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## Turbulent Velocity Structure in Molecular Clouds

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Português

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We compare velocity structure in the Polaris Flare molecular cloud at scales
ranging from 0.015 pc to 20 pc to simulations of supersonic hydrodynamic and
MHD turbulence computed with the ZEUS MHD code. We use several different
statistical methods to compare models and observations. The Delta-variance
wavelet transform is most sensitive to characteristic scales and scaling laws,
but is limited by a lack of intensity weighting. The scanning-beam
size-linewidth relation is more robust with respect to noisy data. Obtaining
the global velocity scaling behaviour requires that large-scale trends in the
maps not be removed but treated as part of the turbulent cascade. We compare
the true velocity PDF in our models to velocity centroids and average line
profiles in optically thin lines, and find that the line profiles reflect the
true PDF better unless the map size is comparable to the total line-of-sight
thickness of the cloud. Comparison of line profiles to velocity centroid PDFs
can thus be used to measure the line-of-sight depth of a cloud. The observed
density and velocity structure is consistent with supersonic turbulence with a
driving scale at or above the size of the molecular cloud and dissipative
processes below 0.05 pc. Ambipolar diffusion could explain the dissipation. The
velocity PDFs exclude small-scale driving such as that from stellar outflows as
a dominant process in the observed region. In the models...

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## Closure testing NNPDF3.0 with LHC observables

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 24/06/2015
Português

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A thorough understanding of PDFs and their uncertainties is important for the
LHC and for future collider experiments. The recently released NNPDF3.0 set was
presented alongside results from closure tests, where PDF fits were performed
on pseudo-data generated from a chosen input PDF set. The results there
demonstrate the validity of the NNPDF methodology and also provide some
information about different contributions to the PDF uncertainties. Here I
present a number of additional closure test results, specifically an
investigation into the effect of using cross-validation in the fits, and an
assessment of the successful reproduction of LHC cross-sections in closure
tests. The results are consistent with those previously shown in the NNPDF3.0
paper.; Comment: 6 pages, 4 figures, to appear in the proceedings of the XXIII
International Workshop on Deep-Inelastic Scattering and Related Subjects
(DIS2015), Dallas, Texas, 27 April - 1 May 2015

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## Nonequilibrium Green's function approach to the pair distribution function of quantum many-body systems out of equilibrium

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 01/11/2012
Português

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The pair distribution function (PDF) is a key quantity for the analysis of
correlation effects of a quantum system both in equilibrium and far from
equilibrium. We derive an expression for the PDF in terms of the
single-particle Green's functions---the solutions of the Keldysh/Kadanoff-Baym
equations in the two-time plane---for a one- or two-component system. The
result includes initial correlations and generalizes previous density matrix
expressions from single-time quantum kinetic theory. Explicit expressions for
the PDF are obtained in second Born approximation.

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## Interpretations of some parameter dependent generalizations of classical matrix ensembles

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 18/11/2002
Português

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Two types of parameter dependent generalizations of classical matrix
ensembles are defined by their probability density functions (PDFs). As the
parameter is varied, one interpolates between the eigenvalue PDF for the
superposition of two classical ensembles with orthogonal symmetry and the
eigenvalue PDF for a single classical ensemble with unitary symmetry, while the
other interpolates between a classical ensemble with orthogonal symmetry and a
classical ensemble with symplectic symmetry. We give interpretations of these
PDFs in terms of probabilities associated to the continuous
Robinson-Schensted-Knuth correspondence between matrices, with entries chosen
from certain exponential distributions, and non-intersecting lattice paths, and
in the course of this probability measures on partitions and pairs of
partitions are identified. The latter are generalized by using Macdonald
polynomial theory, and a particular continuum limit -- the Jacobi limit -- of
the resulting measures is shown to give PDFs related to those appearing in the
work of Anderson on the Selberg integral. By interpreting Anderson's work as
giving the PDF for the zeros of a certain rational function, it is then
possible to identify random matrices whose eigenvalue PDFs realize the original
parameter dependent PDFs. This line of theory allows sampling of the original
parameter dependent PDFs...

