53,563 research outputs found
Soft Pion Emission in Semileptonic -Meson Decays
An analysis of semileptonic decays of mesons with the emission of a
single soft pion is presented in the framework of the heavy-quark limit using
an effective Lagrangian which implements chiral and heavy-quark symmetries. The
analysis is performed at leading order of the chiral and inverse heavy mass
expansions. In addition to the ground state heavy mesons some of their
resonances are included. The estimates of the various effective coupling
constants and form factors needed in the analysis are obtained using a chiral
quark model. As the main result, a clear indication is found that the
and resonances substantially affect the decay mode with a in
the final state, and a less dramatic effect is also noticed in the mode. An
analysis of the decay spectrum in the squared invariant mass
is carried out, showing the main effects of including the resonances. The
obtained rates show promising prospects for studies of soft pion emission in
semileptonic -meson decays in a -meson factory where, modulo experimental
cuts, about such decays in the meson mode and in the
mode could be observed per year.Comment: 41 pages, uses revtex, epsf. 16 uuencoded postscript figures appended
after `\end{document
NASA/Pratt and Whitney experimental clean combustor program: Engine test results
A two-stage vorbix (vortex burning and mixing) combustor and associated fuel system components were successfully tested in an experimental JT9D engine at steady-state and transient operating conditions, using ASTM Jet-A fuel. Full-scale JT9D experimental engine tests were conducted in a phase three aircraft experimental clean combustor program. The low-pollution combustor, fuel system, and fuel control concepts were derived from phase one and phase two programs in which several combustor concepts were evaluated, refined, and optimized in a component test rig. Significant pollution reductions were achieved with the combustor which meets the performance, operating, and installation requirements of the engine
A model for evolution and extinction
We present a model for evolution and extinction in large ecosystems. The
model incorporates the effects of interactions between species and the
influences of abiotic environmental factors. We study the properties of the
model by approximate analytic solution and also by numerical simulation, and
use it to make predictions about the distribution of extinctions and species
lifetimes that we would expect to see in real ecosystems. It should be possible
to test these predictions against the fossil record. The model indicates that a
possible mechanism for mass extinction is the coincidence of a large
coevolutionary avalanche in the ecosystem with a severe environmental
disturbance.Comment: Postscript (compressed etc. using uufiles), 16 pages, with 15
embedded figure
Semileptonic Decays of Heavy Omega Baryons in a Quark Model
The semileptonic decays of and are treated in the
framework of a constituent quark model developed in a previous paper on the
semileptonic decays of heavy baryons. Analytic results for the form
factors for the decays to ground states and a number of excited states are
evaluated. For to the form factors obtained are shown to
satisfy the relations predicted at leading order in the heavy-quark effective
theory at the non-recoil point. A modified fit of nonrelativistic and
semirelativistic Hamiltonians generates configuration-mixed baryon wave
functions from the known masses and the measured \lcle rate, with wave
functions expanded in both harmonic oscillator and Sturmian bases. Decay rates
of \ob to pairs of ground and excited \omc states related by heavy-quark
symmetry calculated using these configuration-mixed wave functions are in the
ratios expected from heavy-quark effective theory, to a good approximation. Our
predictions for the semileptonic elastic branching fraction of vary
minimally within the models we use. We obtain an average value of (84 2%)
for the fraction of decays to ground states, and 91%
for the fraction of decays to the ground state
. The elastic fraction of \ob \to \omc ranges from about 50%
calculated with the two harmonic-oscillator models, to about 67% calculated
with the two Sturmian models.Comment: 52 pages, 8 figure
On the elastic approximation to the vacancy formation energy in metals
Isotropic elastic continuum model application to calculate energy and entropy of vacancy formation in metal crystal
On the size of approximately convex sets in normed spaces
Let X be a normed space. A subset A of X is approximately convex if
for all and where is
the distance of to . Let \Co(A) be the convex hull and \diam(A) the
diameter of . We prove that every -dimensional normed space contains
approximately convex sets with \mathcal{H}(A,\Co(A))\ge \log_2n-1 and
\diam(A) \le C\sqrt n(\ln n)^2, where denotes the Hausdorff
distance. These estimates are reasonably sharp. For every , we construct
worst possible approximately convex sets in such that
\mathcal{H}(A,\Co(A))=\diam(A)=D. Several results pertaining to the
Hyers-Ulam stability theorem are also proved.Comment: 32 pages. See also http://www.math.sc.edu/~howard
An ultra-low frequency electromagnetic wave force mechanism for the ionosphere
Ultra-low frequency electromagnetic wave force mechanism for ionospheric anomalie
Extremal Approximately Convex Functions and Estimating the Size of Convex Hulls
A real valued function defined on a convex is anemconvex function iff
it satisfies A thorough study of
approximately convex functions is made. The principal results are a sharp
universal upper bound for lower semi-continuous approximately convex functions
that vanish on the vertices of a simplex and an explicit description of the
unique largest bounded approximately convex function~ vanishing on the
vertices of a simplex.
A set in a normed space is an approximately convex set iff for all
the distance of the midpoint to is . The bounds
on approximately convex functions are used to show that in with the
Euclidean norm, for any approximately convex set , any point of the
convex hull of is at a distance of at most
from . Examples are given to show
this is the sharp bound. Bounds for general norms on are also given.Comment: 39 pages. See also http://www.math.sc.edu/~howard
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