2,784 research outputs found
A model realizing the Harrison-Perkins-Scott lepton mixing matrix
We present a supersymmetric model in which the lepton mixing matrix
obeys, at the seesaw scale, the Harrison--Perkins--Scott
\textit{Ansatz}--vanishing , maximal atmospheric neutrino mixing, and
( is the solar mixing angle). The
model features a permutation symmetry among the three lepton multiplets
of each type--left-handed doublets, right-handed charged leptons, and
right-handed neutrinos--and among three Higgs doublets and three
zero-hypercharge scalar singlets; a fourth right-handed neutrino, a fourth
Higgs doublet, and a fourth scalar singlet are invariant under . In
addition, the model has seven \mathbbm{Z}_2 symmetries, out of which six do
not commute with . Supersymmetry is needed in order to eliminate some
quartic terms from the scalar potential, quartic terms which would make
impossible to obtain the required vacuum expectation values of the three Higgs
doublets and three scalar singlets. The Yukawa couplings to the charged leptons
are flavour diagonal, so that flavour-changing neutral Yukawa interactions only
arise at loop level.Comment: 16 pages, plain LaTeX, no figures; some clarifying remarks in the
conclusions and references added, version accepted for publication in JHE
Specific Heat-Coefficient of YbAl3 Studied by Combined Nearly Free Electron Conduction Band Hybridized with Localized f Electrons with Correlation Effect
Based on the recently proposed band model, the electronic specific heat of
moderately heavy electron compound YbAl are investigated. The band term of
the Hamiltonian consists of three parts; conduction electrons described by the
nearly free electron method, localized 4f electrons of Yb ions and the
hybridization term between these electrons. Extracting several bands near the
Fermi level, we reconstruct the low-energy effective Hamiltonian in order to
consider the correlation effect, which is studied by using the self-consistent
second order perturbation theory combined with local approximation. The
temperature dependence of the specific heat is calculated as a
function of temperature from the numerical derivative of the internal
energy. Sommerfeld coefficient is also calculated from the direct
formula. The overall structure of is in quantitative agreement
with the experimental results, which have the characteristic two-peak
structures. They originate from the correlation effect and the structure of the
non-interacting density of states, respectively. We show that our effective
Hamiltonian yielding the realistic band structure may describe quantitatively
heavy electron compounds with conduction bands composed of s- or p- electrons.Comment: 7 pages, 6 figures, To appear in J. Phys. Soc. Jp
The Broadband Afterglow of GRB980329
We present radio observations of the afterglow of the bright gamma-ray burst
GRB980329 made between one month and several years after the burst, a
re-analysis of previously published submillimeter data, and late-time optical
and near-infrared (NIR) observations of the host galaxy. From the absence of a
spectral break in the optical/NIR colors of the host galaxy, we exclude the
earlier suggestion that GRB980329 lies at a redshift of z >~5. We combine our
data with the numerous multi-wavelength observations of the early afterglow,
fit a comprehensive afterglow model to the entire broadband dataset, and derive
fundamental physical parameters of the blast-wave and its host environment.