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## Time evolution of probability density function of gamma ray burst (GRB) - a possible indication of turbulence origin of GRB

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 11/11/2011
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#Astrophysics - High Energy Astrophysical Phenomena#Physics - Data Analysis, Statistics and Probability

Gamma ray burst (GRB) time series is a non-stationary time series with all
its statistical properties varying with time. Considering that each GRB is a
different manifestation of the same stochastic process we studied the time
dependent as well as time averaged probability density function (\emph{pdf})
characterizing the underlying stochastic process. The \emph{pdf}s are fitted
with Gaussian distribution function and it has been argued that the Gaussian
\emph{pdf}s possibly indicate the turbulence origin of GRB. The spectral and
temporal evolution of GRBs are also studied through the evolution of spectral
forms, color-color diagrams and hysteresis loops. The results do not contradict
the turbulence interpretation of GRB.; Comment: Accepted for publication in MNRAS

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## Spatial Extent of Branching Brownian Motion

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Português

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We study the one dimensional branching Brownian motion starting at the origin
and investigate the correlation between the rightmost ($X_{\max}\geq 0$) and
leftmost ($X_{\min} \leq 0$) visited sites up to time $t$. At each time step
the existing particles in the system either diffuse (with diffusion constant
$D$), die (with rate $a$) or split into two particles (with rate $b$). We focus
on the regime $b \leq a$ where these two extreme values $X_{\max}$ and
$X_{\min}$ are strongly correlated. We show that at large time $t$, the joint
probability distribution function (PDF) of the two extreme points becomes
stationary $P(X,Y,t \to \infty) \to p(X,Y)$. Our exact results for $p(X,Y)$
demonstrate that the correlation between $X_{\max}$ and $X_{\min}$ is nonzero,
even in the stationary state. From this joint PDF, we compute exactly the
stationary PDF $p(\zeta)$ of the (dimensionless) span $\zeta = {(X_{\max} -
X_{\min})}/{\sqrt{D/b}}$, which is the distance between the rightmost and
leftmost visited sites. This span distribution is characterized by a linear
behavior ${p}(\zeta) \sim \frac{1}{2} \left(1 + \Delta \right) \zeta$ for small
spans, with $\Delta = \left(\frac{a}{b} -1\right)$. In the critical case
($\Delta = 0$) this distribution has a non-trivial power law tail ${p}(\zeta)
\sim 8 \pi \sqrt{3} /\zeta^3$ for large spans. On the other hand...

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## Non-Gaussianity in Cosmic Microwave Background Temperature Fluctuations from Cosmic (Super-)Strings

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Português

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We compute analytically the small-scale temperature fluctuations of the
cosmic microwave background from cosmic (super-)strings and study the
dependence on the string intercommuting probability $P$. We develop an
analytical model which describes the evolution of a string network and
calculate the numbers of string segments and kinks in a horizon volume. Then we
derive the probability distribution function (pdf) which takes account of
finite angular resolution of observation. The resultant pdf consists of a
Gaussian part due to frequent scatterings by long string segments and a
non-Gaussian tail due to close encounters with kinks. The dispersion of the
Gaussian part is reasonably consistent with that obtained by numerical
simulations by Fraisse et al.. On the other hand, the non-Gaussian tail
contains two phenomenological parameters which are determined by comparison
with the numerical results for P=1. Extrapolating the pdf to the cases with
$P<1$, we predict that the non-Gaussian feature is suppressed for small $P$.; Comment: 6 pages, 2 figure, version accepted by JCAP

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## Single-file dynamics with different diffusion constants

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Português

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We investigate the single-file dynamics of a tagged particle in a system
consisting of N hardcore interacting particles (the particles cannot pass each
other) which are diffusing in a one-dimensional system where the particles have
different diffusion constants. For the two particle case an exact result for
the conditional probability density function (PDF) is obtained for arbitrary
initial particle positions and all times. The two-particle PDF is used to
obtain the tagged particle PDF. For the general N-particle case (N large) we
perform stochastic simulations using our new computationally efficient
stochastic simulation technique based on the Gillespie algorithm. We find that
the mean square displacement for a tagged particle scales as the square root of
time (as for identical particles) for long times, with a prefactor which
depends on the diffusion constants for the particles; these results are in
excellent agreement with very recent analytic predictions in the mathematics
literature.; Comment: 9 pages, 5 figures. Journal of Chemical Physics (in press)

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## Crystal structure solution from experimentally determined atomic pair distribution functions

