965 research outputs found
Slepton Flavor Physics at Linear Colliders
If low energy supersymmetry is realized in nature it is possible that a first
generation linear collider will only have access to some of the superpartners
with electroweak quantum numbers. Among these, sleptons can provide sensitive
probes for lepton flavor violation through potentially dramatic lepton
violating signals. Theoretical proposals to understand the absence of low
energy quark and lepton flavor changing neutral currents are surveyed and many
are found to predict observable slepton flavor violating signals at linear
colliders. The observation or absence of such sflavor violation will thus
provide important indirect clues to very high energy physics. Previous analyses
of slepton flavor oscillations are also extended to include the effects of
finite width and mass differences.Comment: 7 pages, no figures, uses RevTeX4. Contribution to Snowmass 200
Double-Diffusive Mixing-Length Theory, Semiconvection, and Massive Star Evolution
Double-diffusive convection refers to mixing where the effects of thermal and
composition gradients compete to determine the stability of a fluid. In
addition to the familiar fast convective instability, such fluids exhibit the
slow, direct salt finger instability and the slow, overstable semiconvective
instability. Previous approaches to this subject usually have been based on
linear stability analyses. We develop here the nonlinear mixing-length theory
(MLT) of double-diffusive convection, in analogy to the more familiar MLT for a
fluid of homogeneous composition. We present approximate solutions for the
mixing rate in the various regimes, and show that the familiar Schwarzschild
and Ledoux stability criteria are good approximations to the precise criteria
in stellar interiors.
We have implemented the self-consistent computation of the temperature
gradient and turbulent mixing rate in a stellar evolution code and solved a
diffusion equation to mix composition at the appropriate rate. We have evolved
15\msun and 30\msun stars from the zero-age main sequence to the end of
core He-burning. Semiconvective mixing is fast enough to alter stellar
composition profiles on relevant time scales, but not so fast that
instantaneous readjustment is appropriate.Comment: 17 pages, 6 postscript figures, to appear in MNRA
Recommended from our members
Contemporary Seriality: A Roundtable
A panel discussion about seriality in film, television, literature, and podcasting
Cooling of Dense Gas by H2O Line Emission and an Assessment of its Effects in Chondrule-Forming Shocks
We consider gas at densities appropriate to protoplanetary disks and
calculate its ability to cool due to line radiation emitted by H2O molecules
within the gas. Our work follows that of Neufeld & Kaufman (1993; ApJ, 418,
263), expanding on their work in several key aspects, including use of a much
expanded line database, an improved escape probability formulism, and the
inclusion of dust grains, which can absorb line photons. Although the escape
probabilities formally depend on a complicated combination of optical depth in
the lines and in the dust grains, we show that the cooling rate including dust
is well approximated by the dust-free cooling rate multiplied by a simple
function of the dust optical depth. We apply the resultant cooling rate of a
dust-gas mixture to the case of a solar nebula shock pertinent to the formation
of chondrules, millimeter-sized melt droplets found in meteorites. Our aim is
to assess whether line cooling can be neglected in chondrule-forming shocks or
if it must be included. We find that for typical parameters, H2O line cooling
shuts off a few minutes past the shock front; line photons that might otherwise
escape the shocked region and cool the gas will be absorbed by dust grains.
During the first minute or so past the shock, however, line photons will cool
the gas at rates ~ 10,000 K/hr, dropping the temperature of the gas (and most
likely the chondrules within the gas) by several hundred K. Inclusion of H2O
line cooling therefore must be included in models of chondrule formation by
nebular shocks.Comment: Accepted for publication in The Astrophysical Journa
Corporate financing decisions: UK survey evidence
Despite theoretical developments in recent years, our understanding of corporate capital structure remains incomplete. Prior empirical research has been dominated by archival regression studies which are limited in their ability to fully reflect the diversity found in practice. The present paper reports on a comprehensive survey of corporate financing decision-making in UK listed companies. A key finding is that firms are heterogeneous in their capital structure policies. About half of the firms seek to maintain a target debt level, consistent with trade-off theory, but 60 per cent claim to follow a financing hierarchy, consistent with pecking order theory. These two theories are not viewed by respondents as either mutually exclusive or exhaustive. Many of the theoretical determinants of debt levels are widely accepted by respondents, in particular the importance of interest tax shield, financial distress, agency costs and also, at least implicitly, information asymmetry. Results also indicate that cross-country institutional differences have a significant impact on financial decisions
Characterisation of myocardial structure and function in adult-onset growth hormone deficiency using cardiac magnetic resonance.
