3,848 research outputs found
Homotheties and topology of tangent sphere bundles
We prove a Theorem on homotheties between two given tangent sphere bundles
of a Riemannian manifold of , assuming different
variable radius functions and weighted Sasaki metrics induced by the
conformal class of . New examples are shown of manifolds with constant
positive or with constant negative scalar curvature, which are not Einstein.
Recalling results on the associated almost complex structure and
symplectic structure on the manifold , generalizing the
well-known structure of Sasaki by admitting weights and connections with
torsion, we compute the Chern and the Stiefel-Whitney characteristic classes of
the manifolds and .Comment: 15 pages, to appear in Journal of Geometr
Weak interactions and quasi-stable particle energy loss
We discuss the interplay between electromagnetic energy loss and weak
interactions in the context of quasistable particle particle propagation
through materials. As specific examples, we consider staus, where weak
interactions may play a role, and taus, where they don't.Comment: 4 pages, 4 figures, to appear in the proceedings of the Second
Workshop on TeV Particle Astrophysics (August 2006, Madison, WI
Effective Spin Couplings in the Mott Insulator of the Honeycomb Lattice Hubbard Model
Motivated by the recent discovery of a spin liquid phase for the Hubbard
model on the honeycomb lattice at half-filling, we apply both perturbative and
non-perturbative techniques to derive effective spin Hamiltonians describing
the low-energy physics of the Mott-insulating phase of the system. Exact
diagonalizations of the so-derived models on small clusters are performed, in
order to assess the quality of the effective low-energy theory in the
spin-liquid regime. We show that six-spin interactions on the elementary loop
of the honeycomb lattice are the dominant sub-leading effective couplings. A
minimal spin model is shown to reproduce most of the energetic properties of
the Hubbard model on the honeycomb lattice in its spin-liquid phase.
Surprisingly, a more elaborate effective low-energy spin model obtained by a
systematic graph expansion rather disagrees beyond a certain point with the
numerical results for the Hubbard model at intermediate couplings.Comment: 20 pages, 10 figure
Neutrino Telescopes' Sensitivity to Dark Matter
The nature of the dark matter of the Universe is yet unknown and most likely
is connected with new physics. The search for its composition is under way
through direct and indirect detection. Fundamental physical aspects such as
energy threshold, geometry and location are taken into account to investigate
proposed neutrino telescopes of km^3 volume sensitivities to dark matter. These
sensitivities are just sufficient to test a few WIMP scenarios. Telescopes of
km^3 volume, such as IceCube, can definitely discover or exclude superheavy (M
> 10^10 GeV) Strong Interacting Massive Particles (Simpzillas). Smaller
neutrino telescopes such as ANTARES, AMANDA-II and NESTOR can probe a large
region of the Simpzilla parameter space.Comment: 28 pages, 9 figure
Astrophysical Neutrino Event Rates and Sensitivity for Neutrino Telescopes
Spectacular processes in astrophysical sites produce high-energy cosmic rays
which are further accelerated by Fermi-shocks into a power-law spectrum. These,
in passing through radiation fields and matter, produce neutrinos. Neutrino
telescopes are designed with large detection volumes to observe such
astrophysical sources. A large volume is necessary because the fluxes and
cross-sections are small. We estimate various telescopes' sensitivities and
expected event rates from astrophysical sources of high-energy neutrinos. We
find that an ideal detector of km^2 incident area can be sensitive to a flux of
neutrinos integrated over energy from 10^5 and 10^{7} GeV as low as 1.3 *
10^(-8) * E^(-2) (GeV/cm^2 s sr) which is three times smaller than the
Waxman-Bachall conservative upper limit on potential neutrino flux. A real
detector will have degraded performance. Detection from known point sources is
possible but unlikely unless there is prior knowledge of the source location
and neutrino arrival time.Comment: Section added +modification
Magnetic Properties of the Metamagnet Ising Model in a three-dimensional Lattice in a Random and Uniform Field
By employing the Monte Carlo technique we study the behavior of Metamagnet
Ising Model in a random field. The phase diagram is obtained by using the
algorithm of Glaubr in a cubic lattice of linear size with values ranging
from 16 to 42 and with periodic boundary conditions.Comment: 4 pages, 6 figure
Yang-Lee Zeros of the Two- and Three-State Potts Model Defined on Feynman Diagrams
We present both analytic and numerical results on the position of the
partition function zeros on the complex magnetic field plane of the
(Ising) and states Potts model defined on Feynman diagrams
(thin random graphs). Our analytic results are based on the ideas of
destructive interference of coexisting phases and low temperature expansions.
