509 research outputs found
Singularities of equidistants and global centre symmetry sets of Lagrangian submanifolds
We define the Global Centre Symmetry set (GCS) of a smooth closed
m-dimensional submanifold M of R^n, , which is an affinely invariant
generalization of the centre of a k-sphere in R^{k+1}. The GCS includes both
the centre symmetry set defined by Janeczko and the Wigner caustic defined by
Berry. We develop a new method for studying generic singularities of the GCS
which is suited to the case when M is lagrangian in R^{2m} with canonical
symplectic form. The definition of the GCS, which slightly generalizes one by
Giblin and Zakalyukin, is based on the notion of affine equidistants, so, we
first study singularities of affine equidistants of Lagrangian submanifolds,
classifying all the stable ones. Then, we classify the affine-Lagrangian stable
singularities of the GCS of Lagrangian submanifolds and show that, already for
smooth closed convex curves in R^2, many singularities of the GCS which are
affine stable are not affine-Lagrangian stable.Comment: 26 pages, 2 figure
High precision astrometry mission for the detection and characterization of nearby habitable planetary systems with the Nearby Earth Astrometric Telescope (NEAT)
(abridged) A complete census of planetary systems around a volume-limited
sample of solar-type stars (FGK dwarfs) in the Solar neighborhood with uniform
sensitivity down to Earth-mass planets within their Habitable Zones out to
several AUs would be a major milestone in extrasolar planets astrophysics. This
fundamental goal can be achieved with a mission concept such as NEAT - the
Nearby Earth Astrometric Telescope. NEAT is designed to carry out space-borne
extremely-high-precision astrometric measurements sufficient to detect
dynamical effects due to orbiting planets of mass even lower than Earth's
around the nearest stars. Such a survey mission would provide the actual
planetary masses and the full orbital geometry for all the components of the
detected planetary systems down to the Earth-mass limit. The NEAT performance
limits can be achieved by carrying out differential astrometry between the
targets and a set of suitable reference stars in the field. The NEAT instrument
design consists of an off-axis parabola single-mirror telescope, a detector
with a large field of view made of small movable CCDs located around a fixed
central CCD, and an interferometric calibration system originating from
metrology fibers located at the primary mirror. The proposed mission
architecture relies on the use of two satellites operating at L2 for 5 years,
flying in formation and offering a capability of more than 20,000
reconfigurations (alternative option uses deployable boom). The NEAT primary
science program will encompass an astrometric survey of our 200 closest F-, G-
and K-type stellar neighbors, with an average of 50 visits. The remaining time
might be allocated to improve the characterization of the architecture of
selected planetary systems around nearby targets of specific interest (low-mass
stars, young stars, etc.) discovered by Gaia, ground-based high-precision
radial-velocity surveys.Comment: Accepted for publication in Experimental Astronomy. The full member
list of the NEAT proposal and the news about the project are available at
http://neat.obs.ujf-grenoble.fr. The final publication is available at
http://www.springerlink.co
Decoherence, Entanglement and Irreversibility in Quantum Dynamical Systems with Few Degrees of Freedom
This review summarizes and amplifies on recent investigations of coupled
quantum dynamical systems in the short wavelength limit. We formulate and
attempt to answer three fundamental questions: (i) What drives a dynamical
quantum system to behave classically ? (ii) What determines the rate at which
two coupled quantum--mechanical systems become entangled ? (iii) How does
irreversibility occur in quantum systems with few degrees of freedom ? We embed
these three questions in the broader context of the quantum--classical
correspondence, which motivates the use of short--wavelength approximations to
quantum mechanics such as the trajectory-based semiclassical methods and random
matrix theory. Doing so, we propose a novel investigative procedure towards
decoherence and the emergence of classicality out of quantumness in dynamical
systems coupled to external degrees of freedom. We reproduce known results
derived using master equation or Lindblad approaches but also generate novel
ones. In particular we show how local exponential instability also affects the
temporal evolution of quantum chaotic dynamical systems. We extensively rely on
numerical experiments to illustrate our findings and briefly comment on
possible extensions to more complex problems involving environments with interacting dynamical systems, going beyond the uncoupled harmonic
oscillator model of Caldeira and Leggett.Comment: Final version, to appear in Advances in Physic
Measurement of the Z/gamma* + b-jet cross section in pp collisions at 7 TeV
The production of b jets in association with a Z/gamma* boson is studied
using proton-proton collisions delivered by the LHC at a centre-of-mass energy
of 7 TeV and recorded by the CMS detector. The inclusive cross section for
Z/gamma* + b-jet production is measured in a sample corresponding to an
integrated luminosity of 2.2 inverse femtobarns. The Z/gamma* + b-jet cross
section with Z/gamma* to ll (where ll = ee or mu mu) for events with the
invariant mass 60 < M(ll) < 120 GeV, at least one b jet at the hadron level
with pT > 25 GeV and abs(eta) < 2.1, and a separation between the leptons and
the jets of Delta R > 0.5 is found to be 5.84 +/- 0.08 (stat.) +/- 0.72 (syst.)
+(0.25)/-(0.55) (theory) pb. The kinematic properties of the events are also
studied and found to be in agreement with the predictions made by the MadGraph
event generator with the parton shower and the hadronisation performed by
PYTHIA.Comment: Submitted to the Journal of High Energy Physic
Performance of the CMS Cathode Strip Chambers with Cosmic Rays
The Cathode Strip Chambers (CSCs) constitute the primary muon tracking device
in the CMS endcaps. Their performance has been evaluated using data taken
during a cosmic ray run in fall 2008. Measured noise levels are low, with the
number of noisy channels well below 1%. Coordinate resolution was measured for
all types of chambers, and fall in the range 47 microns to 243 microns. The
efficiencies for local charged track triggers, for hit and for segments
reconstruction were measured, and are above 99%. The timing resolution per
layer is approximately 5 ns
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