7,819 research outputs found
Stochastic Semi-Classical Description of Fusion at Near-Barrier Energies
Fusion reactions of heavy ions are investigated by employing a simple
stochastic semi-classical model which includes the coupling between relative
motion and low frequency collective surface modes of colliding ions similarly
to the quantal coupled-channels description. The quantal effect enters into the
calculation through the initial zero-point fluctuations of the surface
vibrations. Good agreement with the result of coupled-channels calculations as
well as data is obtained for the fusion cross sections of nickel isotopes. The
internal excitations in non-fusing events as well as the fusion time are
investigated.Comment: 8 pages, 8 figures, Published in Phys. Rev.
Measuring the Effectiveness of Research Grant Getting
To a very large extent, the national and international reputation of major research universities depends upon their research performance. That explains why competition is so fierce among them to get as much as they can from the three Canadian government major granting agencies. This study demonstrates how performance indicators were developed to measure the effectiveness of research grant getting among eleven Canadian universities. It shows how amount of money received, size of teaching staff, and disciplinary characteristics were standardized to yield objective disciplinary and institutional rankings.La rĂ©putation nationale et internationale des principales universitĂ©s oĂč il se fait beaucoup de recherche dĂ©pend dans une trĂšs large mesure de leur rendement dans ce domaine .C'est pourquoi ces institutions mĂšnent une lutte serrĂ©e pour obtenir le maximum de subventions des trois principaux organismes subventionnaires du gouvernement canadien. L'article porte sur l'approche utilisĂ©e pour mesurer, Ă l'aide des indices de rendement, l'efficacitĂ© des subventions de recherche rĂ©parties entre onze universitĂ©s canadiennes. Il montre que pour en arriver Ă un classement objectif des disciplines et des institutions, il a fallu standardiser les sommes versĂ©es, l'importance numĂ©rique du corps professoral et les caractĂ©ris-tiques disciplinaires
The XENON100 exclusion limit without considering Leff as a nuisance parameter
In 2011, the XENON100 experiment has set unprecedented constraints on dark
matter-nucleon interactions, excluding dark matter candidates with masses down
to 6 GeV if the corresponding cross section is larger than 10^{-39} cm^2. The
dependence of the exclusion limit in terms of the scintillation efficiency
(Leff) has been debated at length. To overcome possible criticisms XENON100
performed an analysis in which Leff was considered as a nuisance parameter and
its uncertainties were profiled out by using a Gaussian likelihood in which the
mean value corresponds to the best fit Leff value smoothly extrapolated to zero
below 3 keVnr. Although such a method seems fairly robust, it does not account
for more extreme types of extrapolation nor does it enable to anticipate on how
much the exclusion limit would vary if new data were to support a flat
behaviour for Leff below 3 keVnr, for example. Yet, such a question is crucial
for light dark matter models which are close to the published XENON100 limit.
To answer this issue, we use a maximum Likelihood ratio analysis, as done by
the XENON100 collaboration, but do not consider Leff as a nuisance parameter.
Instead, Leff is obtained directly from the fits to the data. This enables us
to define frequentist confidence intervals by marginalising over Leff.Comment: 10 pages;, 9 figures; references adde
Bimodality as a signal of Liquid-Gas phase transition in nuclei?
