of some comments at the end of
the fourth sectio
Building materials with historical and heritage interest – A database in progress
Conservation and restoration of architectural heritage requires knowledge of the conservation state of its constituent materials in order to provide recommendations concerning the intervention plan, and the materials and the techniques to be used. Information on physical, chemical and mechanical characterization of materials, when available, is usually shared by different actors, but is not normally accessible by the general public. In this context, the DB-HERITAGE project intends to build a reference sample collection or repository for historical materials and an IT-tool to collect the related data on construction materials history, properties, and performance. This database is being developed to categorize materials by type and function, addressing the relevant issues in the historical context of construction materials. An overview of the database tools will be presented in this paper, exemplifying some of its outputs
- …