1,280 research outputs found
From attosecond to zeptosecond coherent control of free-electron wave functions using semi-infinite light fields
Light-electron interaction in empty space is the seminal ingredient for
free-electron lasers and also for controlling electron beams to dynamically
investigate materials and molecules. Pushing the coherent control of free
electrons by light to unexplored timescales, below the attosecond, would enable
unprecedented applications in light-assisted electron quantum circuits and
diagnostics at extremely small timescales, such as those governing
intramolecular electronic motion and nuclear phenomena. We experimentally
demonstrate attosecond coherent manipulation of the electron wave function in a
transmission electron microscope, and show that it can be pushed down to the
zeptosecond regime with existing technology. We make a relativistic pulsed
electron beam interact in free space with an appropriately synthesized
semi-infinite light field generated by two femtosecond laser pulses reflected
at the surface of a mirror and delayed by fractions of the optical cycle. The
amplitude and phase of the resulting coherent oscillations of the electron
states in energymomentum space are mapped via momentum-resolved ultrafast
electron energy-loss spectroscopy. The experimental results are in full
agreement with our theoretical framework for light-electron interaction, which
predicts access to the zeptosecond timescale by combining semi-infinite X-ray
fields with free electrons.Comment: 22 pages, 6 figure
Dark Matter detection via lepton cosmic rays
Recent observations of lepton cosmic rays, coming from the PAMELA and FERMI
experiments, have pushed our understanding of the interstellar medium and
cosmic rays sources to unprecedented levels. The imprint of dark matter on
lepton cosmic rays is the most exciting explanation of both PAMELA's positron
excess and FERMI's total flux of electrons. Alternatively, supernovae are
astrophysical objects with the same potential to explain these observations. In
this work, we present an updated study of the astrophysical sources of lepton
cosmic rays and the possible trace of a dark matter signal on the positron
excess and total flux of electrons.Comment: 6 pages and 3 figures. Proceedings for PASCOS 2010, Valencia, Spai
The Society for Implementation Research Collaboration Instrument Review Project: A methodology to promote rigorous evaluation
Abstract Background Identification of psychometrically strong instruments for the field of implementation science is a high priority underscored in a recent National Institutes of Health working meeting (October 2013). Existing instrument reviews are limited in scope, methods, and findings. The Society for Implementation Research Collaboration Instrument Review Project’s objectives address these limitations by identifying and applying a unique methodology to conduct a systematic and comprehensive review of quantitative instruments assessing constructs delineated in two of the field’s most widely used frameworks, adopt a systematic search process (using standard search strings), and engage an international team of experts to assess the full range of psychometric criteria (reliability, construct and criterion validity). Although this work focuses on implementation of psychosocial interventions in mental health and health-care settings, the methodology and results will likely be useful across a broad spectrum of settings. This effort has culminated in a centralized online open-access repository of instruments depicting graphical head-to-head comparisons of their psychometric properties. This article describes the methodology and preliminary outcomes. Methods The seven stages of the review, synthesis, and evaluation methodology include (1) setting the scope for the review, (2) identifying frameworks to organize and complete the review, (3) generating a search protocol for the literature review of constructs, (4) literature review of specific instruments, (5) development of an evidence-based assessment rating criteria, (6) data extraction and rating instrument quality by a task force of implementation experts to inform knowledge synthesis, and (7) the creation of a website repository. Results To date, this multi-faceted and collaborative search and synthesis methodology has identified over 420 instruments related to 34 constructs (total 48 including subconstructs) that are relevant to implementation science. Despite numerous constructs having greater than 20 available instruments, which implies saturation, preliminary results suggest that few instruments stem from gold standard development procedures. We anticipate identifying few high-quality, psychometrically sound instruments once our evidence-based assessment rating criteria have been applied. Conclusions The results of this methodology may enhance the rigor of implementation science evaluations by systematically facilitating access to psychometrically validated instruments and identifying where further instrument development is needed
Hb Long Island: a hemoglobin variant with a methionyl extension at the NH2 terminus and a prolyl substitution for the normal histidyl residue 2 of the beta chain.
