1,741 research outputs found
Two-Photon Total Annihilation of Molecular Positronium
The rate for complete two-photon annihilation of molecular positronium
Ps is reported. This decay channel involves a four-body collision among
the fermions forming Ps, and two photons of 1.022 MeV, each, as the final
state. The quantum electrodynamics result for the rate of this process is found
to be = 9.0
s. This decay channel completes the most comprehensive decay chart for
Ps up to date.Comment: 4 pages, 2figures and 1 tabl
Characterizing lab instructors' self-reported learning goals to inform development of an experimental modeling skills assessment
The ability to develop, use, and refine models of experimental systems is a
nationally recognized learning outcome for undergraduate physics lab courses.
However, no assessments of students' model-based reasoning exist for
upper-division labs. This study is the first step toward development of
modeling assessments for optics and electronics labs. In order to identify test
objectives that are likely relevant across many institutional contexts, we
interviewed 35 lab instructors about the ways they incorporate modeling in
their course learning goals and activities. The study design was informed by
the Modeling Framework for Experimental Physics. This framework conceptualizes
modeling as consisting of multiple subtasks: making measurements, constructing
system models, comparing data to predictions, proposing causes for
discrepancies, and enacting revisions to models or apparatus. We found that
each modeling subtask was identified by multiple instructors as an important
learning outcome for their course. Based on these results, we argue that test
objectives should include probing students' competence with most modeling
subtasks, and test items should be designed to elicit students' justifications
for choosing particular modeling pathways. In addition to discussing these and
other implications for assessment, we also identify future areas of research
related to the role of modeling in optics and electronics labs.Comment: 24 pages, 2 figures, 5 tables; submitted to Phys. Rev. PE
Why metallic surfaces with grooves a few nanometers deep and wide may strongly absorb visible light
It is theoretically shown that nanometric silver lamellar gratings present
very strong visible light absorption inside the grooves, leading to electric
field intensities by several orders of magnitude larger than that of the
impinging light. This effect, due to the excitation of long wave vector surface
plasmon polaritons with particular small penetration depth in the metal, may
explain the abnormal optical absorption observed a long time ago on almost flat
Ag films. Surface enhanced Raman scattering in rough metallic films could also
be due to the excitation of such plasmon polaritons in the grain boundaries or
notches of the films.Comment: 5 pages, 5 figure, submitted to Phys. Rev. Let
Efficient excitation of cavity resonances of subwavelength metallic gratings
One dimensional rectangular metallic gratings enable enhanced transmission of
light for specific resonance frequencies. Two kinds of modes participating to
enhanced transmission have already been demonstrated : (i) waveguide modes and
(ii) surface plasmon polaritons (SPP). Since the original paper of Hessel and
Oliner \cite{hessel} pointing out the existence of (i), no progress was made in
their understanding. We present here a carefull analysis, and show that the
coupling between the light and such resonances can be tremendously improved
using an {\it evanescent} wave. This leads to enhanced localisation of light in
cavities, yielding, in particular, to a very selective light transmission
through these gratings.Comment: 4 pages, 5 figures, submitted to Phys. Rev. Let
Intrinsic point defects and volume swelling in ZrSiO4 under irradiation
The effects of high concentration of point defects in crystalline ZrSiO4 as
originated by exposure to radiation, have been simulated using first principles
density functional calculations. Structural relaxation and vibrational studies
were performed for a catalogue of intrinsic point defects, with different
charge states and concentrations. The experimental evidence of a large
anisotropic volume swelling in natural and artificially irradiated samples is
used to select the subset of defects that give similar lattice swelling for the
concentrations studied, namely interstitials of O and Si, and the anti-site
Zr(Si), Calculated vibrational spectra for the interstitials show additional
evidence for the presence of high concentrations of some of these defects in
irradiated zircon.Comment: 9 pages, 7 (color) figure
Lifetimes of ultralong-range strontium Rydberg molecules in a dense BEC
The lifetimes and decay channels of ultralong-range Rydberg molecules created
in a dense BEC are examined by monitoring the time evolution of the Rydberg
population using field ionization. Studies of molecules with values of
principal quantum number, , in the range to that contain tens
to hundreds of ground state atoms within the Rydberg electron orbit show that
their presence leads to marked changes in the field ionization characteristics.
