283 research outputs found
Polarization in Top Quark Pair Production near Threshold
The polarization dependent momentum distributions of top quarks and their
decay products are calculated for production at future
colliders with polarized beams. The Green function formalism is applied to this
reaction near energy threshold. The Lippmann--Schwinger equations for the
-wave and -wave Green functions are solved numerically for the QCD
chromostatic potential given by the two-loop formula at large momentum
transfers and Richardson ansatz at intermediate and small ones. It is
demonstrated that for the longitudinally polarized electron beam an optimally
polarized sample of top quarks can be produced.Comment: 11 pages, LaTex (uses epsf.sty, 3 ps-figures appended as extra
uuencoded file). The complete paper, including figures, is also available via
anonymous ftp at ftp://ttpux2.physik.uni-karlsruhe.de/ttp94-28/ttp94-28.ps,
or via www at http://ttpux2.physik.uni-karlsruhe.de/preprints.htm
Particle Production and Gravitino Abundance after Inflation
Thermal history after inflation is studied in a chaotic inflation model with
supersymmetric couplings of the inflaton to matter fields. Time evolution
equation is solved in a formalism that incorporates both the back reaction of
particle production and the cosmological expansion. The effect of the
parametric resonance gives rise to a rapid initial phase of the inflaton decay
followed by a slow stage of the Born term decay. Thermalization takes place
immediately after the first explosive stage for a medium strength of the
coupling among created particles. As an application we calculate time evolution
of the gravitino abundance that is produced by ordinary particles directly
created from the inflaton decay, which typically results in much more enhanced
yield than what a naive estimate based on the Born term would suggest.Comment: 23 pages + 13 figure
String Cosmology: A Review
We give an overview of the status of string cosmology. We explain the
motivation for the subject, outline the main problems, and assess some of the
proposed solutions. Our focus is on those aspects of cosmology that benefit
from the structure of an ultraviolet-complete theory.Comment: 55 pages. v2: references adde
On the Initial Conditions for Brane Inflation
String theory gives rise to various mechanisms to generate primordial
inflation, of which ``brane inflation'' is one of the most widely considered.
In this scenario, inflation takes place while two branes are approaching each
other, and the modulus field representing the separation between the branes
plays the role of the inflaton field. We study the phase space of initial
conditions which can lead to a sufficiently long period of cosmological
inflation, and find that taking into account the possibility of nonvanishing
initial momentum can significantly change the degree of fine tuning of the
required initial conditions.Comment: 11 pages, 2 figure
Cosmology at the Millennium
One hundred years ago we did not know how stars generate energy, the age of
the Universe was thought to be only millions of years, and our Milky Way galaxy
was the only galaxy known. Today, we know that we live in an evolving and
expanding Universe comprising billions of galaxies, all held together by dark
matter. With the hot big-bang model, we can trace the evolution of the Universe
from the hot soup of quarks and leptons that existed a fraction of a second
after the beginning to the formation of galaxies a few billion years later, and
finally to the Universe we see today 13 billion years after the big bang, with
its clusters of galaxies, superclusters, voids, and great walls. The attractive
force of gravity acting on tiny primeval inhomogeneities in the distribution of
matter gave rise to all the structure seen today. A paradigm based upon deep
connections between cosmology and elementary particle physics -- inflation +
cold dark matter -- holds the promise of extending our understanding to an even
more fundamental level and much earlier times, as well as shedding light on the
unification of the forces and particles of nature. As we enter the 21st
century, a flood of observations is testing this paradigm.Comment: 44 pages LaTeX with 14 eps figures. To be published in the Centennial
Volume of Reviews of Modern Physic
Growth of Inflaton Perturbations and the Post-Inflation Era in Supersymmetric Hybrid Inflation Models
It has been shown that hybrid inflation may end with the formation of
non-topological solitons of inflaton field. As a first step towards a fully
realistic picture of the post-inflation era and reheating in supersymmetric
hybrid inflation models, we study the classical scalar field equations of a
supersymmetric hybrid inflation model using a semi-analytical ansatz for the
spatial dependence of the fields. Using the minimal D-term inflation model as