Observability of Earth-skimming Ultra-high Energy Neutrinos
Neutrinos with energies above 10^8 GeV are expected from cosmic ray
interactions with the microwave background and are predicted in many
speculative models. Such energetic neutrinos are difficult to detect, as they
are shadowed by the Earth, but rarely interact in the atmosphere. Here we
propose a novel detection strategy: Earth-skimming neutrinos convert to charged
leptons that escape the Earth, and these leptons are detected in ground level
fluorescence detectors. With the existing HiRes detector, neutrinos from some
proposed sources are marginally detectable, and improvements of two orders of
magnitude are possible at the proposed Telescope Array.Comment: 4 pages, 3 figure
Observations of the Askaryan Effect in Ice
We report on the first observations of the Askaryan effect in ice: coherent
impulsive radio Cherenkov radiation from the charge asymmetry in an
electromagnetic (EM) shower. Such radiation has been observed in silica sand
and rock salt, but this is the first direct observation from an EM shower in
ice. These measurements are important since the majority of experiments to date
that rely on the effect for ultra-high energy neutrino detection are being
performed using ice as the target medium. As part of the complete validation
process for the Antarctic Impulsive Transient Antenna (ANITA) experiment, we
performed an experiment at the Stanford Linear Accelerator Center (SLAC) in
June 2006 using a 7.5 metric ton ice target, yielding results fully consistent
with theoretical expectations.Comment: 4 pages, 5 figures, minor correction
Majorana Neutrinos and Gravitational Oscillation
We analyze the possibility of encountering resonant transitions of high
energy Majorana neutrinos produced in Active Galactic Nuclei (AGN). We consider
gravitational, electromagnetic and matter effects and show that the latter are
ignorable. Resonant oscillations due to the gravitational interactions are
shown to occur at energies in the PeV range for magnetic moments in the
range. Coherent precession will dominate for larger magnetic
moments. The alllowed regions for gravitational resonant transitions are
obtained.Comment: 11 pages, 8 figures, Latex; requires revtex and epsf.tex submitted to
Physical Review
Neutrino production through hadronic cascades in AGN accretion disks
We consider the production of neutrinos in active galactic nuclei (AGN)
through hadronic cascades. The initial, high energy nucleons are accelerated in
a source above the accretion disk around the central black hole. From the
source, the particles diffuse back to the disk and initiate hadronic cascades.
The observable output from the cascade are electromagnetic radiation and
neutrinos. We use the observed diffuse background X-ray luminosity, which
presumably results {}from this process, to predict the diffuse neutrino flux
close to existing limits from the Frejus experiment. The resulting neutrino
spectrum is down to the \GeV region. We discuss modifications of
this scenario which reduce the predicted neutrino flux.Comment: 12 Pages, LaTeX, TK 92 0
Status of ANITA and ANITA-lite
We describe a new experiment to search for neutrinos with energies above 3 x
10^18 eV based on the observation of short duration radio pulses that are
emitted from neutrino-initiated cascades. The primary objective of the
ANtarctic Impulse Transient Antenna (ANITA) mission is to measure the flux of
Greisen-Zatsepin-Kuzmin (GZK) neutrinos and search for neutrinos from Active
Galactic Nuclei (AGN). We present first results obtained from the successful
launch of a 2-antenna prototype instrument (called ANITA-lite) that circled
Antarctica for 18 days during the 03/04 Antarctic campaign and show preliminary
results from attenuation length studies of electromagnetic waves at radio
frequencies in Antarctic ice. The ANITA detector is funded by NASA, and the
first flight is scheduled for December 2006.Comment: 9 pages, 8 figures, to be published in Proceedings of International
School of Cosmic Ray Astrophysics, 14th Course: "Neutrinos and Explosive
Events in the Universe", Erice, Italy, 2-13 July 200
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