8,445 research outputs found
Studies of CP-conserving and CP-violating Bs mixing parameters with the D0 experiment
This paper summarises the recent results of the Run IIa D0 experiment at the
Tevatron Collider at Fermilab on the observable parameters of the
meson. A measurement of the branching fraction is
reported, which provides an estimate of the width difference
. Through the decay the width difference is extracted, and for the first
time a constraint is set on the CP-violating phase , although a
four-fold ambiguity remains. This result is combined with other D0 measurements
to yield , .Comment: Submitted for the SUSY07 proceeding
Accuracy requirements to test the applicability of the random cascade model to supersonic turbulence
A model, which is widely used for inertial rang statistics of supersonic
turbulence in the context of molecular clouds and star formation, expresses
(measurable) relative scaling exponents Z_p of two-point velocity statistics as
a function of two parameters, beta and Delta. The model relates them to the
dimension D of the most dissipative structures, D=3-Delta/(1-beta). While this
description has proved most successful for incompressible turbulence
(beta=Delta=2/3, and D=1), its applicability in the highly compressible regime
remains debated. For this regime, theoretical arguments suggest D=2 and
Delta=2/3, or Delta=1. Best estimates based on 3D periodic box simulations of
supersonic isothermal turbulence yield Delta=0.71 and D=1.9, with uncertainty
ranges of Delta in [0.67, 0.78] and D in [2.04,1.60]. With these 5-10\%
uncertainty ranges just marginally including the theoretical values of
Delta=2/3 and D=2, doubts remain whether the model indeed applies and, if it
applies, for what values of beta and Delta. We use a Monte Carlo approach to
mimic actual simulation data and examine what factors are most relevant for the
fit quality. We estimate that 0.1% (0.05%) accurate Z_p, with p=1...5, should
allow for 2% (1%) accurate estimates of beta and Delta in the highly
compressible regime, but not in the mildly compressible regime. We argue that
simulation-based Z_p with such accuracy are within reach of today's computer
resources. If this kind of data does not allow for the expected high quality
fit of beta and Delta, then this may indicate the inapplicability of the model
for the simulation data. In fact, other models than the one we examine here
have been suggested.Comment: 8 pages, 8 figures, accepted by Astronomy and Astrophysic
Study of alumina-trichite reinforcement of a nickel-based matric by means of powder metallurgy
Research was conducted on reinforcing nickel based matrices with alumina trichites by using powder metallurgy. Alumina trichites previously coated with nickel are magnetically aligned. The felt obtained is then sintered under a light pressure at a temperature just below the melting point of nickel. The halogenated atmosphere technique makes it possible to incorporate a large number of additive elements such as chromium, titanium, zirconium, tantalum, niobium, aluminum, etc. It does not appear that going from laboratory scale to a semi-industrial scale in production would create any major problems
Comparison of different nonlinear solvers for 2D time-implicit stellar hydrodynamics
Time-implicit schemes are attractive since they allow numerical time steps
that are much larger than those permitted by the Courant-Friedrich-Lewy
criterion characterizing time-explicit methods. This advantage comes, however,
with a cost: the solution of a system of nonlinear equations is required at
each time step. In this work, the nonlinear system results from the
discretization of the hydrodynamical equations with the Crank-Nicholson scheme.
We compare the cost of different methods, based on Newton-Raphson iterations,
to solve this nonlinear system, and benchmark their performances against
time-explicit schemes. Since our general scientific objective is to model
stellar interiors, we use as test cases two realistic models for the convective
envelope of a red giant and a young Sun. Focusing on 2D simulations, we show
that the best performances are obtained with the quasi-Newton method proposed
by Broyden. Another important concern is the accuracy of implicit calculations.
Based on the study of an idealized problem, namely the advection of a single
vortex by a uniform flow, we show that there are two aspects: i) the nonlinear
solver has to be accurate enough to resolve the truncation error of the
numerical discretization, and ii) the time step has be small enough to resolve
the advection of eddies. We show that with these two conditions fulfilled, our
implicit methods exhibit similar accuracy to time-explicit schemes, which have
lower values for the time step and higher computational costs. Finally, we
discuss in the conclusion the applicability of these methods to fully implicit
3D calculations.Comment: Accepted for publication in A&
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