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 04/03/2010
Português

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The paper describes an extension of the Liga algorithm for structure solution
from atomic pair distribution function (PDF), to handle periodic crystal
structures with multiple elements in the unit cell. The procedure is performed
in 2 separate steps - at first the Liga algorithm is used to find unit cell
sites consistent with pair distances extracted from the experimental PDF. In
the second step the assignment of atom species over cell sites is solved by
minimizing the overlap of their empirical atomic radii. The procedure has been
demonstrated on synchrotron x-ray PDF data from 16 test samples. The structure
solution was successful for 14 samples including cases with enlarged super
cells. The algorithm success rate and the reasons for failed cases are
discussed together with enhancements that should improve its convergence and
usability.; Comment: 16 pages, 1 figure, submitted to Journal of Applied Crystallography

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## Notes on the sum and maximum of independent exponentially distributed random variables with different scale parameters

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 15/07/2013
Português

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We consider the distribution of the sum and the maximum of a collection of
independent exponentially distributed random variables. The focus is laid on
the explicit form of the density functions (pdf) of non-i.i.d. sequences. Those
are recovered in a simple and direct way based on conditioning. A connection
between the pdf and a representation of the convolution characteristic function
as a linear combination of the single characteristic functions is drawn. It is
demonstrated how the results on the pdf of order statistics and the convolution
merge.

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## The stellar population histories of early-type galaxies. III. The Coma Cluster

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 04/03/2008
Português

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We present stellar population parameters of twelve early-type galaxies (ETGs)
in the Coma Cluster based on spectra obtained using the Low Resolution Imaging
Spectrograph on the Keck II Telescope. Our data allow us to examine in detail
the zero-point and scatter in their stellar population properties. Our ETGs
have SSP-equivalent ages of on average 5-8 Gyr with the models used here, with
the oldest galaxies having ages of ~10 Gyr old. This average age is identical
to the mean age of field ETGs. Our ETGs span a large range in velocity
dispersion but are consistent with being drawn from a population with a single
age. Specifically, ten of the twelve ETGs are consistent within their formal
errors of having the same age, 5.2+/-0.2 Gyr, over a factor of more than 750 in
mass. We therefore find no evidence for downsizing of the stellar populations
of ETGs in the core of the Coma Cluster. We suggest that Coma Cluster ETGs may
have formed the majority of their mass at high redshifts but suffered small but
detectable star formation events at z~0.1-0.3. Previous detections of
'downsizing' from stellar populations of local ETGs may not reflect the same
downsizing seen in lookback studies of RSGs, as the young ages of the local
ETGs represent only a small fraction of their total masses. (abridged); Comment: 49 pages...

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## On the Initial Conditions for Star Formation and the IMF

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 25/02/2011
Português

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Density probability distribution functions (PDFs) for turbulent
self-gravitating clouds should be convolutions of the local log-normal PDF,
which depends on the local average density rho-ave and Mach number M, and the
probability distribution functions for rho-ave and M, which depend on the
overall cloud structure. When self-gravity drives a cloud to increased central
density, the total PDF develops an extended tail. If there is a critical
density or column density for star formation, then the fraction of the local
mass exceeding this threshold becomes higher near the cloud center. These
elements of cloud structure should be in place before significant star
formation begins. Then the efficiency is high so that bound clusters form
rapidly, and the stellar initial mass function (IMF) has an imprint in the gas
before destructive radiation from young stars can erase it. The IMF could arise
from a power-law distribution of mass for cloud structure. These structures
should form stars down to the thermal Jeans mass MJ at each density in excess
of a threshold. The high-density tail of the PDF, combined with additional
fragmentation in each star-forming core, extends the IMF into the Brown Dwarf
regime. The core fragmentation process is distinct from the cloud structuring
process and introduces an independen core fragmentation mass function (CFMF).
The CFMF would show up primarily below the IMF peak.; Comment: 16 pages...

