42,960 research outputs found
A Monte Carlo Evaluation of Some Common Panel Data Estimators when Serial Correlation and Cross-sectional Dependence are Both Present
This study employs Monte Carlo experiments to evaluate the performances of a number of common panel data estimators when serial correlation and cross-sectional dependence are both present. It focuses on fixed effects models with less than 100 cross-sectional units and between 10 and 25 time periods (such as are commonly employed in empirical growth studies). Estimator performance is compared on two dimensions: (i) root mean square error and (ii) accuracy of estimated confidence intervals. An innovation of our study is that our simulated panel data sets are designed to look like “real-world” panel data. We find large differences in the performances of the respective estimators. Further, estimators that perform well on efficiency grounds may perform poorly when estimating confidence intervals, and vice versa. Our experimental results form the basis for a set of estimator recommendations. These are applied to “out of sample” simulated panel data sets and found to perform well.Panel Data estimation; Monte Carlo analysis; FGLS; PCSE; Groupwise Heteroscedasticity; Serial Correlation; Cross-sectional Dependence; Stata; EViews
Pressure Dependence of Wall Relaxation in Polarized He Gaseous Cells
We have observed a linear pressure dependence of longitudinal relaxation time
() at 4.2 K and 295 K in gaseous He cells made of either bare pyrex
glass or Cs/Rb-coated pyrex due to paramagnetic sites in the cell wall. The
paramagnetic wall relaxation is previously thought to be independent of He
pressure. We develop a model to interpret the observed wall relaxation by
taking into account the diffusion process, and our model gives a good
description of the data
Medium effects in the production and decay of vector mesons in pion-nucleus reactions
The -, - and -resonance production and their dileptonic
decay in reactions at 1.1 - 1.7 GeV/c are calculated within the
intranuclear cascade (INC) approach. The invariant mass distribution of the
dilepton pair for each resonance can be decomposed in two components which
correspond to their decay 'inside' the target nucleus and in the vacuum,
respectively. The first components are strongly distorted by the nuclear medium
due to resonance-nucleon scattering and a possible mass shift at finite baryon
density. These medium modifications are compared to background sources in the
dilepton spectrum from bremsstrahlung as well as the Dalitz decays of
and mesons produced in the reaction. Detailed predictions for
reactions at 1.3 and 1.7 GeV/c are made within several momentum bins
for the lepton pair.Comment: 29 pages, LaTeX, including 12 postscript figures, UGI-97-07, Nucl.
Phys. A., in pres
Probing the interactions of charmed mesons with nuclei in induced reactions
We study the perspectives of resonant and nonresonant charmed meson
production in reactions within the Multiple Scattering Monte
Carlo (MSMC) approach. We calculate the production of the resonances
and on various nuclei, their propagation
and decay to in the medium and
vacuum, respectively. The modifications of the open charm vector mesons in the
nuclear medium are found to be rather moderate or even small such that dilepton
spectroscopy will require an invariant mass resolution of a few MeV.
Furthermore, the elastic and inelastic interactions of the open charm mesons in
the medium are taken into account, which can be related to -, - or
-quark exchange with nucleons. It is found that by studying the
ratio for low momenta in the laboratory ( GeV/c) as a function of
target mass stringent constraints on the -quark exchange cross section
can be obtained. On the other hand, the ratios
as well as and at low momenta as a function
of will permit to fix independently the strength of the -quark exchange
reaction in scattering.Comment: 9 pages, LaTeX, including 9 postscript figures, submitted to Eur.
Phys. J.
Gauge-invariant Green function in 3+1 dimensional QED (QCD) and 2+1 dimensional Abelian (Non-Abelian) Chern-Simon theory
By applying the simple and effective method developed to study the the
gauge-invariant fermion Green function in dimensional non-compact QED,
we study the gauge-invariant Green function in dimensional QED and dimensional non-compact Chern-Simon theory. We also extend our results to
the corresponding non-Abelian gauge theories. Implications for
Fractional Quantum Hall effect are briefly discussed.Comment: 8 pages, 4 figures, published versio
On gauge-invariant Green function in 2+1 dimensional QED
Both the gauge-invariant fermion Green function and gauge-dependent
conventional Green function in dimensional QED are studied in the large
limit. In temporal gauge, the infra-red divergence of gauge-dependent
Green function is found to be regulariable, the anomalous dimension is found to
be . This anomalous dimension was argued to be
the same as that of gauge-invariant Green function. However, in Coulomb gauge,
the infra-red divergence of the gauge-dependent Green function is found to be
un-regulariable, anomalous dimension is even not defined, but the infra-red
divergence is shown to be cancelled in any gauge-invariant physical quantities.
The gauge-invariant Green function is also studied directly in Lorentz
covariant gauge and the anomalous dimension is found to be the same as that
calculated in temporal gauge.Comment: 8 pages, 6 figures, to appear in Phys. Rev.
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