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Resultados filtrados por Publicador: Universidade Cornell

## The Real and Redshift Space Density Distribution Function for Large-Scale Structure in the Spherical Collapse Approximation

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Português

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We use the spherical collapse (SC) approximation to derive expressions for
the smoothed redshift-space probability distribution function (PDF), as well as
the $p$-order hierarchical amplitudes $S_p$, in both real and redshift space.
We compare our results with numerical simulations, focusing on the $\Omega=1$
standard CDM model, where redshift distortions are strongest. We find good
agreement between the SC predictions and the numerical PDF in real space even
for $\sigma_L \simgt 1$, where $\sigma_L$ is the linearly-evolved rms
fluctuation on the smoothing scale. In redshift space, reasonable agreement is
possible only for $\sigma_L \simlt 0.4$. Numerical simulations also yield a
simple empirical relation between the real-space PDF and redshift-space PDF: we
find that for $\sigma \simlt 1$, the redshift space PDF, P[\delta_z], is, to a
good approximation, a simple rescaling of the real space PDF, P[\delta], i.e.,
P[\delta/\sigma] d[\delta/\sigma] = P[\delta_z/\sigma_z] d[\delta_z/\sigma_z],
where $\sigma$ and \sigma_z are the real-space and redshift-space rms
fluctuations, respectively. This result applies well beyond the validity of
linear perturbation theory, and it is a good fit for both the standard CDM
model and the Lambda-CDM model. It breaks down for SCDM at $\sigma \approx 1$...

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## Uncertainty induced by QCD coupling in the CTEQ global analysis of parton distributions

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

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We examine the dependence of parton distribution functions (PDFs) on the
value of the QCD coupling strength $\alpha_{s}(M_{Z})$. We explain a simple
method that is rigorously valid in the quadratic approximation normally applied
in PDF fitting, and fully reproduces the correlated dependence of theoretical
cross sections on $\alpha_s$ and PDF parameters. This method is based on a
statistical relation that allows one to add the uncertainty produced by
$\alpha_s$, computed with some special PDF sets, in quadrature with the PDF
uncertainty obtained for the fixed $\alpha_s$ value (such as the CTEQ6.6 PDF
set). A series of four CTEQ6.6AS PDFs realizing this approach, for $\alpha_s$
values in the interval $0.116 \leq \alpha_{s}(M_{Z}) \leq 0.120$, is presented.
Using these PDFs, the combined $\alpha_{s}$ and PDF uncertainty is assessed for
theoretical predictions at the Fermilab Tevatron and Large Hadron Collider.; Comment: minor changes for publication

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## TMD PDFs in the Laguerre polynomial basis

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Português

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We suggest the modified matching procedure for TMD PDF to the integrated PDF
aimed to increase the amount of perturbative information in the TMD PDF
expression. The procedure consists in the selection and usage of the
non-minimal operator basis, which restricts the expansion to desired general
behavior. The implication of OPE allows to systematic account of the higher
order corrections. In the case of TMD PDF we assume the Gaussian behavior,
which suggests Laguerre polynomial basis as the best for the convergence of
OPE. We present the leading and next-to-leading expression of TMD PDF in this
basis. The obtained perturbative expression for the TMD PDF is valid in the
wide region of $b_T$ (we estimate this region as $b_T\lesssim 2-3$ GeV$^{-1}$
depending on $x$).; Comment: 19 pages, 6 figures; corrected abstract, conclusion and various
misprints; version submitted to JHEP

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## What determines the density structure of molecular clouds ? A case study of Orion B with Herschel

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 01/04/2013
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A key parameter to the description of all star formation processes is the
density structure of the gas. In this letter, we make use of probability
distribution functions (PDFs) of Herschel column density maps of Orion B,
Aquila, and Polaris, obtained with the Herschel Gould Belt survey (HGBS). We
aim to understand which physical processes influence the PDF shape, and with
which signatures. The PDFs of Orion B (Aquila) show a lognormal distribution
for low column densities until Av 3 (6), and a power-law tail for high column
densities, consistent with a rho r^-2 profile for the equivalent spherical
density distribution. The PDF of Orion B is broadened by external compression
due to the nearby OB stellar aggregates. The PDF of a quiescent subregion of
the non-star-forming Polaris cloud is nearly lognormal, indicating that
supersonic turbulence governs the density distribution. But we also observe a
deviation from the lognormal shape at Av>1 for a subregion in Polaris that
includes a prominent filament. We conclude that (i) the point where the PDF
deviates from the lognormal form does not trace a universal Av-threshold for
star formation, (ii) statistical density fluctuations, intermittency and
magnetic fields can cause excess from the lognormal PDF at an early cloud
formation stage...