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## Finite N corrections to the limiting distribution of the smallest eigenvalue of Wishart complex matrices

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 08/06/2015
Português

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We study the probability distribution function (PDF) of the smallest
eigenvalue of Laguerre-Wishart matrices $W = X^\dagger X$ where $X$ is a random
$M \times N$ ($M \geq N$) matrix, with complex Gaussian independent entries. We
compute this PDF in terms of semi-classical orthogonal polynomials, which are
deformations of Laguerre polynomials. By analyzing these polynomials, and their
associated recurrence relations, in the limit of large $N$, large $M$ with $M/N
\to 1$ -- i.e. for quasi-square large matrices $X$ -- we show that this PDF, in
the hard edge limit, can be expressed in terms of the solution of a Painlev\'e
III equation, as found by Tracy and Widom, using Fredholm operators techniques.
Furthermore, our method allows us to compute explicitly the first $1/N$
corrections to this limiting distribution at the hard edge. Our computations
confirm a recent conjecture by Edelman, Guionnet and P\'ech\'e. We also study
the soft edge limit, when $M-N \sim {\cal O}(N)$, for which we conjecture the
form of the first correction to the limiting distribution of the smallest
eigenvalue.; Comment: 28 pages, 2 Figures

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## Universal features and tail analysis of the order-parameter distribution of the two-dimensional Ising model: An entropic sampling Monte Carlo study

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 15/09/2008
Português

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We present a numerical study of the order-parameter probability density
function (PDF) of the square Ising model for lattices with linear sizes
$L=80-140$. A recent efficient entropic sampling scheme, combining the
Wang-Landau and broad histogram methods and based on the high-levels of the
Wang-Landau process in dominant energy subspaces is employed. We find that for
large lattices there exists a stable window of the scaled order-parameter in
which the full ansatz including the pre-exponential factor for the tail regime
of the universal PDF is well obeyed. This window is used to estimate the
equation of state exponent and to observe the behavior of the universal
constants implicit in the functional form of the universal PDF. The probability
densities are used to estimate the universal Privman-Fisher coefficient and to
investigate whether one could obtain reliable estimates of the universal
constants controlling the asymptotic behavior of the tail regime.; Comment: 24 pages, 5 figures

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## Statistics of surface gravity wave turbulence in the space and time domains

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 16/11/2008
Português

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We present experimental results on simultaneous space-time measurements for
the gravity wave turbulence in a large laboratory flume. We compare these
results with predictions of the weak turbulence theory (WTT) based on random
waves, as well as with predictions based on the coherent singular wave crests.
We see that both wavenumber and the frequency spectra are not universal and
dependent on the wave strength, with some evidence in favor of WTT at larger
wave intensities when the finite flume effects are minimal. We present further
theoretical analysis of the role of the random and coherent waves in the wave
probability density function (PDF) and the structure functions (SFs). Analyzing
our experimental data we found that the random waves and the coherent
structures/breaks coexist: the former show themselves in a quasi-gaussian PDF
core and in the low-order SFs, and the latter - in the PDF tails and the
high-order SF's. It appears that the x-space signal is more intermittent than
the t-space signal, and the x-space SFs capture more singular coherent
structures than do the t-space SFs. We outline an approach treating the
interactions of these random and coherent components as a turbulence cycle
characterized by the turbulence fluxes in both the wavenumber and the amplitude
spaces.; Comment: 29 pages...

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## Uncertainties of the Inclusive Higgs Production Cross Section at the Tevatron and the LHC

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 20/08/2005
Português

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We study uncertainties of the predicted inclusive Higgs production cross
section due to the uncertainties of parton distribution functions (PDF).
Particular attention is given to bbH Yukawa coupling enhanced production
mechanisms in beyond SM scenarios, such as MSSM. The PDF uncertainties are
determined by the robust Lagrange Multiplier method within the CTEQ global
analysis framework. We show that PDF uncertainties dominate over theoretical
uncertainties of the perturbative calculation (usually estimated by the scale
dependence of the calculated cross sections), except for low Higgs masses at
LHC. Thus for the proper interpretation of any Higgs signal, and for better
understanding of the underlying electroweak symmetry breaking mechanism, it is
important to gain better control of the uncertainties of the PDFs.; Comment: LaTeX, JHEP, 19 pages, 14 figures

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## Evaluation of Symmetric Mutual Information of the Simplified TDMR Channel Model

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 31/03/2015
Português

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In the present paper, a simplified two-dimensional magnetic recording (TDMR)
channel model is proposed in order to capture the qualitative features of
writing and read-back processes of TDMR systems.The proposed channel model
incorporates the effects of both linear interference from adjacent bit-cells
and signal-dependent noise due to irregular grain boundaries between adjacent
bit-cells. The simplicity of the proposed model enables us to derive the closed
form of the conditional PDF representing the probabilistic nature of the
channel. The conditional PDF is Gaussian distributed and is parameterized by a
signal-dependent covariance matrix. Based on this conditional PDF, a Monte
Carlo method for approximating the symmetric mutual information of this channel
is developed. The symmetric mutual information is closely related to the areal
density limit for TDMR systems. The numerical results suggest that we may need
low-rate coding, e.g., 2/3 or 1/2, when the jitter-like noise becomes dominant.

