8,237 research outputs found
From Vacuum Fluctuations to Radiation: Accelerated Detectors and Black Holes
The vacuum fluctuations that induce the transitions and the thermalisation of
a uniformly accelerated two level atom are studied in detail. Their energy
content is revealed through the weak measurement formalism of Aharonov et al.
It is shown that each time the detector makes a transition it radiates a
Minkowski photon. The same analysis is then applied to the conversion of vacuum
fluctuations into real quanta in the context of black hole radiation. Initially
these fluctuations are located around the light like geodesic that shall
generate the horizon and carry zero total energy. However upon exiting from the
star they break up into two pieces one of which gradually acquires positive
energy and becomes a Hawking quantum, the other, its ''partner", ends up in the
singularity. As time goes by the vacuum fluctuations generating Hawking quanta
have exponentially large energy densities. This implies that back reaction
effects are large.Comment: definitive version, 39 pages and 5 figures available upon request
from S.M., ULB-TH 94/0
Study of Leading Hadrons in Gluon and Quark Fragmentation
The study of quark jets in e+e- reactions at LEP has demonstrated that the
hadronisation process is reproduced well by the Lund string model. However, our
understanding of gluon fragmentation is less complete. In this study enriched
quark and gluon jet samples of different purities are selected in three-jet
events from hadronic decays of the Z collected by the DELPHI experiment in the
LEP runs during 1994 and 1995. The leading systems of the two kinds of jets are
defined by requiring a rapidity gap and their sum of charges is studied. An
excess of leading systems with total charge zero is found for gluon jets in all
cases, when compared to Monte Carlo Simulations with JETSET (with and without
Bose-Einstein correlations included) and ARIADNE. The corresponding leading
systems of quark jets do not exhibit such an excess. The influence of the gap
size and of the gluon purity on the effect is studied and a concentration of
the excess of neutral leading systems at low invariant masses (<~ 2 GeV/c^2) is
observed, indicating that gluon jets might have an additional hitherto
undetected fragmentation mode via a two-gluon system. This could be an
indication of a possible production of gluonic states as predicted by QCD.Comment: 19 pages, 6 figures, Accepted by Phys. Lett.
Parametric Inference for Biological Sequence Analysis
One of the major successes in computational biology has been the unification,
using the graphical model formalism, of a multitude of algorithms for
annotating and comparing biological sequences. Graphical models that have been
applied towards these problems include hidden Markov models for annotation,
tree models for phylogenetics, and pair hidden Markov models for alignment. A
single algorithm, the sum-product algorithm, solves many of the inference
problems associated with different statistical models. This paper introduces
the \emph{polytope propagation algorithm} for computing the Newton polytope of
an observation from a graphical model. This algorithm is a geometric version of
the sum-product algorithm and is used to analyze the parametric behavior of
maximum a posteriori inference calculations for graphical models.Comment: 15 pages, 4 figures. See also companion paper "Tropical Geometry of
Statistical Models" (q-bio.QM/0311009
Minkowski Tensors in Two Dimensions - Probing the Morphology and Isotropy of the Matter and Galaxy Density Fields
We apply the Minkowski Tensor statistics to two dimensional slices of the
three dimensional density field. The Minkowski Tensors are a set of functions
that are sensitive to directionally dependent signals in the data, and
furthermore can be used to quantify the mean shape of density peaks. We begin
by introducing our algorithm for constructing bounding perimeters around
subsets of a two dimensional field, and reviewing the definition of Minkowski
Tensors. Focusing on the translational invariant statistic - a matrix - we calculate its eigenvalues for both the entire excursion
set () and for individual connected regions and holes
within the set (). The ratio of eigenvalues
informs us of the presence of global anisotropies in
the data, and is a measure of the
mean shape of peaks and troughs in the density field. We study these quantities
for a Gaussian field, then consider how they are modified by the effect of
gravitational collapse using the latest Horizon Run 4 cosmological simulation.
We find are essentially independent of gravitational collapse,
as the process maintains statistical isotropy. However, the mean shape of peaks
is modified significantly - overdensities become relatively more circular
compared to underdensities of the same area. When applying the statistic to a
redshift space distorted density field, we find a significant signal in the
eigenvalues , suggesting that they can be used to probe the
large-scale velocity field.Comment: 17 pages, accepted for publication in AP
Gluonic Meson Production
The existence of glueballs is predicted in QCD, the lightest one with quantum
numbers J^{PC}=0^{++}, but different calculations do not well agree on its mass
in the range below 1800 MeV. Several theoretical schemes have been proposed to
cope with the experimental data which often have considerable uncertainties.
Further experimental studies of the scalar meson sector are therefore important
and we discuss recent proposals to study leading clusters in gluon jets and
charmless B-decays to serve this purpose.Comment: Talk at Ringberg Workshop "New Trens in HERA Physics 2003",
Sept.28-Oct.3, 2003 (by W.O.), to appear in Proceedings, 12 pages, 2 figure
- …