284 research outputs found
Out of Equilibrium Dynamics of Supersymmetry at High Energy Density
We investigate the out of equilibrium dynamics of global chiral supersymmetry
at finite energy density. We concentrate on two specific models. The first is
the massive Wess-Zumino model which we study in a selfconsistent one-loop
approximation. We find that for energy densities above a certain threshold, the
fields are driven dynamically to a point in field space at which the fermionic
component of the superfield is massless. The state, however is found to be
unstable, indicating a breakdown of the one-loop approximation. To investigate
further, we consider an O(N) massive chiral model which is solved exactly in
the large limit. For sufficiently high energy densities, we find that for
late times the fields reach a nonperturbative minimum of the effective
potential degenerate with the perturbative minimum. This minimum is a true
attractor for O(N) invariant states at high energy densities, and this provides
a mechanism for determining which of the otherwise degenerate vacua is chosen
by the dynamics. The final state for large energy density is a cloud of
massless particles (both bosons and fermions) around this new nonperturbative
supersymmetric minimum. By introducing boson masses which softly break the
supersymmetry, we demonstrate a see-saw mechanism for generating small fermion
masses. We discuss some of the cosmological implications of our results.Comment: 31 pages, 15 figure
One-loop corrections to the metastable vacuum decay
We evaluate the one-loop prefactor in the false vacuum decay rate in a theory
of a self interacting scalar field in 3+1 dimensions. We use a numerical
method, established some time ago, which is based on a well-known theorem on
functional determinants. The proper handling of zero modes and of
renormalization is discussed. The numerical results in particular show that
quantum corrections become smaller away from the thin-wall case. In the
thin-wall limit the numerical results are found to join into those obtained by
a gradient expansion.Comment: 31 pages, 7 figure
Gauge Fields Out-Of-Equilibrium: A Gauge Invariant Formulation and the Coulomb Gauge
We study the abelian Higgs model out-of-equilibrium in two different
approaches, a gauge invariant formulation, proposed by Boyanovsky et al.
\cite{Boyanovsky:1996dc} and in the Coulomb gauge. We show that both approaches
become equivalent in a consistent one loop approximation. Furthermore, we carry
out a proper renormalization for the model in order to prepare the equations
for a numerical implementation. The additional degrees of freedom, which arise
in gauge theories, influence the behavior of the system dramatically. A
comparison with results in the 't Hooft-Feynman background gauge found by us
recently, shows very good agreement.Comment: 32 pages, 8 figure
Nonequilibrium dynamics: a renormalized computation scheme
We present a regularized and renormalized version of the one-loop nonlinear
relaxation equations that determine the non-equilibrium time evolution of a
classical (constant) field coupled to its quantum fluctuations. We obtain a
computational method in which the evaluation of divergent fluctuation integrals
and the evaluation of the exact finite parts are cleanly separated so as to
allow for a wide freedom in the choice of regularization and renormalization
schemes. We use dimensional regularization here. Within the same formalism we
analyze also the regularization and renormalization of the energy-momentum
tensor. The energy density serves to monitor the reliability of our numerical
computation. The method is applied to the simple case of a scalar phi^4 theory;
the results are similar to the ones found previously by other groups.Comment: 15 pages, 9 postscript figures, revtex; version published in Phys.
