3,447 research outputs found
A Study of Meteoroid Impact Phenomena
Process of crater formation resulting from impact of hypervelocity projectile - meteoroid impac
Constraining quasar host halo masses with the strength of nearby Lyman-alpha forest absorption
Using cosmological hydrodynamic simulations we measure the mean transmitted
flux in the Lyman alpha forest for quasar sightlines that pass near a
foreground quasar. We find that the trend of absorption with pixel-quasar
separation distance can be fitted using a simple power law form including the
usual correlation function parameters r_{0} and \gamma so that ( = \sum
exp(-tau_eff*(1+(r/r_{0})^(-\gamma)))). From the simulations we find the
relation between r_{0} and quasar mass and formulate this as a way to estimate
quasar host dark matter halo masses, quantifying uncertainties due to
cosmological and IGM parameters, and redshift errors. With this method, we
examine data for ~3000 quasars from the Sloan Digital Sky Survey (SDSS) Data
Release 3, assuming that the effect of ionizing radiation from quasars (the
so-called transverse proximity effect) is unimportant (no evidence for it is
seen in the data.) We find that the best fit host halo mass for SDSS quasars
with mean redshift z=3 and absolute G band magnitude -27.5 is log10(M/M_sun) =
12.48^{+0.53}_{-0.89}. We also use the Lyman-Break Galaxy (LBG) and Lyman alpha
forest data of Adelberger et al in a similar fashion to constrain the halo mass
of LBGs to be log10(M/M_sun) = 11.13^{+0.39}_{-0.55}, a factor of ~20 lower
than the bright quasars. In addition, we study the redshift distortions of the
Lyman alpha forest around quasars, using the simulations. We use the quadrupole
to monopole ratio of the quasar-Lyman alpha forest correlation function as a
measure of the squashing effect. We find that this does not have a measurable
dependence on halo mass, but may be useful for constraining cosmic geometry.Comment: 10 pages, 11 figures, submitted to MNRA
First Order Premelting Transition of Vortex Lattices
Vortex lattices in the high temperature superconductors undergo a first order
phase transition which has thus far been regarded as melting from a solid to a
liquid. We point out an alternative possibility of a two step process in which
there is a first order transition from an ordinary vortex lattice to a soft
vortex solid followed by another first order melting transition from the soft
vortex solid to a vortex liquid. We focus on the first step. This premelting
transition is induced by vacancy and interstitial vortex lines. We obtain good
agreement with the experimental transition temperature versus field, latent
heat, and magnetization jumps for YBCO and BSCCO.Comment: revised version replaces 9705092, 5 pages, Latex, 2 postscript
figures, defect line wandering is included, 2 step melting is propose
Statistics of Solar Wind Electron Breakpoint Energies Using Machine Learning Techniques
Solar wind electron velocity distributions at 1 au consist of a thermal
"core" population and two suprathermal populations: "halo" and "strahl". The
core and halo are quasi-isotropic, whereas the strahl typically travels
radially outwards along the parallel and/or anti-parallel direction with
respect to the interplanetary magnetic field. With Cluster-PEACE data, we
analyse energy and pitch angle distributions and use machine learning
techniques to provide robust classifications of these solar wind populations.
Initially, we use unsupervised algorithms to classify halo and strahl
differential energy flux distributions to allow us to calculate relative number
densities, which are of the same order as previous results. Subsequently, we
apply unsupervised algorithms to phase space density distributions over ten
years to study the variation of halo and strahl breakpoint energies with solar
wind parameters. In our statistical study, we find both halo and strahl
suprathermal breakpoint energies display a significant increase with core
temperature, with the halo exhibiting a more positive correlation than the
strahl. We conclude low energy strahl electrons are scattering into the core at
perpendicular pitch angles. This increases the number of Coulomb collisions and
extends the perpendicular core population to higher energies, resulting in a
larger difference between halo and strahl breakpoint energies at higher core
temperatures. Statistically, the locations of both suprathermal breakpoint
energies decrease with increasing solar wind speed. In the case of halo
breakpoint energy, we observe two distinct profiles above and below 500 km/s.