We use the HIPSE (Heavy-Ion Phase-Space Exploration) Model to discuss the
origin of the bimodality in charge asymmetry observed in nuclear reactions
around the Fermi energy. We show that it may be related to the important
angular momentum (spin) transferred into the quasi-projectile before secondary
decay. As the spin overcomes the critical value, a sudden opening of decay
channels is induced and leads to a bimodal distribution for the charge
asymmetry. In the model, it is not assigned to a liquid-gas phase transition
but to specific instabilities in nuclei with high spin. Therefore, we propose
to use these reactions to study instabilities in rotating nuclear droplets.Comment: 4 pages, 4 figures Accepted to PR
Partial Disorder and Metal-Insulator Transition in the Periodic Anderson Model on a Triangular Lattice
Ground state of the periodic Anderson model on a triangular lattice is
systematically investigated by the mean-field approximation. We found that the
model exhibits two different types of partially disordered states: one is at
half filling and the other is at other commensurate fillings. In the latter
case, the kinetic energy is lowered by forming an extensive network involving
both magnetic and nonmagnetic sites, in sharp contrast to the former case in
which the nonmagnetic sites are rather isolated. This spatially extended nature
of nonmagnetic sites yields a metallic partially-disordered state by hole
doping. We discuss the mechanism of the metal-insulator transition by the
change of electronic structure.Comment: 4 pages, 4 figures, accepted for publication in J. Phys. Soc. Jp
Stochastic mean-field dynamics for fermions in the weak coupling limit
Assuming that the effect of the residual interaction beyond mean-field is
weak and has a short memory time, two approximate treatments of correlation in
fermionic systems by means of Markovian quantum jump are presented. A
simplified scenario for the introduction of fluctuations beyond mean-field is
first presented. In this theory, part of the quantum correlations between the
residual interaction and the one-body density matrix are neglected and jumps
occur between many-body densities formed of pairs of states where and are
antisymmetrized products of single-particle states. The underlying Stochastic
Mean-Field (SMF) theory is discussed and applied to the monopole vibration of a
spherical Ca nucleus under the influence of a statistical ensemble of
two-body contact interaction. This framework is however too simplistic to
account for both fluctuation and dissipation. In the second part of this work,
an alternative quantum jump method is obtained without making the approximation
on quantum correlations. Restricting to two particles-two holes residual
interaction, the evolution of the one-body density matrix of a correlated
system is transformed into a Lindblad equation. The associated dissipative
dynamics can be simulated by quantum jumps between densities written as is a normalized Slater determinant. The
associated stochastic Schroedinger equation for single-particle wave-functions
is given.Comment: Enlarged version, 10 pages, 2 figure
Les charges d'enseignement dans les universités québécoises : une réplique au Rapport Gobeil
This article responds to the recommendation in the "Gobeil Report" (Rapport Gobeil) which proposes an increase in teaching workload of 50% in all Quebec universities in order to conform to what "exists" elsewhere in Canada. After having undertaken a survey among 39 Canadian universities of all sizes, and after having analysed the number of students at the graduate and/or undergraduate levels as well as the amount of grant assistance provided to research, the authors conclude that it is necessary to distinguish between two types of university establishments: research universities and teaching universities. Results demonstrate clearly that course hours represent but one component of the teaching load and that it is also necessary to consider the graduate studies program of the institution as well as the research activities being undertaken by the university before drawing a conclusion as general as that presented in the Gobeil Report. The authors of this study conclude that there is a net difference in the teaching responsibilities in Canadian universities outside of Quebec â whether it be a research or a teaching university. In the francophone universities of Quebec, teaching loads are uniform no matter what the commitment to research or advanced studies happens to be. An indiscriminate increase in the teaching assignment would compromise graduate studies and the research carried out in research universities.Cet article rĂ©pond Ă la recommandation du Rapport Gobeil qui propose indis- tinctement /'augmentation de 50% de la charge des