15,370 research outputs found
The scattering map in two coupled piecewise-smooth systems, with numerical application to rocking blocks
We consider a non-autonomous dynamical system formed by coupling two
piecewise-smooth systems in \RR^2 through a non-autonomous periodic
perturbation. We study the dynamics around one of the heteroclinic orbits of
one of the piecewise-smooth systems. In the unperturbed case, the system
possesses two normally hyperbolic invariant manifolds of dimension two
with a couple of three dimensional heteroclinic manifolds between them. These
heteroclinic manifolds are foliated by heteroclinic connections between
tori located at the same energy levels. By means of the {\em impact map} we
prove the persistence of these objects under perturbation. In addition, we
provide sufficient conditions of the existence of transversal heteroclinic
intersections through the existence of simple zeros of Melnikov-like functions.
The heteroclinic manifolds allow us to define the {\em scattering map}, which
links asymptotic dynamics in the invariant manifolds through heteroclinic
connections. First order properties of this map provide sufficient conditions
for the asymptotic dynamics to be located in different energy levels in the
perturbed invariant manifolds. Hence we have an essential tool for the
construction of a heteroclinic skeleton which, when followed, can lead to the
existence of Arnol'd diffusion: trajectories that, on large time scales,
destabilize the system by further accumulating energy. We validate all the
theoretical results with detailed numerical computations of a mechanical system
with impacts, formed by the linkage of two rocking blocks with a spring
Gravitational Wave Propagation in Isotropic Cosmologies
We study the propagation of gravitational waves carrying arbitrary
information through isotropic cosmologies. The waves are modelled as small
perturbations of the background Robertson-Walker geometry. The perfect fluid
matter distribution of the isotropic background is, in general, modified by
small anisotropic stresses. For pure gravity waves, in which the perturbed Weyl
tensor is radiative (i.e. type N in the Petrov classification), we construct
explicit examples for which the presence of the anisotropic stress is shown to
be essential and the histories of the wave-fronts in the background
Robertson-Walker geometry are shear-free null hypersurfaces. The examples
derived in this case are analogous to the Bateman waves of electromagnetic
theory.Comment: 27 pages, accepted for publication in Phys.Rev.
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Estimating drizzle drop size and precipitation rate using two-colour lidar measurements
A method to estimate the size and liquid water content of drizzle drops using lidar measurements at two wavelengths is described. The method exploits the differential absorption of infrared light by liquid water at 905 nm and 1.5 μm, which leads to a different backscatter cross section for water drops larger than ≈50 μm. The ratio of backscatter measured from drizzle samples below cloud base at these two wavelengths (the colour ratio) provides a measure of the median volume drop diameter D0. This is a strong effect: for D0=200 μm, a colour ratio of ≈6 dB is predicted. Once D0 is known, the measured backscatter at 905 nm can be used to calculate the liquid water content (LWC) and other moments of the drizzle drop distribution.
The method is applied to observations of drizzle falling from stratocumulus and stratus clouds. High resolution (32 s, 36 m) profiles of D0, LWC and precipitation rate R are derived. The main sources of error in the technique are the need to assume a value for the dispersion parameter μ in the drop size spectrum (leading to at most a 35% error in R) and the influence of aerosol returns on the retrieval (≈10% error in R for the cases considered here). Radar reflectivities are also computed from the lidar data, and compared to independent measurements from a colocated cloud radar, offering independent validation of the derived drop size distributions
Light-like Signals in General relativity and Cosmology
The modelling of light-like signals in General Relativity taking the form of
impulsive gravitational waves and light-like shells of matter is examined.
Systematic deductions from the Bianchi identities are made. These are based
upon Penrose's hierarchical classification of the geometry induced on the null
hypersurface history of the surface by its imbedding in the space-times to the
future and to the past of it. The signals are not confined to propagate in a
vacuum and thus their interaction with matter (a burst of radiation propagating
through a cosmic fluid, for example) is also studied. Results are accompanied
by illustrative examples using cosmological models, vacuum space-times, the de
sitter univers and Minkowskian space-time.Comment: 21 pages, latex, no figure
Inclusive production of the state in collisions at D0
We present a study of the inclusive production of the with the
decay to the final state in hadronic collisions. Based on
of collision data collected by the D0
experiment at the Fermilab Tevatron collider, we report the first evidence for
the prompt production of and find the fraction of events
originating from hadrons to be . The ratio of the non-prompt
production rate to the yield in the same channel is . The values of the
mass ~MeV
and width ~MeV are consistent with previous measurements. 8 pages, 2 figuesComment: Submitted to PRL. * pages, 2 figure
Simultaneous measurement of forward-backward asymmetry and top polarization in dilepton final states from production at the Tevatron
We present a simultaneous measurement of the forward-backward asymmetry and
the top-quark polarization in production in dilepton final states
using 9.7 fb of proton-antiproton collisions at TeV with
the D0 detector. To reconstruct the distributions of kinematic observables we
employ a matrix element technique that calculates the likelihood of the
possible kinematic configurations. After accounting for the presence
of background events and for calibration effects, we obtain a forward-backward
asymmetry of and a top-quark polarization times spin analyzing power in the beam
basis of ,
with a correlation of between the measurements. If we constrain the
forward-backward asymmetry to its expected standard model value, we obtain a
measurement of the top polarization of If we constrain the top polarization to its
expected standard model value, we measure a forward-backward asymmetry of
A
combination with the D0 measurement in the lepton+jets final
state yields an asymmetry of Within their respective uncertainties, all these
results are consistent with the standard model expectations.Comment: 16 pages, 16 figures, published versio
Precise measurement of the top quark mass in dilepton decays using optimized neutrino weighting
We measure the top quark mass in dilepton final states of top-antitop events
in proton-antiproton collisions at sqrt(s) = 1.96 TeV, using data corresponding
to an integrated luminosity of 9.7 fb^-1 at the Fermilab Tevatron Collider. The
analysis features a comprehensive optimization of the neutrino weighting method
to minimize the statistical uncertainties. We also improve the calibration of
jet energies using the calibration determined in top-antitop to lepton+jets
events, which reduces the otherwise limiting systematic uncertainty from the
jet energy scale. The measured top quark mass is mt = 173.32 +/- 1.36(stat) +/-
0.85(syst) GeV.Comment: 10 pages, 2 figures, 2 tables, subm. to Phys. Lett.
Geodesics in spacetimes with expanding impulsive gravitational waves
We study geodesic motion in expanding spherical impulsive gravitational waves
propagating in a Minkowski background. Employing the continuous form of the
metric we find and examine a large family of geometrically preferred geodesics.
For the special class of axially symmetric spacetimes with the spherical
impulse generated by a snapping cosmic string we give a detailed physical
interpretation of the motion of test particles.Comment: 12 pages, Revtex, final versio
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