Models for which the ejecta expand isotropically require both a high
circumburst density and extreme radiative losses from the shock. No low density
model (n << 10 cm^{-3}) fits the data. A burst with a total energy of ~ 10^{51}
erg, with the ejecta narrowly collimated to an opening angle of a few degrees,
driven into a surrounding medium with density ~ 20 cm^{-3}, provides a
satisfactory fit to the lightcurves over a range of redshifts.Comment: 27 pages, incl. 6 figures, minor revisions (e.g. added/updated
references) Accepted by Ap
Precision Astrometry with the Very Long Baseline Array: Parallaxes and Proper Motions for 14 Pulsars
Astrometry can bring powerful constraints to bear on a variety of scientific
questions about neutron stars, including their origins, astrophysics,
evolution, and environments. Using phase-referenced observations at the VLBA,
in conjunction with pulsar gating and in-beam calibration, we have measured the
parallaxes and proper motions for 14 pulsars. The smallest measured parallax in
our sample is 0.13+-0.02 mas for PSR B1541+09, which has a most probable
distance of 7.2+1.3-1.1 kpc. We detail our methods, including initial VLA
surveys to select candidates and find in-beam calibrators, VLBA
phase-referencing, pulsar gating, calibration, and data reduction. The use of
the bootstrap method to estimate astrometric uncertainties in the presence of
unmodeled systematic errors is also described. Based on our new
model-independent estimates for distance and transverse velocity, we
investigate the kinematics and birth sites of the pulsars and revisit models of
the Galactic electron density distribution. We find that young pulsars are
moving away from the Galactic plane, as expected, and that age estimates from
kinematics and pulsar spindown are generally in agreement, with certain notable
exceptions. Given its present trajectory, the pulsar B2045-16 was plausibly
born in the open cluster NGC 6604. For several high-latitude pulsars, the
NE2001 electron density model underestimates the parallax distances by a factor
of two, while in others the estimates agree with or are larger than the
parallax distances, suggesting that the interstellar medium is irregular on
relevant length scales. The VLBA astrometric results for the recycled pulsar
J1713+0747 are consistent with two independent estimates from pulse timing,
enabling a consistency check between the different reference frames.Comment: 16 pages, 9 figures, 4 tables; results unchanged; revised version
accepted by Ap
The electronic structure of amorphous silica: A numerical study
We present a computational study of the electronic properties of amorphous
SiO2. The ionic configurations used are the ones generated by an earlier
molecular dynamics simulations in which the system was cooled with different
cooling rates from the liquid state to a glass, thus giving access to
glass-like configurations with different degrees of disorder [Phys. Rev. B 54,
15808 (1996)]. The electronic structure is described by a tight-binding
Hamiltonian. We study the influence of the degree of disorder on the density of
states, the localization properties, the optical absorption, the nature of
defects within the mobility gap, and on the fluctuations of the Madelung
potential, where the disorder manifests itself most prominently. The
experimentally observed mismatch between a photoconductivity threshold of 9 eV
and the onset of the optical absorption around 7 eV is interpreted by the
picture of eigenstates localized by potential energy fluctuations in a mobility
gap of approximately 9 eV and a density of states that exhibits valence and
conduction band tails which are, even in the absence of defects, deeply located
within the former band gap.Comment: 21 pages of Latex, 5 eps figure
Broadband Observations of the Afterglow of GRB 000926: Observing the Effect of Inverse Compton Scattering and Evidence for a High-Density Environment
GRB 000926 has one of the best-studied afterglows to-date, with multiple
X-ray observations, as well as extensive multi-frequency optical and radio
coverage. Broadband afterglow observations, spanning from X-ray to radio
frequencies, provide a probe of the density structure of the circumburst
medium, as well as of the ejecta energetics, geometry, and the physical
parameters of the relativistic blastwave resulting from the explosion. We
present an analysis of {\em Chandra X-ray Observatory} observations of this
event, along with {\em Hubble Space Telescope} and radio monitoring. We combine
these data with ground-based optical and IR observations and fit the
synthesized afterglow lightcurve using models where collimated ejecta expand
into a surrounding medium. We find that we can explain the broadband lightcurve
with reasonable physical parameters only if the cooling is dominated by inverse
Compton scattering. Excess X-ray emission in the broadband spectrum indicates
that we are directly observing a contribution from inverse Compton scattering.
