20,698 research outputs found
Gluon saturation and the Froissart bound: a simple approach
At very high energies we expect that the hadronic cross sections satisfy the
Froissart bound, which is a well-established property of the strong
interactions. In this energy regime we also expect the formation of the Color
Glass Condensate, characterized by gluon saturation and a typical momentum
scale: the saturation scale . In this paper we show that if a saturation
window exists between the nonperturbative and perturbative regimes of Quantum
Chromodynamics (QCD), the total cross sections satisfy the Froissart bound.
Furthermore, we show that our approach allows us to describe the high energy
experimental data on total cross sections.Comment: 6 pages, 5 figures. Includes additional figures, discussion and
reference
Controlling entanglement by direct quantum feedback
We discuss the generation of entanglement between electronic states of two
atoms in a cavity using direct quantum feedback schemes. We compare the effects
of different control Hamiltonians and detection processes in the performance of
entanglement production and show that the quantum-jump-based feedback proposed
by us in Phys. Rev. A {\bf 76} 010301(R) (2007) can protect highly entangled
states against decoherence. We provide analytical results that explain the
robustness of jump feedback, and also analyse the perspectives of experimental
implementation by scrutinising the effects of imperfections and approximations
in our model.Comment: 10 pages, 8 figures. To appear in PR
Avaliação de cultivares de Coffea arabica L. ao parasitismo de Meloidogyne exigua.
Os nematóides podem causar grandes danos em cafezais no Brasil, dependendo da espécie e do tipo de solo. A espécie Meloidogyne exigua é a mais disseminada nas regiões cafeeiras, especialmente em lavouras mais velhas do Sul de Minas Gerais. As áreas com nematóides mostram redução tanto no desenvolvimento das plantas como na produção. Dentre as medidas de controle do nematóide, a que tem-se mostrado mais adequada é o uso de cultivares resistentes. O objetivo deste ensaio foi avaliar o comportamento de diferentes cultivares de Coffea arabica L. quando inoculados com M. exigua, visando a identificação de fontes de resistência. O ensaio foi instalado no setor de Nematologia da UFLA /Lavras-MG, em casa de vegetação, sob delineamento experimental de blocos ao acaso, com quatro repetições e três plantas por parcela. Foram avaliadas 22 cultivares, utilizando-se a cultivar Acaiá 474/19 e IAPAR 59, como padrões de suscetibilidade e de resistência, respsctivamente. Cada planta foi inoculada 80 dias após transplantio com 5000 ovos de M. exigua e avaliadas 40 e 120 dias após inoculação quanto ao número de galhas por grama de raiz. A cultivar IAPAR 59 confirmou ser imune, já que nenhuma galha foi observada em seu sistema radicular. As cultivares Catucaà Vermelho 785/15 e Acauã apresentaram segregação sendo algumas plantas resistentes e outras de reduzida susceptibilidade. As cultivares Catucaà Vermelho 36/6, Siriema, Soledade, Bem-te-vi Amarelo, Catucaà Vermelho 20/15- cv 395, Catucaà Amarelo 3-5, Cultivar oriunda de Franca, Catucaà Vermelho 36/6- cv 470, Catucaà Amarelo 24/137, Sabiá Tardio, Catucaà Amarelo 2SL- cv 446, Catucaà Amarelo 3 SM, Catucaà Amarelo 20/15- 479, Icatu IAC 2944, IBC-Palma II, Canário, Catucaà Açu Vermelho, IBC-Palma I e Acaiá-IAC 474/19 foram classificadas como susceptÃveis, embora tivessem apresentado na primeira avaliação baixos Ãndices de infestação, quando comparadas a cultivar Acaiá-IAC 474/19, o que indica a possibilidade de diferentes nÃveis de susceptibilidade entre as mesmas
On topological spin excitations on a rigid torus
We study Heisenberg model of classical spins lying on the toroidal support,
whose internal and external radii are and , respectively. The isotropic
regime is characterized by a fractional soliton solution. Whenever the torus
size is very large, , its charge equals unity and the soliton
effectively lies on an infinite cylinder. However, for R=0 the spherical
geometry is recovered and we obtain that configuration and energy of a soliton
lying on a sphere. Vortex-like configurations are also supported: in a ring
torus () such excitations present no core where energy could blow up. At
the limit we are effectively describing it on an infinite
cylinder, where the spins appear to be practically parallel to each other,
yielding no net energy. On the other hand, in a horn torus () a singular
core takes place, while for (spindle torus) two such singularities
appear. If is further diminished until vanish we recover vortex
configuration on a sphere.Comment: 11 pages, 9 figure
Stellar equilibrium configurations of white dwarfs in the gravity
In this work we investigate the equilibrium configurations of white dwarfs in
a modified gravity theory, na\-mely, gravity, for which and
stand for the Ricci scalar and trace of the energy-momentum tensor,
respectively. Considering the functional form , with
being a constant, we obtain the hydrostatic equilibrium equation for
the theory. Some physical properties of white dwarfs, such as: mass, radius,
pressure and energy density, as well as their dependence on the parameter
are derived. More massive and larger white dwarfs are found for
negative values of when it decreases. The equilibrium configurations
predict a maximum mass limit for white dwarfs slightly above the Chandrasekhar
limit, with larger radii and lower central densities when compared to standard
gravity outcomes. The most important effect of theory for massive
white dwarfs is the increase of the radius in comparison with GR and also
results. By comparing our results with some observational data of
massive white dwarfs we also find a lower limit for , namely, .Comment: To be published in EPJ
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