The Rydberg molecules have lifetimes of s, their destruction
being attributed to two main processes: formation of Sr ions through
associative ionization, and dissociation induced through -changing
collisions. The observed loss rates are consistent with a reaction model that
emphasizes the interaction between the Rydberg core ion and its nearest
neighbor ground-state atom. The measured lifetimes place strict limits on the
time scales over which studies involving Rydberg species in cold, dense atomic
gases can be undertaken and limit the coherence times for such measurements.Comment: 9 pages, 8 figure
Principales marcadores moleculares utilizados para la identificación de Babesia bovis y Babesia bigemina
This paper describes the principal molecular markers used to identify B. bovis and B. bigemina reported in the scientific literature. A systematic review was designed based on the application of the PICO methodologic modified strategy to define the nucleotide sequences detected in different geographical locations and their diagnostic utility. An advanced search was made in data bases ScienceDirect, SpringerLink and PubMed. Using the terms “Babesia bovis” and “DNA” and “Babesia bigemina” and “DNA”. A total of 68 original articles were selected after the information was filtered. Both included and excluded articles were registered in tables, where its position of their status, within the study, was represented. The 68 articles were evaluated using a previously defined criteria of inclusion or exclusion. A total of 21 articles met the inclusion criteria and were included in this study. It was described the usefulness of molecular markers referenced in the scientific literature from 1995 to 2010: small-subunit ribosomal rna, cytochrome b gene, msa-1 and msa-2c gene, bv gene, elongation factor –1 alpha (ef-1α), beta-tubulin gene, sbp 1-2-3, and rap genes, its diagnostic application and its use in different geographical locations. Molecular markers used for detection of bovine babesia vary by geographic region, degree of genetic conservation and results of previous studies that conclude their diagnostic utility. Este artículo describe los principales marcadores moleculares utilizados para la identificación de B. bovis y B. bigemina reportados en la literatura científica. Para ello se diseñó una revisión sistemática a partir de la aplicación de la estrategia metodológica PICO modificada con el objetivo de definir las secuencias nucleotídicas detectadas en los diferentes sitios geográficos y su utilidad diagnóstica. Se realizó una búsqueda avanzada con los términos “Babesia bovis” y “DNA” y “Babesia bigemina” y “DNA” en las bases de datos ScienceDirect, SpringerLink y PubMed que después de ser filtradas permitieron obtener un resultado total de 68 artículos originales. Tanto los artículos incluidos como los excluidos fueron almacenados en tablas, en las cuales se presenta la justificación de su condición dentro del estudio. A los 68 artículos seleccionados se les aplicó una evaluación con criterios de inclusión y exclusión previamente definidos, de este modo, 21 artículos originales cumplieron con los criterios de inclusión y se incluyeron en el estudio. Se describe la utilidad de los marcadores moleculares referenciados en la literatura científica desde 1995 hasta el 2010: la subunidad pequeña RNAr, el gen citocromo b, gen msa-1 and msa-2c, el gen Bv, el factor de elongación alfa (EF-1α), el gen de la beta-Tubulina, SBP 1-2-3, y los RAP; su aplicación diagnóstica y su utilización en los diferentes sitios geográficos. Los marcadores moleculares utilizados para la detección de las babesias bovinas varían dependiendo de la región geográfica, grado de conservación genética y resultados de estudios previos que concluyen su utilidad diagnóstica
Nature of the f_0(600) from its N_c dependence at two loops in unitarized Chiral Perturbation Theory
By using unitarized two-loop Chiral Perturbation Theory partial waves to
describe pion-pion scattering we find that the dominant component of the
lightest scalar meson does not follow the q-qbar dependence on the number of
colors that, in contrast, is obeyed by the lightest vectors. The method
suggests that a subdominant q-qbar component of the f_0(600) possibly
originates around 1 GeV.Comment: 4 pages, 1 Figure. To appear in Phys. Rev. Let
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