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## HERAFitter, Open Source QCD Fit Project

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Português

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HERAFitter is an open-source package that provides a framework for the
determination of the parton distribution functions (PDFs) of the proton and for
many different kinds of analyses in Quantum Chromodynamics (QCD). It encodes
results from a wide range of experimental measurements in lepton-proton deep
inelastic scattering and proton-proton (proton-antiproton) collisions at hadron
colliders. These are complemented with a variety of theoretical options for
calculating PDF-dependent cross section predictions corresponding to the
measurements. The framework covers a large number of the existing methods and
schemes used for PDF determination. The data and theoretical predictions are
brought together through numerous methodological options for carrying out PDF
fits and plotting tools to help visualise the results. While primarily based on
the approach of collinear factorisation, HERAFitter also provides facilities
for fits of dipole models and transverse-momentum dependent PDFs. The package
can be used to study the impact of new precise measurements from hadron
colliders. This paper describes the general structure of HERAFitter and its
wide choice of options.; Comment: 18 pages, 7 figures

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## The Decay of Passive Scalars Under the Action of Single Scale Smooth Velocity Fields in Bounded 2D Domains : From non self similar pdf's to self similar eigenmodes

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Português

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#Nonlinear Sciences - Chaotic Dynamics#Physics - Atmospheric and Oceanic Physics#Physics - Fluid Dynamics

We examine the decay of passive scalars with small, but non zero, diffusivity
in bounded 2D domains. The velocity fields responsible for advection are smooth
(i.e., they have bounded gradients) and of a single large scale. Moreover, the
scale of the velocity field is taken to be similar to the size of the entire
domain. The importance of the initial scale of variation of the scalar field
with respect to that of the velocity field is strongly emphasized. If these
scales are comparable and the velocity field is time periodic, we see the
formation of a periodic scalar eigenmode. The eigenmode is numerically realized
by means of a deterministic 2D map on a lattice. Analytical justification for
the eigenmode is available from theorems in the dynamo literature. Weakening
the notion of an eigenmode to mean statistical stationarity, we provide
numerical evidence that the eigenmode solution also holds for aperiodic flows
(represented by random maps). Turning to the evolution of an initially small
scale scalar field, we demonstrate the transition from an evolving (i.e., {\it
non} self similar) pdf to a stationary (self similar) pdf as the scale of
variation of the scalar field progresses from being small to being comparable
to that of the velocity field (and of the domain). Furthermore...

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## The 1-Point Cluster Distribution Function and its Moments

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 02/09/1994
Português

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We derive the 1-point probability density function of the smoothed 3-D
Abell-ACO cluster density field and we compare it with that of artificial
cluster samples, generated as high peaks of a Gaussian field in such a way that
they reproduce the low-order (2- and 3-point) correlation functions and the
observed cluster selection functions. We find that both real and simulated {\em
pdf}'s are well approximated by a log-normal distribution even when the
Gaussian smoothing radius is as large as 40 $h^{-1}$ Mpc. Furthermore the
low-order moments of the {\em pdf} are found to obey a relation $\gamma = S_{3}
\sigma^4$, with $\gamma$ being the skewness and $S_{3}\approx 1.8$. These
results are consistent with clusters being high-peaks of an underlying initial
Gaussian density field. A by-product of our analysis is that when we rescale
the {\em pdf} cluster moments to those of the QDOT-IRAS galaxies, using linear
biasing with $b_{cI}\sim 4.5$ and for the common smoothing radius of 20
$h^{-1}$ Mpc, we find them to be significantly smaller than those directly
estimated from the QDOT data by Saunders et al. (1991).; Comment: LaTex file. Submitted in MNRAS June 1994. Figures upon request

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