4,499 research outputs found
Dynamical quark recombination in ultrarelativistic heavy-ion collisions and the proton to pion ratio
We study quark thermal recombination as a function of energy density during
the evolution of a heavy-ion collision in a numerical model that reproduces
aspects of QCD phenomenology. We show that starting with a set of free quarks
(or quarks and antiquarks) the probability to form colorless clusters of three
quarks differs from that to form colorless clusters of quark-antiquark and that
the former has a sharp jump at a critical energy density whereas the latter
transits smoothly from the low to the high energy density domains. We interpret
this as a quantitative difference in the production of baryons and mesons with
energy density. We use this approach to compute the proton and pion spectra in
a Bjorken scenario that incorporates the evolution of these probabilities with
energy density, and therefore with proper time. From the spectra, we compute
the proton to pion ratio and compare to data at the highest RHIC energies. We
show that for a standard choice of parameters, this ratio reaches one, though
the maximum is very sensitive to the initial evolution proper time.Comment: 10 pages, 12 figures, version to appear in Phys. Rev.
Genetic control of soybean resistance to brown spot (Septoria glycines): first studies.
Knowledge of the genetic effects that control a trait is essential to direct the breeding program for gains in the selection process. The main objective of this study was to investigate the genetic control of soybean resistance to brown spot. Two tolerant cultivars (FT-2 and Dourados) and two susceptible cultivars (Davis and Paraná) and the segregant F2 and F3 generations derived from the crosses between these cultivars were assessed for reaction to the disease after inoculation. Genetic models were adjusted to the means and variances of the generations. It could be concluded that the trait is governed by various genes with lesser effect, has low to medium heritability and selection should be made always using progeny from the F3 generation and can be made during the initial stages of soybean growth, based on the mean of at least two trifoliate leaves
Ultramorphology of digestive tract of Anticarsia gemmatalis (Hübner, 1818) (Lepidoptera: Noctuidae) at final larval development.
RESUMO: O trato digestivo dos insetos constitui uma importante barreira físico-química natural contra invasão de patógenos. Algumas larvas de lepidópteros são consideradas pragas agrícolas potenciais e sua biologia tem recebido muita atenção; no entanto, pouco se sabe sobre a morfologia do sistema digestivo. A análise morfológica do trato digestivo de Anticarsia gemmatalis em nível ultraestrutural é um método bastante eficaz para o estudo dos seus mecanismos de defesa. Os materiais foram fixados (solução de glutaraldeído 2,5%; 0.1M tampão fosfato, pH 7.3), pós-fixados (tetróxido de ósmio 1% no mesmo tampão), desidratados em ponto crítico, recobertos com ouro e analisados ao microscópio eletrônico de varredura 515-Philips. O trato digestivo de A. gemmatalis consiste de um tubo retilíneo de diâmetro e comprimento variável, subdividido em três regiões: intestino anterior formado pela cavidade bucal, faringe, esôfago e papo; o intestino médio que é a região mais longa do trato digestivo, sem aparente diferenciação morfológica ao longo do comprimento; e o intestino posterior que é diferenciado em piloro, íleo, cólon, e reto. Embora a morfologia geral do trato digestivo de A. gemmatalis seja bastante semelhante ao de outras espécies de Lepidoptera, o arranjo anatômico das camadas musculares do papo difere do descrito para larvas destes insetos. ABSTRACT: The digestive tract of insects is an important natural, physical, and chemical defense barrier against pathogen invasion. Certain lepidopteran caterpillars are serious pests of agricultural crops and their biology has received much attention, but little is known about the larval noctuid gut. The morphological analysis of the digestive tract in Anticarsia gemmatalis under scanning electron microscopy (SEM) is a good model for studies about its defense mechanism. The material was fixed (2,5% glutaraldehyde solution; 0.1M-phosphate buffer, pH 7.3), post-fixed (1% osmium tetroxide in the same buffer), dried at critical point, gold coated and analyzed in a SEM 515-Philips. A. gemmatalis digestive tract consists of a straight duct of varying length and diameter, subdivided in three main regions: the foregut formed by the oral cavity, pharynx, esophagus, and crop; the midgut that is the largest portion of the digestive tract
On the Issue of the \zeta Series Convergence and Loop Corrections in the Generation of Observable Primordial Non-Gaussianity in Slow-Roll Inflation. Part II: the Trispectrum
We calculate the trispectrum T_\zeta of the primordial curvature perturbation
\zeta, generated during a {\it slow-roll} inflationary epoch by considering a
two-field quadratic model of inflation with {\it canonical} kinetic terms. We
consider loop contributions as well as tree level terms, and show that it is
possible to attain very high, {\it including observable}, values for the level
of non-gaussianity \tau_{NL} if T_\zeta is dominated by the one-loop
contribution. Special attention is paid to the claim in JCAP {\bf 0902}, 017
(2009) [arXiv:0812.0807 [astro-ph]] that, in the model studied in this paper
and for the specific inflationary trajectory we choose, the quantum
fluctuations of the fields overwhelm the classical evolution. We argue that
such a claim actually does not apply to our model, although more research is
needed in order to understand the role of quantum diffusion. We also consider
the probability that an observer in an ensemble of realizations of the density
field sees a non-gaussian distribution. In that respect, we show that the
probability associated to the chosen inflationary trajectory is non-negligible.
