45,981 research outputs found
Impact of radiative corrections on sterile neutrino scenarios
In sterile neutrino scenarios, radiative corrections induce mass splittings
proportional to the top Yukawa coupling, in contrast to the three active
neutrino case where the induced splittings are proportional to the tau Yukawa
coupling. In view of this, we have analyzed the stability of the four-neutrino
schemes favored by oscillation experiments, consisting in two pairs of nearly
degenerate neutrinos separated by the LSND gap. Requiring compatibility with
the measurements of the abundances of primordial elements produced in Big Bang
Nucleosynthesis, we find that when the heaviest pair corresponds to the solar
neutrinos (mainly an admixture of nu_e - nu_s) the natural mass splitting is
3-5 orders of magnitude larger than the observed one, discrediting the scenario
from a theoretical point of view. On the contrary, the scheme where the
heaviest pair corresponds to the atmospheric neutrinos (mainly an admixture of
nu_mu - nu_tau) is safe from radiative corrections due to the small sterile
component of these mass eigenstates.Comment: 14 pages, LaTeX, 2 figures. Discussion enlarged, references added and
typos correcte
Convergence of Scalar-Tensor theories toward General Relativity and Primordial Nucleosynthesis
In this paper, we analyze the conditions for convergence toward General
Relativity of scalar-tensor gravity theories defined by an arbitrary coupling
function (in the Einstein frame). We show that, in general, the
evolution of the scalar field is governed by two opposite mechanisms:
an attraction mechanism which tends to drive scalar-tensor models toward
Einstein's theory, and a repulsion mechanism which has the contrary effect. The
attraction mechanism dominates the recent epochs of the universe evolution if,
and only if, the scalar field and its derivative satisfy certain boundary
conditions. Since these conditions for convergence toward general relativity
depend on the particular scalar-tensor theory used to describe the universe
evolution, the nucleosynthesis bounds on the present value of the coupling
function, , strongly differ from some theories to others. For
example, in theories defined by analytical
estimates lead to very stringent nucleosynthesis bounds on
(). By contrast, in scalar-tensor theories defined by
much larger limits on () are
found.Comment: 20 Pages, 3 Figures, accepted for publication in Class. and Quantum
Gravit
Mechanisms of Action and Cell Death Associated with Clostridium perfringens Toxins.
Clostridium perfringens uses its large arsenal of protein toxins to produce histotoxic, neurologic and intestinal infections in humans and animals. The major toxins involved in diseases are alpha (CPA), beta (CPB), epsilon (ETX), iota (ITX), enterotoxin (CPE), and necrotic B-like (NetB) toxins. CPA is the main virulence factor involved in gas gangrene in humans, whereas its role in animal diseases is limited and controversial. CPB is responsible for necrotizing enteritis and enterotoxemia, mostly in neonatal individuals of many animal species, including humans. ETX is the main toxin involved in enterotoxemia of sheep and goats. ITX has been implicated in cases of enteritis in rabbits and other animal species; however, its specific role in causing disease has not been proved. CPE is responsible for human food-poisoning and non-foodborne C. perfringens-mediated diarrhea. NetB is the cause of necrotic enteritis in chickens. In most cases, hostâ»toxin interaction starts on the plasma membrane of target cells via specific receptors, resulting in the activation of intracellular pathways with a variety of effects, commonly including cell death. In general, the molecular mechanisms of cell death associated with C. perfringens toxins involve features of apoptosis, necrosis and/or necroptosis
The Vector Curvaton
We analyze a massive vector field with a non-canonical kinetic term in the
action, minimally coupled to gravity, where the mass and kinetic function of
the vector field vary as functions of time during inflation. The vector field
is introduced following the same idea of a scalar curvaton, which must not
affect the inflationary dynamics since its energy density during inflation is
negligible compared to the total energy density in the Universe. Using this
hypothesis, the vector curvaton will be solely responsible for generating the
primordial curvature perturbation \zeta. We have found that the spectra of the
vector field perturbations are scale-invariant in superhorizon scales due to
the suitable choice of the time dependence of the kinetic function and the
effective mass during inflation. The preferred direction, generated by the
vector field, makes the spectrum of \zeta depend on the wavevector, i.e. there
exists statistical anisotropy in \zeta. This is discussed principally in the
case where the mass of the vector field increases with time during inflation,
where it is possible to find a heavy field (M >> H) at the end of inflation,
making the particle production be practically isotropic; thus, the longitudinal
and transverse spectra are nearly the same order which in turn causes that the
statistical anisotropy generated by the vector field is within the
observational bounds.Comment: LaTex file in Aipproc style, 6 pages, no figures. Prepared for the
conference proceedings of the IX Mexican School of the DGFM-SMF: Cosmology
for the XXIst Century. This work is entirely based on Refs. [23-26] and is
the result of Andres A. Navarro's MSc thesi
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