180 research outputs found
Current Algebra in the Path Integral framework
In this letter we describe an approach to the current algebra based in the
Path Integral formalism. We use this method for abelian and non-abelian quantum
field theories in 1+1 and 2+1 dimensions and the correct expressions are
obtained. Our results show the independence of the regularization of the
current algebras.Comment: 8 pages, Revtex. One reference added. To appear in Mod. Phys. Lett.
A, Vol. 13, No. 27 (1998
Antagonistic effect of Trichoderma harzianum VSL291 on phytopathogenic fungi isolated from cocoa (Theobroma cacao L.) fruits
In this study we evaluated the antagonism in vitro of Trichoderma harzianum strain VSL291 against 18 pathogens of cocoa fruits in dual culture. T. harzianum VSL291 inhibited the growth of the phytopathogenic fungi tested between 10.54 and 85.43%. The mycoparasitism of Moniliophthora roreri by T. harzianum VSL291 was studied by light and scanning electron microscopy. T. harzianum VSL291 hyphae grew in parallel with the hyphae of M. roreri and in some places these were united with the hyphae of the cocoa pathogen through small structures like apresorious that tangled in the pathogenic fungus preventing its growth. T. harzianum VSL291 produced lytic enzymes: ÎČ-1,3-glucanases, chitinases, proteases, xylanases and lipases, when grown in minimal medium, with fungal cell walls as the sole carbon source. The highest proteolytic activities detected in T. harzianum VSL291 broth with M. roreri, Penicillium expansum and Byssochlamys spectabilis cell walls appear to be associated with increased activities of ÎČ-1,3 glucanases, chitinases, lipases, proteases and xylanases and biocontrol index derived from the experiments of confrontation. These results suggest that proteolytic enzymes according to their degree of induction could participate in the antagonistic effect of T. harzianum VSL291 against the fungi tested.Key words: Antagonism, Trichoderma harzianum, mycoparasitism, phytopathogenic fung
Scattering of scalar particles by a black hole
The absorption cross section for scalar particle impact on a Schwarzschild
black hole is found. The process is dominated by two physical phenomena. One of
them is the well-known greybody factor that arises from the energy-dependent
potential barrier outside the horizon that filters the incoming and outgoing
waves. The other is related to the reflection of particles on the horizon
(Kuchiev 2003). This latter effect strongly diminishes the cross section for
low energies, forcing it to vanish in the infrared limit. It is argued that
this is a general property, the absorption cross section vanishes in the
infrared limit for scattering of particles of arbitrary spin.Comment: 7 pages, revtex, 1 figur
Quasinormal Modes of Extremal BTZ Black Hole
Motivated by several pieces of evidence, in order to show that extreme black
holes cannot be obtained as limits of non-extremal black holes, in this article
we calculate explicitly quasinormal modes for Ba\~{n}ados, Teitelboim and
Zanelli (BTZ) extremal black hole and we showed that the imaginary part of the
frequency is zero. We obtain exact result for the scalar an fermionic
perturbations. We also showed that the frequency is bounded from below for the
existence of the normal modes (non-dissipative modes).Comment: 6 pp. Accepted Classical and Quantum Gravity. Typos corrected and
some references was added. Final Versio
The phase transition and the Quasi-Normal Modes of black Holes
We reexamined the argument that the quasinormal modes could be a probe of the
phase transition of a topological black hole to a hairy configuration by
investigating general scalar perturbations. We found further evidence in the
quasinormal modes for this phase transition. For the general black hole
configurations, we observed that although the quasinormal modes can present us
different phases of different configurations, there is no dramatic change in
the slope of quasinormal frequencies at the critical point of the phase
transition. More detailed studies of quasinormal modes are needed to reveal the
subtle behavior of the phase transition.Comment: Revised version, accepted for publication in JHE
Absorption and quasinormal modes of classical fields propagating on 3D and 4D de Sitter spacetime
We extensively study the exact solutions of the massless Dirac equation in 3D
de Sitter spacetime that we published recently. Using the Newman-Penrose
formalism, we find exact solutions of the equations of motion for the massless
classical fields of spin s=1/2,1,2 and to the massive Dirac equation in 4D de
Sitter metric. Employing these solutions, we analyze the absorption by the
cosmological horizon and de Sitter quasinormal modes. We also comment on the
results given by other authors.Comment: 31 page
BTZ Black Hole as Solution of 3d Higher Spin Gauge Theory
BTZ black hole is interpreted as exact solution of 3d higher spin gauge
theory. Solutions for free massless fields in BTZ black hole background are
constructed with the help of the star-product algebra formalism underlying the
formulation of 3d higher spin theory. It is shown that a part of higher spin
symmetries remains unbroken for special values of the BTZ parameters.Comment: 31 pages, LaTeX; references correcte
Reflection, radiation and interference for black holes
Black holes are capable of reflection: there is a finite probability for any
particle that approaches the event horizon to bounce back. The albedo of the
black hole depends on its temperature and the energy of the incoming particle.
The reflection shares its physical origins with the Hawking process of
radiation, both of them arise as consequences of the mixing of the incoming and
outgoing waves that takes place on the event horizon.Comment: 10 pages, 1 figure, Revte
Intermediate Asymptotics of the Kerr Quasinormal Spectrum
We study analytically the quasinormal mode spectrum of near-extremal
(rotating) Kerr black holes. We find an analytic expression for these
black-hole resonances in terms of the black-hole physical parameters: its
Bekenstein-Hawking temperature T_{BH} and its horizon's angular velocity
\Omega, which is valid in the intermediate asymptotic regime
1<<\omega<<1/T_{BH}.Comment: 4 page
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