For the case of the Ising model an argument based on a symmetry of the saddle
point equations leads us to a nonperturbative proof that the Yang-Lee zeros are
located on the unit circle, although no circle theorem is known in this case of
random graphs. For the states Potts model our perturbative results
indicate that the Yang-Lee zeros lie outside the unit circle. Both analytic
results are confirmed by finite lattice numerical calculations.Comment: 16 pages, 2 figures. Third version: the title was slightly changed.
To be published in Physical Review
New diseases affecting black pepper crop in Brazil.
Paper presented in the 26th Peppertech Meeting of IPC, held at Belém, Pará, Brazil, 30th Oct. 2001
Enhanced signal of astrophysical tau neutrinos propagating through Earth
Earth absorbs \nue and \numu of energies above about 100 TeV. As is
well-known, although \nutau will also disappear through charged-current
interactions, the \nutau flux will be regenerated by prompt tau decays. We
show that this process also produces relatively large fluxes of secondary
\nube and \nubmu, greatly enhancing the detectability of the initial
\nutau. This is particularly important because at these energies \nutau is
a significant fraction of the expected astrophysical neutrino flux, and only a
tiny portion of the atmospheric neutrino flux.Comment: Four pages, two inline figure
Avaliação da inoculação de fungos micorrízicos arbusculares sobre a incidência da fusariose da pimenta-do-reino.
A fusariose (Fusarlum solani f. sp. piperis) da pimenta-do-reino (Piper nigrum) é a doença mais devastadora à cultura na região amazônica. As medidas de controle preconizadas são dispendiosas e não apresentam controle adequado da doença, sendo necessário estudos de novos métodos de controle. Objetiva-se avaliar a possibilidade de reduzir a incidência da fusariose, através da inoculação com fungos micorrízicos arbusculares (FMAs). Foi realizado um experimento com plântulas de pimenta-do-reino, cv. Guajarina, em casa de plástico. Utilizaram-se quatro espécies de FMA: Scutellospora sp., S_ heterogama, S. gilmorei e Entropliospora colombiana. A pré-inoculação foi feita pela deposição de solo-inóculo em contato direto com as raízes das plântulas, na fase cotiledônea, em copos de 500 mJ, contendo solo fumigado. Após três meses e meio, as plântulas foram transferidas para vasos de plásticos de quatro quilos, contendo mistura de solo fumigado e inóculo de F. solani f. sp. piperis na proporção de 0,2% (v/v). Durante o transplantio, foi feita urna reinoculação de FMA, colocando solo-inoculo das mesmas espécies no fundo do vaso de plástico. A incidência da doença foi avaliada quatro meses após o transplantio. A incidência da fusariose foi de 5%,10%,15%,35% e 85% para os tratamentos de S. gilmorei, E. colombiana, S. heterogama, Scutellospora sp. e a testemunha, respectivamente. A inoculação com FMA reduziu de 50 a 80% a incidência da doença, em relação à testemunha, sendo que S. gilmorei apresentou o maior potencial para redução da doença (80%), maior produção de matéria seca das plantas sobreviventes (11,7 g por planta) e maior percentagem do comprimento das raízes colonizadas pelo FMA (55,18%). Pelos resultados obtidos, há perspectiva de se incorporar o uso de FMA num programa de controle da fusariose de pimenta-do-reino
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