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## The Optimal Input Distribution for Partial Decode-and-Forward in the MIMO Relay Channel

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 30/09/2014
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This paper considers the partial decode-and-forward (PDF) strategy for the
Gaussian multiple-input multiple-output (MIMO) relay channel. Unlike for the
decode-and-forward (DF) strategy or point-to-point (P2P) transmission, for
which Gaussian channel inputs are known to be optimal, the input distribution
that maximizes the achievable PDF rate for the Gaussian MIMO relay channel has
remained unknown so far. For some special cases, e.g., for relay channels where
the optimal PDF strategy reduces to DF or P2P transmission, it could be deduced
that Gaussian inputs maximize the PDF rate. For the general case, however, the
problem has remained open until now. In this work, we solve this problem by
proving that the maximum achievable PDF rate for the Gaussian MIMO relay
channel is always attained by Gaussian channel inputs. Our proof relies on the
channel enhancement technique, which was originally introduced by Weingarten et
al. to derive the (private message) capacity region of the Gaussian MIMO
broadcast channel. By combining this technique with a primal decomposition
approach, we first establish that jointly Gaussian source and relay inputs
maximize the achievable PDF rate for the aligned Gaussian MIMO relay channel.
Subsequently, we use a limiting argument to extend this result from the aligned
to the general Gaussian MIMO relay channel.; Comment: 23 pages...

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## The PDF4LHC report on PDFs and LHC data: Results from Run I and preparation for Run II

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 02/07/2015
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The accurate determination of the Parton Distribution Functions (PDFs) of the
proton is an essential ingredient of the Large Hadron Collider (LHC) program.
PDF uncertainties impact a wide range of processes, from Higgs boson
characterisation and precision Standard Model measurements to New Physics
searches. A major recent development in modern PDF analyses has been to exploit
the wealth of new information contained in precision measurements from the LHC
Run I, as well as progress in tools and methods to include these data in PDF
fits. In this report we summarise the information that PDF-sensitive
measurements at the LHC have provided so far, and review the prospects for
further constraining PDFs with data from the recently started Run II. This
document aims to provide useful input to the LHC collaborations to prioritise
their PDF-sensitive measurements at Run II, as well as a comprehensive
reference for the PDF-fitting collaborations.; Comment: 55 pages, 13 figures

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## Cosmological and astrophysical constraints from the Lyman-alpha forest flux probability distribution function

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 16/07/2009
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We use the probability distribution function (PDF) of the lya forest flux at
z=2-3, measured from high-resolution UVES/VLT data, and hydrodynamical
simulations to obtain constraints on cosmological parameters and the thermal
state of the intergalactic medium (IGM) at z 2-3. The observed flux PDF at z=3
alone results in constraints on cosmological parameters in good agreement with
those obtained from the WMAP data, albeit with about a factor two larger
errors. The observed flux PDF is best fit with simulations with a matter
fluctuation amplitude of sigma_8=0.8-0.85 \pm 0.07 and an inverted IGM
temperature-density relation (gamma ~ 0.5-0.75), consistent with our previous
results obtained using a simpler analysis. These results appear to be robust to
uncertainties in the quasar (QSO) continuum placement. We further discuss
constraints obtained by a combined analysis of the high-resolution flux PDF and
the power spectrum measured from the Sloan Digital Sky Survey (SDSS) lya forest
data. The joint analysis confirms the suggestion of an inverted
temperature-density relation, but prefers somewhat higher values (sigma_8 ~
0.9) of the matter fluctuation amplitude than the WMAP data and the best fit to
the flux PDF alone. The joint analysis of the flux PDF and power spectrum (as
well as an analysis of the power spectrum data alone) prefers rather large
values for the temperature of the IGM...

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## A posteriori inclusion of PDFs in NLO QCD final-state calculations

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 25/10/2005
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Any NLO calculation of a QCD final-state observable involves Monte Carlo
integration over a large number of events. For DIS and hadron colliders this
must usually be repeated for each new PDF set, making it impractical to
consider many `error' PDF sets, or carry out PDF fits. Here we discuss ``a
posteriori'' inclusion of PDFs, whereby the Monte Carlo run calculates a grid
(in x and Q) of cross section weights that can subsequently be combined with an
arbitrary PDF. The procedure is numerically equivalent to using an interpolated
form of the PDF. The main novelty relative to prior work is the use of
higher-order interpolation, which substantially improves the tradeoff between
accuracy and memory use. An accuracy of about 0.01% has been reached for the
single inclusive cross-section in the central rapidity region |y|<0.5 for jet
transverse momenta from 100 to 5000 GeV. This method should facilitate the
consistent inclusion of final-state data from HERA, Tevatron and LHC in PDF
fits, thus helping to increase the sensitivity of LHC to deviations from
standard Model predictions.; Comment: contribution to the CERN DESY workshop on "HERA and LHC"