Finally, the levels of non-gaussianity f_{NL} and \tau_{NL} in the bispectrum
B_\zeta and trispectrum T_\zeta of \zeta, respectively, are also studied for
the case in which \zeta is not generated during inflation.Comment: LaTex File, 27 pages, 8 figures. v2: Previous Section 2 has been
removed. Two new sections (3 and 4) discussing the classicality condition
given by Byrnes, Choi, and Hall, in JCAP 0902, 017 (2009), and the
probability that an observer sees a non-gaussian distribution have been
added. v3: Version accepted for publication in Physical Review
From Mendel to genomics, plant breeding milestones: a review.
Plant breeding has evolved from an intuitive strategy of choosing the best plants in the field to an exciting science of combining genes to improve desirable traits. Our objective with this paper is to review some of the most important discoveries and developments that had an impact on plant breeding. First, we present background information and then the milestones themselves. The topics were grouped into chapters to facilitate comprehension of their contribution to plant breeding. They are discussed within each chapter in a plant-breeding-oriented chronological way. When applicable, the definition, importance and evaluation of the topics are presente
Coherence lifetimes of excitations in an atomic condensate due to the thin spectrum
We study the quantum coherence properties of a finite sized atomic condensate
using a toy-model and the thin spectrum model formalism. The decoherence time
for a condensate in the ground state, nominally taken as a variational symmetry
breaking state, is investigated for both zero and finite temperatures. We also
consider the lifetimes for Bogoliubov quasi-particle excitations, and contrast
them to the observability window determined by the ground state coherence time.
The lifetimes are shown to exhibit a general characteristic dependence on the
temperature, determined by the thin spectrum accompanying the spontaneous
symmetry breaking ground state
Proton fraction in the inner neutron-star crust
Monte Carlo simulations of neutron-rich matter of relevance to the inner
neutron-star crust are performed for a system of A=5,000 nucleons. To determine
the proton fraction in the inner crust, numerical simulations are carried out
for a variety of densities and proton fractions. We conclude---as others have
before us using different techniques---that the proton fraction in the inner
stellar crust is very small. Given that the purported "nuclear pasta" phase in
stellar crusts develops as a consequence of the long-range Coulomb interaction
among protons, we question whether pasta formation is possible in such
proton-poor environments. To answer this question, we search for physical
observables sensitive to the transition between spherical nuclei and exotic
pasta structures. Of particular relevance is the static structure factor
S(k)---an observable sensitive to density fluctuations. However, no dramatic
behavior was observed in S(k). We regard the identification of physical
observables sensitive to the existence---or lack-thereof---of a pasta phase in
proton-poor environments as an open problem of critical importance.Comment: 24 pages and 7 figure
Effective Widths and Effective Number of Phonons of Multiphonon Giant Resonances
We discuss the origin of the difference between the harmonic value of the
width of the multiphonon giant resonances and the smaller observed value.
Analytical expressions are derived for both the effective width and the average
cross-section. The contribution of the Brink-Axel mechanism in resolving the
discrepancy is pointed out.Comment: 9 pages, 4 figure
- …