Rev, with minor corrections; improves the first version of 1996 by including
the discussion of energy momentum tenso
Quantum Fluctuations around the Electroweak Sphaleron
We present an analysis of the quantum fluctuations around the electroweak
sphaleron and calculate the associated determinant which gives the 1--loop
correction to the sphaleron transition rate. The calculation differs in various
technical aspects from a previous analysis by Carson et al. so that it can be
considered as independent. The numerical results differ also -- by several
orders of magnitude -- from those of this previous analysis; we find that the
sphaleron transition rate is much less suppressed than found previously.Comment: DO-TH-93/19 39 pages, 5 figures (available on request as Postscript
files or via Fax or mail), LaTeX, no macros neede
One-loop corrections to the instanton transition in the two-dimensional Abelian Higgs model
We present an evaluation of the fluctuation determinant which appears as a
prefactor in the instanton transition rate for the two-dimensional Abelian
Higgs model. The corrections are found to change the rate at most by a factor
of 2 for 0.4 < M_W/M_H < 2.0.Comment: DO-TH-94/17, 20 pages, 4 figures appended as uucompressed .eps files,
LaTeX, needs epsfig.st
Nonequilibrium dynamics: preheating in the SU(2) Higgs model
The term `preheating' has been introduced recently to denote the process in
which energy is transferred from a classical inflaton field into fluctuating
field (particle) degrees of freedom without generating yet a real thermal
ensemble. The models considered up to now include, besides the inflaton field,
scalar or fermionic fluctuations. On the other hand the typical ingredient of
an inflationary scenario is a nonabelian spontaneously broken gauge theory. So
the formalism should also be developed to include gauge field fluctuations
excited by the inflaton or Higgs field. We have chosen here, as the simplest
nonabelian example, the SU(2) Higgs model. We consider the model at temperature
zero. From the technical point of view we generalize an analytical and
numerical renormalized formalism developed by us recently to coupled channnel
systems. We use the 't Hooft-Feynman gauge and dimensional regularization. We
present some numerical results but reserve a more exhaustive discussion of
solutions within the paramter space of two couplings and the initial value of
the Higgs field to a future publication.Comment: 30 pages, 10 figures in enhanced postscript, 2 unreadable figures
made accessibl
Fluctuation corrections to bubble nucleation
The fluctuation determinant which determines the preexponential factor of the
transition rate for minimal bubbles is computed for the electroweak theory with
. As the basic action we use the three-dimensional
high-temperature action including, besides temperature dependent masses, the one-loop contribution which makes the phase transition first order. The
results show that this contribution (which has then to be subtracted from the
exact result) gives the dominant contribution to the one-loop effective action.
The remaining correction is of the order of, but in general larger than the
critical bubble action and suppresses the transition rate. The results for the
Higgs field fluctuations are compared with those of an approximate heat kernel
computation of Kripfganz et al., good agreement is found for small bubbles,
strong deviations for large thin-wall bubbles.Comment: 19 pages, LaTeX, no macros, no figure
Out-of-equilibrium evolution of scalar fields in FRW cosmology: renormalization and numerical simulations
We present a renormalized computational framework for the evolution of a
self-interacting scalar field (inflaton) and its quantum fluctuations in an FRW
background geometry. We include a coupling of the field to the Ricci scalar
with a general coupling parameter . We take into account the classical and
quantum back reactions, i.e., we consider the the dynamical evolution of the
cosmic scale factor. We perform, in the one-loop and in the large-N
approximation, the renormalization of the equation of motion for the inflaton
field, and of its energy momentum tensor. Our formalism is based on a
perturbative expansion for the mode functions, and uses dimensional
regularization. The renormalization procedure is manifestly covariant and the
counter terms are independent of the initial state. Some shortcomings in the
renormalization of the energy-momentum tensor in an earlier publication are
corrected. We avoid the occurence of initial singularities by constructing a
suitable class of initial states. The formalism is implemented numerically and
we present some results for the evolution in the post-inflationary preheating
era.Comment: 44 pages, uses latexsym, 6 pages with 11 figures in a .ps fil
Dynamics of coupled bosonic systems with applications to preheating
Coupled, multi-field models of inflation can provide several attractive
features unavailable in the case of a single inflaton field. These models have
a rich dynamical structure resulting from the interaction of the fields and
their associated fluctuations. We present a formalism to study the
nonequilibrium dynamics of coupled scalar fields. This formalism solves the
problem of renormalizing interacting models in a transparent way using
dimensional regularization. The evolution is generated by a renormalized
effective Lagrangian which incorporates the dynamics of the mean fields and
their associated fluctuations at one-loop order. We apply our method to two
problems of physical interest: (i) a simple two-field model which exemplifies
applications to reheating in inflation, and (ii) a supersymmetric hybrid
inflation model. This second case is interesting because inflation terminates
via a smooth phase transition which gives rise to a spinodal instability in one
of the fields. We study the evolution of the zero mode of the fields and the
energy density transfer to the fluctuations from the mean fields. We conclude
that back reaction effects can be significant over a wide parameter range. In
particular for the supersymmetric hybrid model we find that particle production
can be suppressed due to these effects.Comment: 23 pages, 16 eps-figures, minor changes in the text, references
added, accepted for publication in PR
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