804,660 research outputs found
1/f noise from nonlinear stochastic differential equations
We consider a class of nonlinear stochastic differential equations, giving
the power-law behavior of the power spectral density in any desirably wide
range of frequency. Such equations were obtained starting from the point
process models of 1/f^b noise. In this article the power-law behavior of
spectrum is derived directly from the stochastic differential equations,
without using the point process models. The analysis reveals that the power
spectrum may be represented as a sum of the Lorentzian spectra. Such a
derivation provides additional justification of equations, expands the class of
equations generating 1/f^b noise, and provides further insights into the origin
of 1/f^b noise
Non-thermal recombination - a neglected source of flare hard X-rays and fast electron diagnostic
Context. Flare Hard X-Rays (HXRs) from non-thermal electrons are commonly
treated as solely bremsstrahlung (f-f), recombination (f-b) being neglected.
This assumption is shown to be substantially in error, especially in hot
sources, mainly due to recombination onto Fe ions.
Aims. We analyse the effects on HXR spectra and electron diagnostics by
including non-thermal recombination onto heavy elements in our model.
Methods. Using Kramers hydrogenic cross sections with effective Z, we
calculate f-f and f-b spectra for power-law electron spectra, in both thin and
thick target limits, and for Maxwellians, with summation over all important
ions.
Results. We find that non-thermal electron recombination, especially onto Fe,
must, in general, be included together with f-f, for reliable spectral
interpretation, when the HXR source is hot. f-b contribution is greatest when
the electron spectral index is large, and any low energy cut-off small. f-b
spectra recombination edges mean a cut-off in F(E) appears as a HXR feature at
Photon energy = Ec + Vz, offering an Ec diagnostic. Including f-b lowers,
greatly in some cases, the F(E) needed for prescribed HXR fluxes and, even when
small, seriously distorts F(E) as inferred by inversion or forward fitting
based on f-f alone.
Conclusions. f-b recombination from non-thermal electrons can be an important
contributor to HXR spectra and should be included in spectral analyses,
especially for hot sources. Accurate results will require use of better cross
sections than ours and consideration of source ionisation structure.Comment: 13 pages, 2 tables, 9 figures, Accepted for publication in A&
A Measurement of the Product Branching Ratio f(b->Lambda_b).BR(Lambda_b->Lambda X) in Z0 Decays
The product branching ratio, f(b->Lambda_b).BR(Lambda_b->Lambda X), where
Lambda_b denotes any weakly-decaying b-baryon, has been measured using the OPAL
detector at LEP. Lambda_b are selected by the presence of energetic Lambda
particles in bottom events tagged by the presence of displaced secondary
vertices. A fit to the momenta of the Lambda particles separates signal from B
meson and fragmentation backgrounds. The measured product branching ratio is
f(b->Lambda_b).BR(Lambda_b->Lambda X) = (2.67+-0.38(stat)+0.67-0.60(sys))%
Combined with a previous OPAL measurement, one obtains
f(b->Lambda_b).BR(Lambda_b->Lambda X) = (3.50+-0.32(stat)+-0.35(sys))%.Comment: 16 pages, LaTeX, 3 eps figs included, submitted to the European
Physical Journal
Transition Form Factor up to within the Factorization Approach
In the paper, we apply the factorization approach to deal with the
transition form factor in the large recoil
regions. The B-meson wave functions and that include the
three-particle Fock states' contributions are adopted to give a consistent PQCD
analysis of the form factor up to . It has been found that
both the wave functions and can give sizable
contributions to the form factor and should be kept for a better understanding
of the meson decays. Then the contributions from different twist structures
of the kaon wavefunction are discussed, including the -breaking
effects. A sizable contribution from the twist-3 wave function is
found, whose model dependence is discussed by taking two group of parameters
that are determined by different distribution amplitude moments obtained in the
literature. It is also shown that and
, which are more
reasonable and consistent with the light-cone sum rule results in the large
recoil regions.Comment: 22 pages and 6 figure
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