1,858 research outputs found
Non-unitary representations of the SU(2) algebra in the Dirac equation with a Coulomb potential
A novel realization of the classical SU(2) algebra is introduced for the
Dirac relativistic hydrogen atom defining a set of operators that, besides,
allow the factorization of the problem. An extra phase is needed as a new
variable in order to define the algebra. We take advantage of the operators to
solve the Dirac equation using algebraic methods. To acomplish this, a similar
path to the one used in the angular momentum case is employed; hence, the
radial eigenfuntions calculated comprise non unitary representations of the
algebra. One of the interesting properties of such non unitary representations
is that they are not labeled by integer nor by half-integer numbers as happens
in the usual angular momentum representation.Comment: 20 pages 1 eps figure in a single zipped file, submitted to J. Math.
Phy
Coherent Radiation from Extensive Air Showers in the Ultra-High Frequency Band
Using detailed Monte Carlo simulations we have characterized the features of
the radio emission of inclined air showers in the Ultra-High Frequency band
(300 MHz - 3 GHz). The Fourier-spectrum of the radiation is shown to have a
sizable intensity well into the GHz frequency range. The emission is mainly due
to transverse currents induced by the geomagnetic field and to the excess
charge produced by the Askaryan effect. At these frequencies only a
significantly reduced volume of the shower around the axis contributes
coherently to the signal observed on the ground. The size of the coherently
emitting volume depends on frequency, shower geometry and observer position,
and is interpreted in terms of the relative time delays. At ground level, the
maximum emission at high frequencies is concentrated in an elliptical ring-like
region around the intersection of a Cherenkov cone with its vertex at shower
maximum and the ground. The frequency spectrum of inclined showers when
observed at positions that view shower maximum in the Cherenkov direction, is
shown to be in broad agreement with the pulses detected by the Antarctic
Impulsive Transient Antenna (ANITA) experiment, making the interpretation that
they are due to Ultra-High Energy Cosmic Ray atmospheric showers consistent
with our simulations. These results are also of great importance for
experiments aiming to detect molecular bremsstrahlung radiation in the GHz
range as they present an important background for its detection.Comment: 8 pages, 8 figure
Challenges and perspectives of veterinary public health
This article presents the conceptual basis for veterinary public health-VPH, including historical aspects of its constitution and development, its fields of action, and its current challenges. It also presents a reflection on VPH within the frame of the veterinary services and it finally proposes that education plays a fundamental role in order to face the challenges of a new era.El artículo presenta la conceptualización de la Salud Pública Veterinaria-SPV, involucrando aspectos históricos de su constitución y desarrollo, los campos de acción y los retos actuales. Igualmente presenta una reflexión de la SPV en el marco de la reestructuración de los servicios veterinarios y finalmente plantea como la educación juega un papel determinante para poder enfrentar los retos de una nueva época
Veterinary public health services in developing countries: guidelines to restructuring
The veterinary public health (VPH) services comprises several plans of action and serve the purpose for improving and protected human health and well-being wich is essential for the socioeconomic development of populations. The reestructuring of public sector as offerent of health services and the increase in participation of private sector are evident. This paper presents a review of the reestructuring process, its justification and framework (administration, delivery and financing) in developing countries. The direction, restrictions, and the restricted delivery view and its effects on veterinary public health services are evaluated.Los servicios de salud pública veterinaria, se ofrecen en diferentes planos de acción y constituyen un factor de importancia desde el punto de vista de la alimentación humana y de la prevención de las zoonosis. La reestructuración del sector público proveedor de servicios y la participación creciente del sector privado son evidentes. En este artículo se presenta un revisión del tema de la reestructuración, su justificación y marco para ejecutar el proceso (administración, suministro y financiación), con énfasis en países en vía de desarrollo. Se evalúan las directrices del proceso, las restricciones al seguir un enfoque centrado en la oferta y la especial proyección sobre los servicios de salud publica veterinaria
Cherenkov radio pulses from electromagnetic showers in the time-domain
The electric field of the Cherenkov radio pulse produced by a single charged
particle track in a dielectric medium is derived from first principles. An
algorithm is developed to obtain the pulse in the time domain for numerical
calculations. The algorithm is implemented in a Monte Carlo simulation of
electromagnetic showers in dense media (specifically designed for coherent
radio emission applications) as might be induced by interactions of ultra-high
energy neutrinos. The coherent Cherenkov radio emission produced by such
showers is obtained simultaneously both in the time and frequency domains. A
consistency check performed by Fourier-transforming the pulse in time and
comparing it to the frequency spectrum obtained directly in the simulations
yields, as expected, fully consistent results. The reversal of the time
structure inside the Cherenkov cone and the signs of the corresponding pulses
are addressed in detail. The results, besides testing algorithms used for
reference calculations in the frequency domain, shed new light into the
properties of the radio pulse in the time domain. The shape of the pulse in the
time domain is directly related to the depth development of the excess charge
in the shower and its width to the observation angle with respect to the
Cherenkov direction. This information can be of great practical importance for
interpreting actual data.Comment: 10 pages, 4 figure
Low-temperature electrical transport in bilayer manganite LaSrMnO
The temperature and magnetic field dependence of anisotropic in-plane
and out-of-plane resistivities have been investigated in
single crystals of the bilayer manganite LaSrMnO.
Below the Curie transition temperature 125 K, and
display almost the same temperature dependence with an up-turn around 50 K. In
the metallic regime (50 K 110 K), both and
follow a dependence, consistent with the two-magnon
scattering. We found that the value of the proportionality coefficient
and the ratio of the exchange interaction obtained
by fitting the data are in excellent agreement with the calculated
based on the two-magnon model and deduced from neutron scattering,
respectively. This provides further support for this scattering mechanism. At
even lower , in the non-metallic regime ( 50 K), {\it both} the in-plane
and out-of-plane conductivities obey a
dependence, consistent with weak localization effects. Hence, this demonstrates
the three-dimensional metallic nature of the bilayer manganite
LaSrMnO at .Comment: 7 pages and 5 figures, accepted for publication in Phys. Rev.
Practical and accurate calculations of Askaryan radiation
An in-depth characterization of coherent radio Cherenkov pulses from particle
showers in dense dielectric media, referred to as the Askaryan effect, is
presented. The time-domain calculation developed in this article is based on a
form factor to account for the lateral dimensions of the shower. It is
computationally efficient and able to reproduce the results of detailed
particle shower simulations with high fidelity in most regions of practical
interest including Fresnel effects due to the longitudinal development of the
shower. In addition, an intuitive interpretation of the characteristics of the
Askaryan pulse is provided. We expect our approach to benefit the analysis of
radio pulses in experiments exploiting the radio technique.Comment: Replaced with version published Phys. Rev.
Miniatures from domestic contexts in Iron age Iberia
This article reviews a set of miniatures from domestic contexts in Iron Age eastern Iberia, and interprets them in terms of their role in forging social personae. After an introduction to the historical case under consideration, the miniatures are described in terms of their typology and their contexts of provenance are outlined. Though not abundant, they tend to occur in central places in the landscape; specifically, they are often found in houses of the powerful. The vast majority are miniatures of pottery and tools, though some miniature weapons are recorded. We contend that these objects were used as a means of enculturation and for the learning of values and norms. It is no coincidence that they emerge in the archaeological record of Iron Age Iberia at the same time as the rise of a social structure based on hereditary power
- …