156,584 research outputs found
Analytical approximation to the multidimensional Fokker--Planck equation with steady state
The Fokker--Planck equation is a key ingredient of many models in physics,
and related subjects, and arises in a diverse array of settings. Analytical
solutions are limited to special cases, and resorting to numerical simulation
is often the only route available; in high dimensions, or for parametric
studies, this can become unwieldy. Using asymptotic techniques, that draw upon
the known Ornstein--Uhlenbeck (OU) case, we consider a mean-reverting system
and obtain its representation as a product of terms, representing short-term,
long-term, and medium-term behaviour. A further reduction yields a simple
explicit formula, both intuitive in terms of its physical origin and fast to
evaluate. We illustrate a breadth of cases, some of which are `far' from the OU
model, such as double-well potentials, and even then, perhaps surprisingly, the
approximation still gives very good results when compared with numerical
simulations. Both one- and two-dimensional examples are considered.Comment: Updated version as publishe
Resonances from meson-meson scattering in U(3) CHPT
In this work, the complete one loop calculation of meson-meson scattering
amplitudes within U(3)\otimes U(3) chiral perturbation theory with explicit
resonance states is carried out for the first time. Partial waves are
unitarized from the perturbative calculation employing a non-perturbative
approach based on the N/D method. Once experimental data are reproduced in a
satisfactory way we then study the resonance properties, such as the pole
positions, corresponding residues and their N_C behaviors. The resulting N_C
dependence is the first one in the literature that takes into account the fact
that the \eta_1 becomes the ninth Goldstone boson in the chiral limit for large
N_C. Within this scheme the vector resonances studied, \rho(770), K^*(892) and
\phi(1020), follow an N_C trajectory in agreement with their standard \bar{q}q
interpretation. The scalars f_0(1370), a_0(1450) and K^*(1430) also have for
large N_C a \bar{q}q pole position trajectory and all of them tend to a bare
octet of scalar resonances around 1.4 GeV. The f_0(980) tends asymptotically to
the bare pole position of a singlet scalar resonance around 1 GeV. The \sigma,
\kappa and a_0(980) scalar resonances have a very different N_C behavior. The
case of the \sigma resonance is analyzed with special detail.Comment: 50 pages, 15 figures, 1 table. Enlarged version with more detail
comparisons with previous results in the literature. To match with accepted
version for publicatio
Orbiter/launch system
The system includes reusable turbojet propelled booster vehicles releasably connected to a reusable rocket powered orbit vehicle. The coupled orbiter-booster combination takes off horizontally and ascends to staging altitude and speed under booster power with both orbiter and booster wings providing lift. After staging, the booster vehicles fly back to Earth for horizontal landing and the orbiter vehicle continues ascending to orbit
List decoding of noisy Reed-Muller-like codes
First- and second-order Reed-Muller (RM(1) and RM(2), respectively) codes are
two fundamental error-correcting codes which arise in communication as well as
in probabilistically-checkable proofs and learning. In this paper, we take the
first steps toward extending the quick randomized decoding tools of RM(1) into
the realm of quadratic binary and, equivalently, Z_4 codes. Our main
algorithmic result is an extension of the RM(1) techniques from Goldreich-Levin
and Kushilevitz-Mansour algorithms to the Hankel code, a code between RM(1) and
RM(2). That is, given signal s of length N, we find a list that is a superset
of all Hankel codewords phi with dot product to s at least (1/sqrt(k)) times
the norm of s, in time polynomial in k and log(N). We also give a new and
simple formulation of a known Kerdock code as a subcode of the Hankel code. As
a corollary, we can list-decode Kerdock, too. Also, we get a quick algorithm
for finding a sparse Kerdock approximation. That is, for k small compared with
1/sqrt{N} and for epsilon > 0, we find, in time polynomial in (k
log(N)/epsilon), a k-Kerdock-term approximation s~ to s with Euclidean error at
most the factor (1+epsilon+O(k^2/sqrt{N})) times that of the best such
approximation
The largest oxigen bearing organic molecule repository
We present the first detection of complex aldehydes and isomers in three
typical molecular clouds located within 200pc of the center of our Galaxy.
We find very large abundances of these complex organic molecules (COMs) in
the central molecular zone (CMZ), which we attribute to the ejection of COMs
from grain mantles by shocks. The relative abundances of the different COMs
with respect to that of CH3OH are strikingly similar for the three sources,
located in very different environments in the CMZ. The similar relative
abundances point toward a unique grain mantle composition in the CMZ. Studying
the Galactic center clouds and objects in the Galactic disk having large
abundances of COMs, we find that more saturated molecules are more abundant
than the non-saturated ones. We also find differences between the relative
abundance between COMs in the CMZ and the Galactic disk, suggesting different
chemical histories of the grain mantles between the two regions in the Galaxy
for the complex aldehydes. Different possibilities for the grain chemistry on
the icy mantles in the GC clouds are briefly discussed. Cosmic rays can play an
important role in the grain chemistry. With these new detections, the molecular
clouds in the Galactic center appear to be one of the best laboratories for
studying the formation of COMs in the Galaxy.Comment: 20 pages, 4 figures, accepted in Ap
On the properties of the transition matrix in bouncing cosmologies
We elaborate further on the evolution properties of cosmological fluctuations
through a bounce. We show this evolution to be describable either by
``transmission'' and ``reflection'' coefficients or by an effective unitary
S-matrix. We also show that they behave in a time reversal invariant way.
Therefore, earlier results are now interpreted in a different perspective and
put on a firmer basis.Comment: 4 pages, 1 figure, to appear in PR
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