professeurs dans toutes les universitĂ©s quĂ©bĂ©coises pour se conformer Ă ce qui "existerait" ailleurs au Canada. AprĂšs avoir menĂ© une enquĂȘte auprĂšs de 39 universitĂ©s canadiennes de toutes tailles, et aprĂšs avoir analysĂ© le nombre d'Ă©tudiants de tous cycles et le niveau de subventions de recherche, les auteurs concluent qu'il faut distinguer deux catĂ©gories d'universitĂ©s: les universitĂ©s de recherche et les universitĂ©s d'enseignement. Les rĂ©sultats dĂ©montrent clairement que les heures de cours ne reprĂ©sentent qu'une partie des charges d'enseignement et qu'il faut considĂ©rer l'encadrement d'Ă©tudiants avancĂ©s et les activitĂ©s de recherche avant de tirer une conclusion aussi gĂ©nĂ©rale que celle prĂ©sentĂ©e par le Rapport Gobeil. Les auteurs concluent qu'il y a une nette diffĂ©rence dans les charges de cours dans les universitĂ©s canadiennes hors QuĂ©bec selon qu'il s'agit d'une universitĂ© de recherche ou d'enseignement. Dans les universitĂ©s francophones du QuĂ©bec, les charges d'enseignement sont uniformisĂ©es quel que soit l'engagement en recherche et dans les Ă©tudes avancĂ©es. Une augmentation indistincte des charges d'enseignement compromettrait fortement les Ă©tudes avancĂ©es et la recherche des universitĂ©s de recherche
Effects of live-bait shrimp trawling on seagrass beds and fish bycatch in Tampa Bay, Florida
The use of live shrimp for bait in
recreational fishing has resulted in
a controversial fishery for shrimp in
Florida. In this fishery, night collections
are conducted over seagrass
beds with roller beam trawls to capture
live shrimp, primarily pink
shrimp, Penaeus duorarum. These
shrimp are culled from the catch on
sorting tables and placed in onboard
aerated âliveâ wells. Beds of
turtlegrass, Thalassia testudinum,
a species that has highest growth
rates and biomass during summer
and lowest during the winter (Fonseca
et al., 1996) are predominant
areas for live-bait shrimp trawling
(Tabb and Kenny, 1969).
Our study objectives were 1) to
determine effects of a roller beam
trawl on turtlegrass biomass and
morphometrics during intensive
(up to 18 trawls over a turtlegrass
bed), short-term (3-hour duration)
use and 2) to examine the mortality
of bycatch finfish following capture
by a trawl
Transfemoral treatment for iliac occlusive disease with endoluminal stent-grafts
Objectives:Percutaneous treatment of iliac artery occlusive disease has replaced open vascular reconstruction for several indications. A balloon angioplasty with or without stent is not an option in the presence of infrainguinal extension of the disease. The authors describe a technique that allows the construction of an aorto- or iliofemoral graft through a single groin incision, using a 4 mm PTFE graft, anchoring it proximally with a Palmaz stent and dilating both to the desired diameter.Design:Retrospective non-randomised study.Materials and Methods:Nineteen procedures were performed in 16 patients mainly because of ischaemic rest pain, often with trophic skin changes or minor gangrene. Three patients had a bilateral procedure. Twelve patients had one or more associated procedures: 10 distal bypasses, one thrombectomy, one reimplantation of a distal bypass on the iliofemoral graft, one contralateral profundaplasty and two stents of the contralateral common iliac artery.Results:Two patients died, one of small bowel ischaemia and the other of a myocardial infarction. During the mean follow-up of 8.8 months, two graft thromboses occurred. In another patient bilateral stenting of a residual stenosis was necessary.Conclusions:Our experience shows that the reported technique is feasible. Whether the procedure is truly âless invasiveâ and the long-term results acceptable remains to be shown
On nucleon exchange mechanism in heavy-ion collisions at near-barrier energies
Nucleon drift and diffusion mechanisms in central collisions of asymmetric
heavy-ions at near-barrier energies are investigated in the framework of a
stochastic mean-field approach. Expressions for diffusion and drift
coefficients for nucleon transfer deduced from the stochastic mean-field
approach in the semiclassical approximation have similar forms familiar from
the phenomenological nucleon exchange model. The variance of fragment mass
distribution agrees with the empirical formula . The comparison with the time-dependent Hartree-Fock calculations
shows that, below barrier energies, the drift coefficient in the semiclassical
approximation underestimates the mean number of nucleon transfer obtained in
the quantal framework. Motion of the window in the dinuclear system has a
significant effect on the nucleon transfer in asymmetric collisions.Comment: 10 pages, 10 figures, submitted for publicatio
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