668 research outputs found
Remote access for NAS: Supercomputing in a university environment
The experiment was designed to assist the Numerical Aerodynamic Simulation (NAS) Project Office in the testing and evaluation of long haul communications for remote users. The objectives of this work were to: (1) use foreign workstations to remotely access the NAS system; (2) provide NAS with a link to a large university-based computing facility which can serve as a model for a regional node of the Long-Haul Communications Subsystem (LHCS); and (3) provide a tail circuit to the University of Colorado a Boulder thereby simulating the complete communications path from NAS through a regional node to an end-user
Long-term evolution of massive star explosions
We examine simulations of core-collapse supernovae in spherical symmetry. Our
model is based on general relativistic radiation hydrodynamics with
three-flavor Boltzmann neutrino transport. We discuss the different supernova
phases, including the long-term evolution up to 20 seconds after the onset of
explosion during which the neutrino fluxes and mean energies decrease
continuously. In addition, the spectra of all flavors become increasingly
similar, indicating the change from charged- to neutral-current dominance.
Furthermore, it has been shown recently by several groups independently, based
on sophisticated supernova models, that collective neutrino flavor oscillations
are suppressed during the early mass-accretion dominated post-bounce evolution.
Here we focus on the possibility of collective flavor flips between electron
and non-electron flavors during the later, on the order of seconds, evolution
after the onset of an explosion with possible application for the
nucleosynthesis of heavy elements.Comment: 12 pages, 7 figures, conference proceeding, HANSE 2011 worksho
Adsorption of Multi-block and Random Copolymer on a Solid Surface: Critical Behavior and Phase Diagram
The adsorption of a single multi-block -copolymer on a solid planar
substrate is investigated by means of computer simulations and scaling
analysis. It is shown that the problem can be mapped onto an effective
homopolymer adsorption problem. In particular we discuss how the critical
adsorption energy and the fraction of adsorbed monomers depend on the block
length of sticking monomers , and on the total length of the polymer
chains. Also the adsorption of the random copolymers is considered and found to
be well described within the framework of the annealed approximation. For a
better test of our theoretical prediction, two different Monte Carlo (MC)
simulation methods were employed: a) off-lattice dynamic bead-spring model,
based on the standard Metropolis algorithm (MA), and b) coarse-grained lattice
model using the Pruned-enriched Rosenbluth method (PERM) which enables tests
for very long chains. The findings of both methods are fully consistent and in
good agreement with theoretical predictions.Comment: 27 pages, 12 figure
On the stability of very massive primordial stars
The stability of metal-free very massive stars ( = 0; M = 120 - 500
\msol) is analyzed and compared with metal-enriched stars. Such zero-metal
stars are unstable to nuclear-powered radial pulsations on the main sequence,
but the growth time scale for these instabilities is much longer than for their
metal-rich counterparts. Since they stabilize quickly after evolving off the
ZAMS, the pulsation may not have sufficient time to drive appreciable mass loss
in Z = 0 stars. For reasonable assumptions regarding the efficiency of
converting pulsational energy into mass loss, we find that, even for the larger
masses considered, the star may die without losing a large fraction of its
mass. We find a transition between the - and -mechanisms for
pulsational instability at Z\sim 2\E{-4} - 2\E{-3}. For the most metal-rich
stars, the -mechanism yields much shorter -folding times, indicating
the presence of a strong instability. We thus stress the fundamental difference
of the stability and late stages of evolution between very massive stars born
in the early universe and those that might be born today.Comment: 7 pages, 5 figures. Minor changes, more results given in Table 1,
accepted for publication in Ap
Rumo a uma política de Estado para inteligência artificial
