63,612 research outputs found
Ricci dark energy in Chern-Simons modified gravity
In this work, we have considered the Ricci dark energy model, where the
energy density of the universe is proportional to the Ricci scalar curvature,
in the dynamic Chern-Simons modified gravity. We show that in this context the
evolution of the scale factor is similar to that displayed by the modified
Chaplygin gas.Comment: 7 pages; to appear in EPJ
On FRW Model in Conformal Teleparallel Gravity
In this paper we use the conformal teleparallel gravity to study an isotropic
and homogeneous Universe which is settled by the FRW metric. We solve the field
equations and we obtain the behavior of some cosmological parameters such as
scale factor, deceleration parameter and the energy density of the perfect
fluid which is the matter field of our model. The field equations, that we
called modified Friedmann equations, allow us to define a dark fluid, with dark
energy density and dark pressure, responsible for the acceleration in the
Universe.Comment: Accepted in EPJ
What if the Masses of the First Two Quark Families are not Generated by the Standard Higgs?
We point out that, in the context of the SM, is
expected to be large, of order one. The fact that motivates the introduction of a symmetry S which
leads to , with only the third generation of
quarks acquiring mass. We consider two scenarios for generating the mass of the
first two quark generations and full quark mixing. One consists of the
introduction of a second Higgs doublet which is neutral under S. The second
scenario consists of assuming New Physics at a high energy scale , contributing
to the masses of light quark generations, in an effective field theory
approach. This last scenario leads to couplings of the Higgs particle to
and which are significantly enhanced with
respect to those of the SM. In both schemes, one has scalar-mediated flavour-
changing neutral currents which are naturally suppressed. Flavour violating top
decays are predicted in the second scenario at the level \mbox{Br} (t
\rightarrow h c ) \geq 5\times 10^{-5}.Comment: 11 pages, 1 figur
Transport Processes in Metal-Insulator Granular Layers
Tunnel transport processes are considered in a square lattice of metallic
nanogranules embedded into insulating host to model tunnel conduction in real
metal/insulator granular layers. Based on a simple model with three possible
charging states (, or 0) of a granule and three kinetic processes
(creation or recombination of a pair, and charge transfer) between
neighbor granules, the mean-field kinetic theory is developed. It describes the
interplay between charging energy and temperature and between the applied
electric field and the Coulomb fields by the non-compensated charge density.
The resulting charge and current distributions are found to be essentially
different in the free area (FA), between the metallic contacts, or in the
contact areas (CA), beneath those contacts. Thus, the steady state dc transport
is only compatible with zero charge density and ohmic resistivity in FA, but
charge accumulation and non-ohmic behavior are \emph{necessary} for conduction
over CA. The approximate analytic solutions are obtained for characteristic
regimes (low or high charge density) of such conduction. The comparison is done
with the measurement data on tunnel transport in related experimental systems.Comment: 10 pages, 11 figures, 1 reference corrected, acknowlegments adde
Gauge fields in a string-cigar braneworld
In this work we investigate the properties of an Abelian gauge vector field
in a thin and in a smoothed string-like braneworld, the so-called string-cigar
model. This thick brane scenario satisfies the regularity conditions and it can
be regarded as an interior and exterior string-like solution. The source
undergoes a geometric Ricci flow which is connected to a variation of the bulk
cosmological constant. The Ricci flow changes the width and amplitude of the
massless mode at the brane core and recover the usual thin string-like behavior
at large distances. By numerical means we obtain the Kaluza-Klein (KK) spectrum
for both the thin brane and the string-cigar. It turns out that both models
exhibit a mass gap between the massless and the massive modes and between the
high and the low mass regimes. The KK modes are smooth near the brane and their
amplitude are enhanced by the string-cigar core. The analogue Schr\"odinger
potential is also tuned by the geometric flow.Comment: The discussion about the Kaluza-Klein spectrum of the gauge field was
improved. Numerical analysis was adapted to the conventional notation on
Kaluza-Klein number. Some graphics were modified for considering other
notation. Results unchanged. References added. Corrected typos. 17 pages. 6
figures. To match version to appears in Physics Letters
Avaliação da produtividade das ordens primária, secundária e terciária de genótipos de mamoneira de porte baixo em Irecê-BA.
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