15,551 research outputs found
Probing the gluon density of the proton in the exclusive photoproduction of vector mesons at the LHC: A phenomenological analysis
The current uncertainty on the gluon density extracted from the global parton
analysis is large in the kinematical range of small values of the Bjorken -
variable and low values of the hard scale . An alternative to reduces this
uncertainty is the analysis of the exclusive vector meson photoproduction in
photon - hadron and hadron - hadron collisions. This process offers a unique
opportunity to constrain the gluon density of the proton, since its cross
section is proportional to the gluon density squared. In this paper we consider
current parametrizations for the gluon distribution and estimate the exclusive
vector meson photoproduction cross section at HERA and LHC using the leading
logarithmic formalism. We perform a fit of the normalization of the
cross section and the value of the hard scale for the process and demonstrate
that the current LHCb experimental data are better described by models that
assume a slow increasing of the gluon distribution at small - and low
.Comment: 8 pages, 6 figures, 1 table. Version published in European Physical
Journal
Phase Transition in a Stochastic Forest Fire Model and Effects of the Definition of Neighbourhood
We present results on a stochastic forest fire model, where the influence of
the neighbour trees is treated in a more realistic way than usual and the
definition of neighbourhood can be tuned by an additional parameter.
This model exhibits a surprisingly sharp phase transition which can be
shifted by redefinition of neighbourhood. The results can also be interpreted
in terms of disease-spreading and are quite unsettling from the epidemologist's
point of view, since variation of one crucial parameter only by a few percent
can result in the change from endemic to epidemic behaviour.Comment: 23 pages, 13 figure
Índices de frutificação em variedades de Malpighia emarginata DC. (Malpighiaceae), em cultivo irrigado, no Vale do Submédio São Francisco.
Predatory lizards perceive plant-derived volatile odorants
Many lizards are olfactory foragers and prey upon herbivorous arthropods, yet their responses to common herbivore-associated plant volatiles remain unknown. As such, their role in mediating plant indirect defenses also remains largely obscured. In this paper, we use a cotton-swab odor presentation assay to ask whether lizards respond to two arthropod-associated plant-derived volatile compounds: 2-(E)-hexenal and hexanoic acid. We studied the response of two lizard species, Sceloporus virgatusand Aspidoscelis exsanguis, because they differ substantially in their foraging behavior. We found that the actively foraging A. exsanguisresponded strongly to hexanoic acid, whereas the ambush foraging S. virgatus responded to 2-(E)-hexenal—an herbivore-associated plant volatile involved in indirect defense against herbivores. These findings indicate that S. virgatus may contribute to plant indirect defense and that a species\u27 response to specific odorants is linked with foraging mode. Future studies can elucidate how lizards use various compounds to locate prey and how these responses impact plant-herbivore interactions
Consequências genéticas da regeneração natural de espécies arbóreas em área antrópica, AC, Brasil.
O cedro (Cedrela odorata L.) e o ipê-amarelo (Tabebuia serratifolia Nichols.) são espécies arbóreas tropicais economicamente valiosas e que têm sido ameaçadas pela exploração madeireira predatória e pela fragmentação florestal. Ambas apresentam dispersão anemocórica e regeneram naturalmente em áreas de pastagem. Esse estudo comparou, para as duas espécies, a diversidade genética de indivíduos estabelecidos em pastagem e em floresta. Trinta indivíduos de ipê-amarelo foram genotipados com cinco locos isoenzimáticos e 54 de cedro, com quatro locos microssatélites. A diversidade genética foi elevada nas duas subpopulações. Para ipê-amarelo, a diversidade genética foi maior na pastagem. Para cedro, observou-se perda de alelos na pastagem (Â = 11,75 alelos/loco) em comparação à floresta (Â = 14,50). Além disso, 31% dos alelos de cedro foram exclusivos da floresta. Não houve divergência genética entre as subpopulações de ipê-amarelo, porém, para cedro, houve divergência significativa, embora baixa (2,2%). Os resultados mostraram que, para as duas espécies, a subpopulação da pastagem não passou por um gargalo genético severo. A colonização de áreas antrópicas mostrou-se eficiente, mas há necessidade de fluxo gênico contínuo, por sucessivas gerações, entre as áreas para restabelecer (cedro) e manter (ipê) os níveis de diversidade genética observados na área de vegetação primária
Scientific Performance of the ISDC Quick Look Analysis
The INTEGRAL Science Data Centre (ISDC) routinely monitors the Near Real Time
data (NRT) from the INTEGRAL satellite. A first scientific analysis is made in
order to check for the detection of new, transient or highly variable sources
in the data. Of primary importance for this work is the Interactive Quick Look
Analysis (IQLA), which produces JEM-X and ISGRI images and monitors them for
interesting astrophysical eventsComment: 4 pages, 3 figures. Proceedings of 5th INTEGRAL Workshop: The
INTEGRAL Universe, Munich, 16-20 February 2004. Accepted for publication in
European Space Agency Special Publication 552. See paper for institute
affiliation
Mating system and fine-scale spatial genetic structure of Solanum lycocarpum St. Hil. (Solanaceae) in the Brazilian cerrado.
METHODOLOGY FOR AUTOMOTIVE AIR-CONDITIONING CONTROL OPTIMIZATION USING ARTIFICIAL NEURAL NETWORKS
The transient nature of automotive air conditioning systems control is generally achieved through proportional–integral–derivative controllers (PID’s) parameters tunning. Due to the vast database available from decades of automotive manufacturers design and expertise, Artificial Neural Networks (ANN) might be able to identify underlying patterns to predict and properly tune the air-conditioning PID control systems under different thermal requirements. Recently, advances in computational capability have enabled compact embarked systems to rapidly solve complex, multi-variable sets of equations, thus allowing for ANN to promptly calculate tunning parameters and act upon PID controllers. As any new application, technical literature is still scarce. On this research, a coupled PID and 6-layers perceptron ANN system was devised, programmed and used to simulate how an air-conditioning system performance can be optimized through proportional–integral–derivative controllers tuning. This proposed setup response was then compared to a conventional heuristic PID tunning method (Ziegler Nichols) to demonstrate how ANN’s can more rapidly stabilize the system output
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