420 research outputs found
Expectations for the Difference Between Local and Global Measurements of the Hubble Constant
There are irreducible differences between the Hubble constant measured
locally and the global value. They are due to density perturbations and finite
sample volume (cosmic variance) and finite number of objects in the sample
(sampling variance). We quantify these differences for a suite of
COBE-normalized CDM models that are consistent with the observed large-scale
structure. For small samples of objects that only extend out to 10,000 km/sec,
the variance can approach 4%. For the largest samples of Type Ia supernovae
(SNeIa), which include about 40 objects and extend out to almost 40,000 km/sec,
the variance is 1-2% and is dominated by sampling variance. Sampling and cosmic
variance may be an important consideration in comparing local determinations of
the Hubble constant with precision determinations of the global value that will
be made from high-resolution maps of CBR anisotropy.Comment: 10 pages, Latex, 2 figures, version accepted for Ap.
Comment on "Does Gluons Carry Half of the Nucleon Momentum?" by X. S. Chen et. al. (PRL103, 062001 (2009))
The authors claim to have found a "proper", "gauge-invariant" definition of a
charged-particle's momentum in gauge theory, which is more "superior" than the
textbook version. I show that their result arises from a misunderstanding of
gauge symmetry by generalizing the Coulomb gauge result indiscriminately and is
not physical
Comparison of Models of Critical Opacity in the Quark-Gluon Plasma
In this work we discuss two methods of calculation of quark propagation in
the quark-gluon plasma. Both methods make use of the Nambu-Jona-Lasinio model.
The essential difference of these calculations is the treatment of
deconfinement. A model of confinement is not included in the work of Gastineau,
Blanquier and Aichelin [hep-ph/0404207], however, the meson states they
consider are still bound for temperatures greater than the deconfinement
temperature T_c. On the other hand, our model deals with unconfined quarks and
includes a description of the q(bar)q resonances found in lattice QCD studies
that make use of the maximum entropy method (MEM). We compare the q{bar)q cross
sections calculated in these models.Comment: 7 pages and 4 figures RevTe
Probing Quark Distribution Amplitudes Through Generalized Parton Distributions at Large Momentum Transfer
In the large momentum transfer limit, generalized parton distributions can be
calculated through a QCD factorization theorem which involves
perturbatively-calculable hard kernels and light-cone parton distribution
amplitudes of hadrons. We illustrate this through the
distribution for the pion and proton, presenting the hard kernels at leading
order. As a result, experimental data on the generalized parton distributions
in this regime can be used to determine the functional form of the parton
distribution amplitudes which has thus far been quite challenging to obtain.
Our result can also be used as a constraint in phenomenological GPD
parametrizations.Comment: 7 pages, 4 figures; new references and figure added, errors correcte
Bayesian variable selection for high dimensional predictors and self-reported outcomes
BACKGROUND: The onset of silent diseases such as type 2 diabetes is often registered through self-report in large prospective cohorts. Self-reported outcomes are cost-effective; however, they are subject to error. Diagnosis of silent events may also occur through the use of imperfect laboratory-based diagnostic tests. In this paper, we describe an approach for variable selection in high dimensional datasets for settings in which the outcome is observed with error.
METHODS: We adapt the spike and slab Bayesian Variable Selection approach in the context of error-prone, self-reported outcomes. The performance of the proposed approach is studied through simulation studies. An illustrative application is included using data from the Women\u27s Health Initiative SNP Health Association Resource, which includes extensive genotypic ( \u3e 900,000 SNPs) and phenotypic data on 9,873 African American and Hispanic American women.
RESULTS: Simulation studies show improved sensitivity of our proposed method when compared to a naive approach that ignores error in the self-reported outcomes. Application of the proposed method resulted in discovery of several single nucleotide polymorphisms (SNPs) that are associated with risk of type 2 diabetes in a dataset of 9,873 African American and Hispanic participants in the Women\u27s Health Initiative. There was little overlap among the top ranking SNPs associated with type 2 diabetes risk between the racial groups, adding support to previous observations in the literature of disease associated genetic loci that are often not generalizable across race/ethnicity populations. The adapted Bayesian variable selection algorithm is implemented in R. The source code for the simulations are available in the Supplement.
CONCLUSIONS: Variable selection accuracy is reduced when the outcome is ascertained by error-prone self-reports. For this setting, our proposed algorithm has improved variable selection performance when compared to approaches that neglect to account for the error-prone nature of self-reports
Absolute negative refraction and imaging of unpolarized electromagnetic waves by two-dimensional photonic crystals
Absolute negative refraction regions for both polarizations of
electromagnetic wave in two-dimensional photonic crystal have been found
through both the analysis and the exact numerical simulation. Especially,
absolute all-angle negative refraction for both polarizations has also been
demonstrated. Thus, the focusing and image of unpolarized light can be realized
by a microsuperlens consisting of the two-dimensional photonic crystals. The
absorption and compensation for the losses by introducing optical gain in these
systems have also been discussed
Criticality in failure under compression: acoustic emission study of coal and charcoal with different microstructures
A systematic study of acoustic emission avalanches in coal and charcoal samples under slow uniaxial compression is presented. The samples exhibit a range of organic composition in terms of chemical elements as well as different degrees of heterogeneity in the microstructure. The experimental analysis focuses on the energies E of the individual acoustic emission events as well as on the time correlations between successive events. The studied samples can be classified into three groups. The more homogeneous samples (group I) with pores in the micro and nanoscales, with signatures of hardening effects in the stress-strain curves, exhibit the cleanest critical power-law behavior for the energy distributions g(E)dE∼E-ϵdE with a critical exponent ϵ=1.4. The more heterogeneous samples with voids, macropores, and granular microstructures (group III), show signatures of weakening effects and a larger effective exponent close to the value ϵ=1.66, but in some cases truncated by exponential damping factors. The rest of the samples (group II) exhibit a mixed crossover behavior still compatible with an effective exponent ϵ=1.4 but clearly truncated by exponential factors. These results suggest the existence of two possible universality classes in the failure of porous materials under compression: one for homogeneous samples and another for highly heterogeneous samples. Concerning time correlations between avalanches, all samples exhibit very similar waiting time distributions although some differences for the Omori aftershock distributions cannot be discarded
Implications of Color Gauge Symmetry For Nucleon Spin Structure
We study the chromodynamical gauge symmetry in relation to the internal spin
structure of the nucleon. We show that 1) even in the helicity eigenstates the
gauge-dependent spin and orbital angular momentum operators do not have
gauge-independent matrix element; 2) the evolution equations for the gluon spin
take very different forms in the Feynman and axial gauges, but yield the same
leading behavior in the asymptotic limit; 3) the complete evolution of the
gauge-dependent orbital angular momenta appears intractable in the light-cone
gauge. We define a new gluon orbital angular momentum distribution
which {\it is} an experimental observable and has a simple scale evolution.
However, its physical interpretation makes sense only in the light-cone gauge
just like the gluon helicity distribution y.Comment: Minor corrections are made in the tex
Perturbative QCD Analysis of the Nucleon's Pauli Form Factor F_2(Q^2)
We perform a perturbative QCD analysis of the nucleon's Pauli form factor
in the asymptotically large limit. We find that the leading
contribution to has a power behavior, consistent with the
well-known result in the literature. Its coefficient depends on the leading-
and subleading-twist light-cone wave functions of the nucleon, the latter
describing the quarks with one unit of orbital angular momentum. We also derive
at the logarithmic accurary the asymptotic scaling which describes recent Jefferson Lab data well.Comment: 4 papes, 3 figures include
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