330 research outputs found
An Exact Calculation of the Energy Density of Cosmological Gravitational Waves
In this paper we calculate the Bogoliubov coefficients and the energy density
of the stochastic gravitational wave background for a universe that undergoes
inflation followed by radiation domination and matter domination, using a
formalism that gives the Bogoliubov coefficients as continous functions of
time. By making a reasonable assumption for the equation of state during
reheating, we obtain in a natural way the expected high frequency cutoff in the
spectral energy density.Comment: 12 pages+5 figures, uuencoded file,DF/IST-2.9
Analytic Perturbation Theory: A New Approach to the Analytic Continuation of the Strong Coupling Constant into the Timelike Region
The renormalization group applied to perturbation theory is ordinarily used
to define the running coupling constant in the spacelike region. However, to
describe processes with timelike momenta transfers, it is important to have a
self-consistent determination of the running coupling constant in the timelike
region. The technique called analytic perturbation theory (APT) allows a
consistent determination of this running coupling constant. The results are
found to disagree significantly with those obtained in the standard
perturbative approach. Comparison between the standard approach and APT is
carried out to two loops, and threshold matching in APT is applied in the
timelike region.Comment: 16 pages, REVTeX, 7 postscript figure
The Kaon-Photoproduction Of Nucleons In The Quark Model
In this paper, we develop a general framework to study the
meson-photoproductions of nucleons in the chiral quark model. The S and U
channel resonance contributions are expressed in terms of the
Chew-Goldberger-Low-Nambu (CGLN) amplitudes. The kaon-photoproduction
processes, , , and , are calculated. The initial results show that the quark
model provides a much improved description of the reaction mechanism for the
kaon-photoproductions of the nucleon with less parameters than the traditional
phenomenological approaches.Comment: 25 pages, 9 postscript figures can be obtained from the author
Cosmic microwave background and parametric resonance in reheating
The variation of the perturbative 3-curvature parameter, \zeta, is
investigated in the period of reheating after inflation. The two-field model
used has the inflaton, with an extra scalar field coupled to it, and non-linear
effects of both fields are included as well as a slow decay mechanism into the
hydrodynamic fluid of the radiation era. Changes in \zeta occur and persist
into the succeeding cosmic eras to influence the generation of the cosmic
microwave background fluctuations.Comment: 21 pages, 6 figures.Corrects misprinted formula and 2 number
Post-Inflationary Reheating
We study a model for reheating that has been much investigated for parametric
resonance, having a quartic interaction of the scalar inflaton with another
scalar field. Attention is particularly on the quantum excitations of the
inflaton field and the metric perturbation with a smooth transition from
quantum to classical stochastic states, followed through from a specific
inflation model to a state including a relativistic fluid. The scalar fields
enter non-perturbatively but the metric enters perturbatively, and the validity
of this latter is assessed. In this model our work seems to point the large
scale curvature parameter changing.Comment: 25 pages, 6 figures. Coding error(misprint) corrected:figures and
some conclusions change
An Unified Approach To Pseudo Scalar Meson Photoproductions Off Nucleons In The Quark Model
An unified approach to the pseudo scalar meson (, and )
photoproduction off nucleons are presented. It begins with the low energy QCD
Lagrangian, and the resonances in the s- and u- channels are treated in the
framework of the quark model
The duality hypothesis is imposed to limit the number of the t-channel
exchanges. The CGLN amplitudes for each reaction are evaluated, which include
both proton and neutron targets. The important role by the S-wave resonances in
the second resonance region is discussed, it is particularly important for the
and photoproductions.Comment: 31 pages in Latex fil
Density and Graviton Perturbations in the Cosmic Microwave Background
This paper evaluates and compares the gravitational wave and density
perturbation contributions to the cosmic microwave background radiation, on the
basis of the same power law inflationary model. The inflation to radiation
transition is treated as instantaneous, but a model is constructed to allow for
a smooth transition from the radiation to the matter dominated eras. The
equations are numerically investigated and integrated, without any basic
approximations being made. Use is made of the synchronous gauge, with
appropriate gauge invariant variables, thus eliminating any confusion arising
from unphysical gauge modes. We find a non- negligible gravitational wave
contribution, which becomes dominant for a power law expansion with exponent . We also explore the dependence of our results with the main
characteristic of the transition region, its length.Comment: 19 page
The eta Photoproduction of Nucleons and The structure of the Resonance S11(1535) in The Quark Model
In this paper, we present our study on the photoproduction based on
the chiral quark model. We find that quark model provides a very good
description of the production with much less parameters, and the
threshold region is not a reliable source to determine the coupling
constant due to its strong dependence on the properties of the resonance
. We suggest that the systematic data in
GeV region may help us to determine the coupling constant more
precisely.
The structure of the resonance is discussed, we find that the
recent data from Mainz group bring the helicity amplitude much closer to the
quark model prediction. However, more studies need to be done to understand the
large branching ratio of the resonance . Our results
show that the quark model is a very good approach to study the underlying
structure of baryon resonances from the meson photoproduction data.Comment: 21 pages in LaTex file, 6 postscript figures can be obtained from the
autho
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