an example, the inflaton field is evolved using the full 1-loop effective
potential from the slow-rolling era to the U(1)_{FI} symmetry-breaking phase
transition. Spatial perturbations of the inflaton corresponding to quantum
fluctuations are introduced for the case where there is spatially coherent
U(1)_{FI} symmetry breaking. The maximal growth of the dominant perturbation is
found to depend only on the ratio of superpotential coupling \lambda to the
gauge coupling g. The inflaton condensate fragments to non-topological solitons
for \lambda/g > 0.09. Possible consequences of non-topological soliton
formation in fully realistic SUSY hybrid inflation models are discussed.Comment: 27 pages LaTeX, 8 figures. Additional references and discussio
First-order cosmological phase transitions in the radiation dominated era
We consider first-order phase transitions of the Universe in the
radiation-dominated era. We argue that in general the velocity of interfaces is
non-relativistic due to the interaction with the plasma and the release of
latent heat. We study the general evolution of such slow phase transitions,
which comprise essentially a short reheating stage and a longer phase
equilibrium stage. We perform a completely analytical description of both
stages. Some rough approximations are needed for the first stage, due to the
non-trivial relations between the quantities that determine the variation of
temperature with time. The second stage, instead, is considerably simplified by
the fact that it develops at a constant temperature, close to the critical one.
Indeed, in this case the equations can be solved exactly, including
back-reaction on the expansion of the Universe. This treatment also applies to
phase transitions mediated by impurities. We also investigate the relations
between the different parameters that govern the characteristics of the phase
transition and its cosmological consequences, and discuss the dependence of
these parameters with the particle content of the theory.Comment: 38 pages, 3 figures; v2: Minor changes, references added; v3: several
typos correcte
From the Big Bang Theory to the Theory of a Stationary Universe
We consider chaotic inflation in the theories with the effective potentials
phi^n and e^{\alpha\phi}. In such theories inflationary domains containing
sufficiently large and homogeneous scalar field \phi permanently produce new
inflationary domains of a similar type. We show that under certain conditions
this process of the self-reproduction of the Universe can be described by a
stationary distribution of probability, which means that the fraction of the
physical volume of the Universe in a state with given properties (with given
values of fields, with a given density of matter, etc.) does not depend on
time, both at the stage of inflation and after it. This represents a strong
deviation of inflationary cosmology from the standard Big Bang paradigm. We
compare our approach with other approaches to quantum cosmology, and illustrate
some of the general conclusions mentioned above with the results of a computer
simulation of stochastic processes in the inflationary Universe.Comment: No changes to the file, but original figures are included. They
substantially help to understand this paper, as well as eternal inflation in
general, and what is now called the "multiverse" and the "string theory
landscape." High quality figures can be found at
http://www.stanford.edu/~alinde/LLMbigfigs
Bianchi type I space and the stability of inflationary Friedmann-Robertson-Walker space
Stability analysis of the Bianchi type I universe in pure gravity theory is
studied in details. We first derive the non-redundant field equation of the
system by introducing the generalized Bianchi type I metric. This non-redundant
equation reduces to the Friedmann equation in the isotropic limit. It is shown
further that any unstable mode of the isotropic perturbation with respect to a
de Sitter background is also unstable with respect to anisotropic
perturbations. Implications to the choice of physical theories are discussed in
details in this paper.Comment: 5 pages, some comment adde
Friedmann Equation and Stability of Inflationary Higher Derivative Gravity
Stability analysis on the De Sitter universe in pure gravity theory is known
to be useful in many aspects. We first show how to complete the proof of an
earlier argument based on a redundant field equation. It is shown further that
the stability condition applies to Friedmann-Robertson-Walker spaces
based on the non-redundant Friedmann equation derived from a simple effective
Lagrangian. We show how to derive this expression for the Friedmann equation of
pure gravity theory. This expression is also generalized to include scalar
field interactions.Comment: Revtex, 6 pages, Add two more references, some typos correcte
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