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## Geometrically Consistent Approach to Stochastic DBI Inflation

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 21/04/2010
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#High Energy Physics - Theory#Astrophysics - Cosmology and Nongalactic Astrophysics#General Relativity and Quantum Cosmology#High Energy Physics - Phenomenology

Stochastic effects during inflation can be addressed by averaging the quantum
inflaton field over Hubble-patch sized domains. The averaged field then obeys a
Langevin-type equation into which short-scale fluctuations enter as a noise
term. We solve the Langevin equation for a inflaton field with Dirac Born
Infeld (DBI) kinetic term perturbatively in the noise and use the result to
determine the field value's Probability Density Function (PDF). In this
calculation, both the shape of the potential and the warp factor are arbitrary
functions, and the PDF is obtained with and without volume effects due to the
finite size of the averaging domain. DBI kinetic terms typically arise in
string-inspired inflationary scenarios in which the scalar field is associated
with some distance within the (compact) extra dimensions. The inflaton's
accessible range of field values therefore is limited because of the extra
dimensions' finite size. We argue that in a consistent stochastic approach the
distance-inflaton's PDF must vanish for geometrically forbidden field values.
We propose to implement these extra-dimensional spatial restrictions into the
PDF by installing absorbing (or reflecting) walls at the respective boundaries
in field space. As a toy model...

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## Probability Distribution Function of Cosmological Density Fluctuations from Gaussian Initial Condition: Comparison of One- and Two-point Log-normal Model Predictions with N-body Simulations

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

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We quantitatively study the probability distribution function (PDF) of
cosmological nonlinear density fluctuations from N-body simulations with
Gaussian initial condition. In particular, we examine the validity and
limitations of one-point and two-point log-normal PDF models against those
directly estimated from the simulations. We find that the one-point log-normal
PDF describes very accurately the cosmological density distribution even in the
nonlinear regime (the rms variance \sigma_{nl} \simlt 4 and the over-density
\delta \simlt 100). Furthermore the two-point log-normal PDFs are also in good
agreement with the simulation data from linear to fairly nonlinear regime,
while slightly deviate from them for \delta \simlt -0.5. Thus the log-normal
PDF can be used as a useful empirical model for the cosmological density
fluctuations. While this conclusion is fairly insensitive to the shape of the
underlying power spectrum of density fluctuations P(k), models with substantial
power on large scales, i.e., n\equiv d\ln P(k)/d \ln k \simlt -1, are better
described by the log-normal PDF. On the other hand, we note that the one-to-one
mapping of the initial and the evolved density fields consistent with the
log-normal model does not approximate the broad distribution of their mutual
correlation even on average. Thus the origin of the phenomenological log-normal
PDF approximation still remains to be understood.; Comment: 25 pages...

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## Density structure of the interstellar medium and the star formation rate in galactic disks

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 21/01/2007
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The probability distribution functions (PDF) of density of the ISM in
galactic disks and global star formation rate are discussed. Three-dimensional
hydrodynamic simulations show that the PDFs in globally stable, inhomogeneous
ISM in galactic disks are well fitted by a single log-normal function over a
wide density range. The dispersion of the log-normal PDF (LN-PDF) is larger for
more gas-rich systems, and whereas the characteristic density of LN-PDF, for
which the volume fraction becomes the maximum, does not significantly depend on
the initial conditions. %At the characteristic density, thermal pressure and
kinetic %pressure due to turbulent motion are comparable, and the material is
%statistically stagnated in a turbulent flow. Supposing the galactic ISM is
characterized by the LN-PDF, we give a global star formation rate (SFR) as a
function of average gas density, a critical local density for star formation,
and star formation efficiency. We find that the observed SFR is well-fitted by
the theoretical SFR in a wide range of the global gas density ($10 - 10^4
M_\odot$ pc$^{-2}$). Star formation efficiency (SFE) for high density gas ($n >
10^3$ cm$^{-3}$) is SFE $= 0.001 - 0.01$ for normal spiral galaxies, and SFE $=
0.01 - 0.1$ for starburst galaxies. The LN-PDF and SFR proposed here could be
applicable for modeling star formation on a kpc-scale in galaxies or numerical
simulations of galaxy formation...