We relate this to the difference in origin of fast and slow solar wind.Comment: Published in Astronomy & Astrophysics, 11 pages, 10 figure
Repeatability of brain phase-based magnetic resonance electric properties tomography methods and effect of compressed SENSE and RF shimming
Magnetic resonance electrical properties tomography (MREPT) is an emerging imaging modality to noninvasively measure tissue conductivity and permittivity. Implementation of MREPT in the clinic requires repeatable measurements at a short scan time and an appropriate protocol. The aim of this study was to investigate the repeatability of conductivity measurements using phase-based MREPT and the effects of compressed SENSE (CS), and RF shimming on the precision of conductivity measurements. Conductivity measurements using turbo spin echo (TSE) and three-dimensional balanced fast field echo (bFFE) with CS factors were repeatable. Conductivity measurement using bFFE phase showed smaller mean and variance that those measured by TSE. The conductivity measurements using bFFE showed minimal deviation with CS factors up to 8, with deviation increasing at CS factors > 8. Subcortical structures produced less consistent measurements than cortical parcellations at higher CS factors. RF shimming using full slice coverage 2D dual refocusing echo acquisition mode (DREAM) and full coverage 3D dual TR approaches further improved measurement precision. BFFE is a more optimal sequence than TSE for phase-based MREPT in brain. Depending on the area of the brain being measured, the scan can be safely accelerated with compressed SENSE without sacrifice of precision, offering the potential to employ MREPT in clinical research and applications. RF shimming with better field mapping further improves precision of the conductivity measures
Nucleon axial form factors from two-flavour Lattice QCD
We present preliminary results on the axial form factor and the
induced pseudoscalar form factor of the nucleon. A systematic
analysis of the excited-state contributions to form factors is performed on the
CLS ensemble `N6' with and lattice spacing . The relevant three-point functions were computed with
source-sink separations ranging from to $t_s \sim \
1.4 \ \text{fm}$. We observe that the form factors suffer from non-trivial
excited-state contributions at the source-sink separations available to us. It
is noted that naive plateau fits underestimate the excited-state contributions
and that the method of summed operator insertions correctly accounts for these
effects.Comment: 7 pages, 12 figures; talk presented at Lattice 2014 -- 32nd
International Symposium on Lattice Field Theory, 23-28 June, 2014, Columbia
University New York, N
Utilitarian Collective Choice and Voting
In his seminal Social Choice and Individual Values, Kenneth Arrow stated that his theory applies to voting. Many voting theorists have been convinced that, on account of Arrow’s theorem, all voting methods must be seriously flawed. Arrow’s theory is strictly ordinal, the cardinal aggregation of preferences being explicitly rejected. In this paper I point out that all voting methods are cardinal and therefore outside the reach of Arrow’s result.
Parallel to Arrow’s ordinal approach, there evolved a consistent cardinal theory of collective choice. This theory, most prominently associated with the work of Harsanyi, continued the older utilitarian tradition in a more formal style. The purpose of this paper is to show that various derivations of utilitarian SWFs can also be used to derive utilitarian voting (UV). By this I mean a voting rule that allows the voter to score each alternative in accordance with a given scale. UV-k indicates a scale with k distinct values. The general theory leaves k to be determined on pragmatic grounds. A (1,0) scale gives approval voting. I prefer the scale (1,0,-1) and refer to the resulting voting rule as evaluative voting.
A conclusion of the paper is that the defects of conventional voting methods result not from Arrow’s theorem, but rather from restrictions imposed on voters’ expression of their preferences.
The analysis is extended to strategic voting, utilizing a novel set of assumptions regarding voter behavior
Bore seal technology topical report
Thermophysical, compatibility, and mechanical properties of ceramic-to-metal bore seal material
Plasma-Induced Frequency Chirp of Intense Femtosecond Lasers and Its Role in Shaping High-Order Harmonic Spectral Lines
We investigate the self-phase modulation of intense femtosecond laser pulses
propagating in an ionizing gas and its effects on collective properties of
high-order harmonics generated in the medium. Plasmas produced in the medium
are shown to induce a positive frequency chirp on the leading edge of the
propagating laser pulse, which subsequently drives high harmonics to become
positively chirped. In certain parameter regimes, the plasma-induced positive
chirp can help to generate sharply peaked high harmonics, by compensating for
the dynamically-induced negative chirp that is caused by the steep intensity
profile of intense short laser pulses.Comment: 5 pages, 5 figure
Interaction of Agulhas filaments with mesoscale turbulence: a case study
The inter-ocean leakage of heat and salt from the South Indian Ocean to the South Atlantic has important consequences for the global thermohaline circulation and in particular for the strength of overturning of the Atlantic Ocean as a whole. This leakage between these two subtropical gyres takes place south of Africa. The main mechanisms are the intermittent shedding of Agulhas rings from the retroflection of the Agulhas Current and the advection of Agulhas filaments from the border of the Agulhas Current, both of which move northwestward into the South Atlantic. The subsequent behaviour and mixing of Agulhas rings has been much studied over the past years, that of Agulhas filaments not at all. We report here on fortuitous hydrographic observations of the behaviour of an Agulhas filament that interacted with a number of mesoscale features shortly after formation. This suggests that Agulhas filaments may be involved in many other circulation elements and not only the Benguela upwelling front, as was surmised previously, and may mix out in a very site-specific region
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