Growth hormone (GH) can profoundly influence cardiac function. While GH excess causes well-defined cardiac pathology, fewer data are available regarding the more subtle cardiac changes seen in GH deficiency (GHD). This preliminary study uses cardiac magnetic resonance imaging (CMR) to assess myocardial structure and function in GHD. Ten adult-onset GHD patients underwent CMR, before and after 6 and 12 months of GH replacement. They were compared to 10 age-matched healthy controls and sex-matched healthy controls. Left ventricular (LV) mass index (LVMi) increased with 1 year of GH replacement (53.8 vs. 57.0 vs. 57.3 g/m2, analysis of variance p = 0.0229). Compared to controls, patients showed a trend towards reduced LVMi at baseline (51.4 vs. 60.0 g/m2, p = 0.0615); this difference was lost by 1 year of GH treatment (57.3 vs. 59.9 g/m2, p = 0.666). Significantly reduced aortic area was observed in GHD (13.2 vs. 19.0 cm2/m2, p = 0.001). This did not change with GH treatment. There were no differences in other LV parameters including end-diastolic volume index (EDVi), end-systolic volume index, stroke volume index (SVi), cardiac index and ejection fraction. There was a trend towards reduced baseline right ventricular (RV)SVi (44.1 vs. 49.1 ml/m2, p = 0.0793) and increased RVEDVi over 1 year (70.3 vs. 74.3 vs. 73.8 ml/m2, p = 0.062). Two patients demonstrated interstitial expansion, for example with fibrosis, and three myocardial ischaemia as assessed by late gadolinium enhancement and stress perfusion. The increased sensitivity of CMR to subtle cardiac changes demonstrates that adult-onset GHD patients have reduced aortic area and LVMi increases after 1 year of GH treatment. These early data should be studied in larger studies in the future
A nonlinear hydrodynamical approach to granular materials
We propose a nonlinear hydrodynamical model of granular materials. We show
how this model describes the formation of a sand pile from a homogeneous
distribution of material under gravity, and then discuss a simulation of a
rotating sandpile which shows, in qualitative agreement with experiment, a
static and dynamic angle of repose.Comment: 17 pages, 14 figures, RevTeX4; minor changes to wording and some
additional discussion. Accepted by Phys. Rev.
North Korean Decisionmaking
Discerning the decisionmaking of Kim Jong-Un and the North Korean
regime on issues of peaceful engagement and warlike actions endures
as a mighty challenge for U.S. intelligence analysts and policymakers.
In this report, we seek to inform analysis of Democratic People’s
Republic of Korea (DPRK) leadership decisionmaking. To do so, we
use three discussion papers that were written to facilitate discussion of
an interagency working group. The three papers are assembled here in
a single report. The first discussion paper describes decisionmaking
among different authoritarian regimes, including North Korea, and
the opening up of those economies to outside engagement. The second
paper outlines two different scenarios that might occur when conventional
deterrence on the Korean Peninsula breaks down and the resulting
decisions that North Korea’s leadership could face. The third paper
assesses DPRK decisionmaking about nuclear weapon use. The report
concludes with some observations, drawn from the issues covered in
these three discussion papers, about DPRK decisionmaking and stability
on the Korean Peninsula
Changing times in England: the influence on geography teachers’ professional practice
School geography in England has been characterised as a pendulum swinging between policies that emphasise curriculum and pedagogy alternately. In this paper, I illustrate the influence of these shifts on geography teacher's professional practice, by drawing on three “moments” from my experience as a student, teacher and teacher educator. Barnett's description of teacher professionalism as a continuous project of “being” illuminates how geography teachers can adapt to competing influences. It reflects teacher professionalism as an unfinished project, which is responsive, but not beholden, to shifting trends, and is informed by how teachers frame and enact policies. I argue that recognising these contextual factors is key to supporting geography teachers in “being” geography education professionals. As education becomes increasingly competitive on a global scale, individual governments are looking internationally for “solutions” to improve educational rankings. In this climate, the future of geography education will rest on how teachers react locally to international trends. Geography teacher educators can support this process by continuing to inform the field through meaningful geography education research, in particular in making the contextual factors of their research explicit. This can be supported through continued successful international collaboration in geography education research
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