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## On the probability distribution function of the mass surface density of molecular clouds. II

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 19/09/2014
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The probability distribution function (PDF) of the mass surface density of
molecular clouds provides essential information about the structure of
molecular cloud gas and condensed structures out of which stars may form. In
general, the PDF shows two basic components: a broad distribution around the
maximum with resemblance to a log-normal function, and a tail at high mass
surface densities attributed to turbulence and self-gravity. In a previous
paper, the PDF of condensed structures has been analyzed and an analytical
formula presented based on a truncated radial density profile, $\rho(r) =
\rho_c/(1+(r/r_0)^2)^{n/2}$ with central density $\rho_c$ and inner radius
$r_0$, widely used in astrophysics as a generalization of physical density
profiles. In this paper, the results are applied to analyze the PDF of
self-gravitating, isothermal, pressurized, spherical (Bonnor-Ebert spheres) and
cylindrical condensed structures with emphasis on the dependence of the PDF on
the external pressure $p_{ext}$ and on the overpressure $q^{-1} =p_c /p_{ext}$,
where $p_c$ is the central pressure. Apart from individual clouds, we also
consider ensembles of spheres or cylinders, where effects caused by a variation
of pressure ratio, a distribution of condensed cores within a turbulent gas...

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## Luminosity Distribution of Gamma-Ray Burst Host Galaxies at redshift z=1 in Cosmological Smoothed Particle Hydrodynamic Simulations: Implications for the Metallicity Dependence of GRBs

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

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We study the relationship between the metallicity of gamma-ray burst (GRB)
progenitors and the probability distribution function (PDF) of GRB host
galaxies as a function of luminosity using cosmological hydrodynamic
simulations of galaxy formation. We impose a maximum limit to the gas
metallicity in which GRBs can occur, and examine how the predicted luminosity
PDF of GRB host galaxies changes in the simulation. We perform the
Kolmogorov-Smirnov test, and show that the result from our simulation agrees
with the observed luminosity PDF of core-collapse supernovae (SNe) host
galaxies when we assume that the core-collapse SNe trace star formation. When
we assume that GRBs occur only in a low-metallicity environment with $Z\lesssim
0.1 \Zsun$, GRBs occur in lower luminosity galaxies, and the simulated
luminosity PDF becomes quantitatively consistent with the observed luminosity
PDF. The observational bias against the host galaxies of optically dark GRBs
owing to dust extinction may be another reason for the lower luminosities of
GRB host galaxies, but the observed luminosity PDF of GRB host galaxies cannot
be reproduced solely by the dust bias in our simulation.; Comment: 11 pages, 14 figures, minor revisions, one added figure, accepted for
publication in ApJ

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## Statistical Equilibria of Uniformly Forced Advection Condensation

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

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#Nonlinear Sciences - Pattern Formation and Solitons#Nonlinear Sciences - Chaotic Dynamics#Physics - Atmospheric and Oceanic Physics#Physics - Fluid Dynamics

We examine the state of statistical equilibrium attained by a uniformly
forced condensable substance subjected to advection in a periodic domain. In
particular, we examine the probability density function (\pdf{}) of the
condensable substance in the limit of rapid condensation. The constraints
imposed by this limit are pointed out and are shown to result in a \pdf{} --
whenever the advecting velocity field admits a diffusive representation -- that
features a peak at small values, decays exponentially and terminates in a rapid
"roll-off" near saturation. Possible physical implications of this feature as
compared to a \pdf{} which continues to decay slowly are pointed out. A set of
simple numerical exercises which employ lattice maps for purposes of advection
are performed to test these features. Despite the simplicity of the model, the
derived \pdf{} is seen to compare favourably with \pdf{}'s constructed from
isentropic specific humidity data. Further, structure functions associated with
the condensable field are seen to scale anomalously with near saturation of the
scaling exponents for high moments -- a feature which agrees with studies of
high resolution aircraft data.; Comment: 10 pages, 5 figs, re-revised version -- clearer formulation of the
pdf equation. Comments on physical implications etc

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## Towards the determination of the photon parton distribution function constrained by LHC data

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 03/07/2013
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We provide a discussion of the impact of a subset of Drell-Yan data from LHC
on the determination of the photon parton distribution function (PDF), using
the NNPDF methodology. In previous work we have shown that the photon PDF
determined from deep-inelastic scattering (DIS) data only has large
uncertainties, suggesting the need for more data from other processes such as
Drell-Yan, which unlike DIS, includes photon-induced contributions at leading
order in QED. We describe the inclusion of ATLAS Drell-Yan W, Z data, which is
a subset of the LHC data used in a final photon PDF determination, by means of
a reweighting procedure. We show the impact of such data by comparing the
reweighted photon PDF with the photon PDF from DIS, highlighting the reduction
of uncertainties at medium/small-x. We conclude that the Drell-Yan data from
LHC allows a reasonably accurate determination of the photon PDF.; Comment: 5 pages, 10 figures, to appear in the proceedings of the XXI
International Workshop on Deep-Inelastic Scattering and Related Subjects
(DIS2013), Marseille, 22-26 April 2013

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