After more than 60 years of evolution, artificial intelligence, driven by new technologies, has accelerated its development and should bring great possibilities to countries that adopt it on a large scale. This article presents an overview of the national strategies launched recently, aiming to highlight the importance for Brazil to define a public policy to promote the adoption of artificial intelligence. Recent studies show the spread of artificial intelligence techniques will further widen inequality among nations, making the delay in this area a path without return.Após mais de 60 anos de evolução, a inteligência artificial acelera seu desenvolvimento, impulsionada por novas tecnologias, devendo trazer grandes possibilidades aos países que a adotarem em larga escala. Este artigo apresenta uma visão geral das estratégias nacionais lançadas nos últimos anos, com o objetivo de ressaltar a importância de que o Brasil defina uma política pública para promover a adoção da inteligência artificial. Estudos recentes mostram que a disseminação das técnicas de inteligência artificial irá alargar ainda mais a desigualdade entre as nações, fazendo com que o atraso nessa área se torne um caminho sem volta
Effect of collective neutrino flavor oscillations on vp-process nucleosynthesis
The vp process is a primary nucleosynthesis process which occurs in core
collapse supernovae. An essential role in this process is being played by
electron antineutrinos. They generate, by absorption on protons, a supply of
neutrons which, by (n,p) reactions, allow to overcome waiting point nuclei with
rather long beta-decay and proton-capture lifetimes. The synthesis of heavy
elements by the vp process depends sensitively on the \bar{\nu}_e luminosity
and spectrum. As has been shown recently, the latter are affected by collective
neutrino flavor oscillations which can swap the \bar{\nu}_e and
\bar{\nu}_{\mu,\tau} spectra above a certain split energy. Assuming such a swap
scenario, we have studied the impact of collective neutrino flavor oscillations
on the vp-process nucleosynthesis. Our results show that the production of
light p-nuclei up to mass number A=108 is very sensitive to collective neutrino
oscillations.Comment: 4 pages, 3 figures, submitted to Physics Letters
Measurements of branching fraction ratios and CP-asymmetries in suppressed B^- -> D(-> K^+ pi^-)K^- and B^- -> D(-> K^+ pi^-)pi^- decays
We report the first reconstruction in hadron collisions of the suppressed
decays B^- -> D(-> K^+ pi^-)K^- and B^- -> D(-> K^+ pi^-)pi^-, sensitive to the
CKM phase gamma, using data from 7 fb^-1 of integrated luminosity collected by
the CDF II detector at the Tevatron collider. We reconstruct a signal for the
B^- -> D(-> K^+ pi^-)K^- suppressed mode with a significance of 3.2 standard
deviations, and measure the ratios of the suppressed to favored branching
fractions R(K) = [22.0 \pm 8.6(stat)\pm 2.6(syst)]\times 10^-3, R^+(K) =
[42.6\pm 13.7(stat)\pm 2.8(syst)]\times 10^-3, R^-(K)= [3.8\pm 10.3(stat)\pm
2.7(syst]\times 10^-3, as well as the direct CP-violating asymmetry A(K) =
-0.82\pm 0.44(stat)\pm 0.09(syst) of this mode. Corresponding quantities for
B^- -> D(-> K^+ pi^-)pi^- decay are also reported.Comment: 8 pages, 1 figure, accepted by Phys.Rev.D Rapid Communications for
Publicatio
Search for the standard model Higgs boson in the H to ZZ to 2l 2nu channel in pp collisions at sqrt(s) = 7 TeV
A search for the standard model Higgs boson in the H to ZZ to 2l 2nu decay
channel, where l = e or mu, in pp collisions at a center-of-mass energy of 7
TeV is presented. The data were collected at the LHC, with the CMS detector,
and correspond to an integrated luminosity of 4.6 inverse femtobarns. No
significant excess is observed above the background expectation, and upper
limits are set on the Higgs boson production cross section. The presence of the
standard model Higgs boson with a mass in the 270-440 GeV range is excluded at
95% confidence level.Comment: Submitted to JHE
Search for New Physics with Jets and Missing Transverse Momentum in pp collisions at sqrt(s) = 7 TeV
A search for new physics is presented based on an event signature of at least
three jets accompanied by large missing transverse momentum, using a data
sample corresponding to an integrated luminosity of 36 inverse picobarns
collected in proton--proton collisions at sqrt(s)=7 TeV with the CMS detector
at the LHC. No excess of events is observed above the expected standard model
backgrounds, which are all estimated from the data. Exclusion limits are
presented for the constrained minimal supersymmetric extension of the standard
model. Cross section limits are also presented using simplified models with new
particles decaying to an undetected particle and one or two jets
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