It is the first time this has been observed in a GRB afterglow, and it implies
that the GRB exploded in a reasonably dense (n~30 cm^{-3}) medium, consistent
with a diffuse interstellar cloud environment.Comment: 16 pages, 5 figures Accepted for publication in the Astrophysical
Journa
Benefits and problems of electronic information exchange as perceived by health care professionals: an interview study
<p>Abstract</p> <p>Background</p> <p>Various countries are currently implementing a national electronic patient record (n-EPR). Despite the assumed positive effects of n-EPRs, their overall adoption remains low and meets resistance from health care providers. This study aims to increase our understanding of health care providers' attitude towards the n-EPR, by investigating their perceptions of the benefits and problems of electronic information exchange in health care and the n-EPR in particular.</p> <p>Methods</p> <p>The study was conducted in three Dutch health care settings: acute care, diabetes care, and ambulatory mental health care. Two health care organisations were included per setting. Between January and June 2010, interviews were conducted with 17 stakeholders working in these organisations. Relevant themes were deduced by means of thematic qualitative analysis.</p> <p>Results</p> <p>Health care providers perceived electronic information exchange to promote the efficiency and quality of care. The problems they perceived in electronic information exchange mainly concerned the confidentiality and safety of information exchange and the reliability and quality of patient data. Many problems perceived by health care providers did not specifically apply to the n-EPR, but to electronic information exchange in general.</p> <p>Conclusions</p> <p>The implementation of the Dutch n-EPR has mainly followed a top-down approach, thereby neglecting the fact that the perceptions and preferences of its users (health care providers) need to be addressed in order to achieve successful implementation. The results of this study provide valuable suggestions about how to promote health care providers' willingness to adopt electronic information exchange, which can be useful for other countries currently implementing an n-EPR. Apart from providing information about the benefits and usefulness of electronic information exchange, efforts should be focused on minimising the problems as perceived by health care providers. The safety and confidentiality of electronic information exchange can be improved by developing tools to evaluate the legitimacy of access to electronic records, by increasing health care providers' awareness of the need to be careful when using patient data, and by measures to limit access to sensitive patient data. Improving health care providers' recording behaviour is important to improve the reliability and quality of electronically exchanged patient data.</p
Static perfect fluids with Pant-Sah equations of state
We analyze the 3-parameter family of exact, regular, static, spherically
symmetric perfect fluid solutions of Einstein's equations (corresponding to a
2-parameter family of equations of state) due to Pant and Sah and
"rediscovered" by Rosquist and the present author. Except for the Buchdahl
solutions which are contained as a limiting case, the fluids have finite radius
and are physically realistic for suitable parameter ranges. The equations of
state can be characterized geometrically by the property that the 3-metric on
the static slices, rescaled conformally with the fourth power of any linear
function of the norm of the static Killing vector, has constant scalar
curvature. This local property does not require spherical symmetry; in fact it
simplifies the the proof of spherical symmetry of asymptotically flat solutions
which we recall here for the Pant-Sah equations of state. We also consider a
model in Newtonian theory with analogous geometric and physical properties,
together with a proof of spherical symmetry of the asymptotically flat
solutions.Comment: 32 p., Latex, minor changes and correction
The Red Sea, Coastal Landscapes, and Hominin Dispersals
This chapter provides a critical assessment of environment, landscape and resources in the Red Sea region over the past five million years in relation to archaeological evidence of hominin settlement, and of current hypotheses about the role of the region as a pathway or obstacle to population dispersals between Africa and Asia and the possible significance of coastal colonization. The discussion assesses the impact of factors such as topography and the distribution of resources on land and on the seacoast, taking account of geographical variation and changes in geology, sea levels and palaeoclimate. The merits of northern and southern routes of movement at either end of the Red Sea are compared. All the evidence indicates that there has been no land connection at the southern end since the beginning of the Pliocene period, but that short sea crossings would have been possible at lowest sea-level stands with little or no technical aids. More important than the possibilities of crossing the southern channel is the nature of the resources available in the adjacent coastal zones. There were many climatic episodes wetter than today, and during these periods water draining from the Arabian escarpment provided productive conditions for large mammals and human populations in coastal regions and eastwards into the desert. During drier episodes the coastal region would have provided important refugia both in upland areas and on the emerged shelves exposed by lowered sea level, especially in the southern sector and on both sides of the Red Sea. Marine resources may have offered an added advantage in coastal areas, but evidence for their exploitation is very limited, and their role has been over-exaggerated in hypotheses of coastal colonization
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