1,652 research outputs found
Perturbative nonequilibrium dynamics of phase transitions in an expanding universe
A complete set of Feynman rules is derived, which permits a perturbative
description of the nonequilibrium dynamics of a symmetry-breaking phase
transition in theory in an expanding universe. In contrast to a
naive expansion in powers of the coupling constant, this approximation scheme
provides for (a) a description of the nonequilibrium state in terms of its own
finite-width quasiparticle excitations, thus correctly incorporating
dissipative effects in low-order calculations, and (b) the emergence from a
symmetric initial state of a final state exhibiting the properties of
spontaneous symmetry breaking, while maintaining the constraint . Earlier work on dissipative perturbation theory and spontaneous symmetry
breaking in Minkowski spacetime is reviewed. The central problem addressed is
the construction of a perturbative approximation scheme which treats the
initial symmetric state in terms of the field , while the state that
emerges at later times is treated in terms of a field , linearly related
to . The connection between early and late times involves an infinite
sequence of composite propagators. Explicit one-loop calculations are given of
the gap equations that determine quasiparticle masses and of the equation of
motion for and the renormalization of these equations is
described. The perturbation series needed to describe the symmetric and
broken-symmetry states are not equivalent, and this leads to ambiguities
intrinsic to any perturbative approach. These ambiguities are discussed in
detail and a systematic procedure for matching the two approximations is
described.Comment: 22 pages, using RevTeX. 6 figures. Submitted to Physical Review
Nonequilibrium perturbation theory for complex scalar fields
Real-time perturbation theory is formulated for complex scalar fields away
from thermal equilibrium in such a way that dissipative effects arising from
the absorptive parts of loop diagrams are approximately resummed into the
unperturbed propagators. Low order calculations of physical quantities then
involve quasiparticle occupation numbers which evolve with the changing state
of the field system, in contrast to standard perturbation theory, where these
occupation numbers are frozen at their initial values. The evolution equation
of the occupation numbers can be cast approximately in the form of a Boltzmann
equation. Particular attention is given to the effects of a non-zero chemical
potential, and it is found that the thermal masses and decay widths of
quasiparticle modes are different for particles and antiparticles.Comment: 15 pages using RevTeX; 2 figures in 1 Postscript file; Submitted to
Phys. Rev.
Dissipation in equations of motion of scalar fields
The methods of non-equilibrium quantum field theory are used to investigate
the possibility of representing dissipation in the equation of motion for the
expectation value of a scalar field by a friction term, such as is commonly
included in phenomenological inflaton equations of motion. A sequence of
approximations is exhibited which reduces the non-equilibrium theory to a set
of local evolution equations. However, the adiabatic solution to these
evolution equations which is needed to obtain a local equation of motion for
the expectation value is not well defined; nor, therefore, is the friction
coefficient. Thus, a non-equilibrium treatment is essential, even for a system
that remains close to thermal equilibrium, and the formalism developed here
provides one means of achieving this numerically.Comment: 17 pages, 5 figure
Nonequilibrium perturbation theory for spin-1/2 fields
A partial resummation of perturbation theory is described for field theories
containing spin-1/2 particles in states that may be far from thermal
equilibrium. This allows the nonequilibrium state to be characterized in terms
of quasiparticles that approximate its true elementary excitations. In
particular, the quasiparticles have dispersion relations that differ from those
of free particles, finite thermal widths and occupation numbers which, in
contrast to those of standard perturbation theory evolve with the changing
nonequilibrium environment. A description of this kind is essential for
estimating the evolution of the system over extended periods of time. In
contrast to the corresponding description of scalar particles, the structure of
nonequilibrium fermion propagators exhibits features which have no counterpart
in the equilibrium theory.Comment: 16 pages; no figures; submitted to Phys. Rev.
Treatment options for recurrent glioblastoma: a network meta-analysis
This is a protocol for a Cochrane Review (Intervention). The objectives are as follows:. To evaluate the effectiveness of further treatment/s for first and subsequent recurrence of glioblastoma multiforme (GBM) among people who have received the standard of care for primary treatment of the disease (chemoradiotherapy) or following development of GBM from a lower grade (radiotherapy with subsequent temozolomide at relapse); and to prepare a brief economic commentary on the available evidence
Large-N transition temperature for superconducting films in a magnetic field
We consider the -component Ginzburg-Landau model in the large limit,
the system being embedded in an external constant magnetic field and confined
between two parallel planes a distance apart from one another. On physical
grounds, this corresponds to a material in the form of a film in the presence
of an external magnetic field. Using techniques from dimensional and
-function regularization, modified by the external field and the
confinement conditions, we investigate the behavior of the system as a function
of the film thickness . This behavior suggests the existence of a minimal
critical thickness below which superconductivity is suppressed.Comment: Revtex, two column, 4 pages, 2 figure
Scaling in high-temperature superconductors
A Hartree approximation is used to study the interplay of two kinds of
scaling which arise in high-temperature superconductors, namely critical-point
scaling and that due to the confinement of electron pairs to their lowest
Landau level in the presence of an applied magnetic field. In the neighbourhood
of the zero-field critical point, thermodynamic functions scale with the
scaling variable , which differs from the variable
suggested by the gaussian approximation.
Lowest-Landau-level (LLL) scaling occurs in a region of high field surrounding
the upper critical field line but not in the vicinity of the zero-field
transition. For YBaCuO in particular, a field of at least 10 T is needed to
observe LLL scaling. These results are consistent with a range of recent
experimental measurements of the magnetization, transport properties and,
especially, the specific heat of high- materials.Comment: 22 pages + 1 figure appended as postscript fil
Critical Casimir Effect in 3He-4He films
Universal aspects of the thermodynamic Casimir effect in wetting films of
3He-4He mixtures near their bulk tricritical point are studied within suitable
models serving as representatives of the corresponding universality class. The
effective forces between the boundaries of such films arising from the
confinement are calculated along isotherms at several fixed concentrations of
3He. Nonsymmetric boundary conditions impose nontrivial concentration profiles
leading to repulsive Casimir forces which exhibit a rich behavior of the
crossover between the tricritical point and the line of critical points. The
theoretical results agree with published experimental data and emphasize the
importance of logarithmic corrections.Comment: 12 pages, 4 figures, submitted to the Phys. Rev. Let
Quantum Rolling Down out of Equilibrium
In a scalar field theory, when the tree level potential admits broken
symmetry ground states, the quantum corrections to the static effective
potential are complex. (The imaginary part is a consequence of an instability
towards phase separation and the static effective potential is not a relevant
quantity for understanding the dynamics). Instead, we study here the equations
of motion obtained from the one loop effective action for slow rollover out of
equilibrium.
We considering the case in which a scalar field theory undergoes a rapid
phase transition from to . We find that, for slow rollover
initial conditions (the field near the maximum of the tree level potential),
the process of phase separation controlled by unstable long-wavelength
fluctuations introduces dramatic corrections to the dynamical evolution of the
field. We find that these effects slow the rollover even furtherComment: 33 pages, Latex,LPTHE-PAR 92-33 PITT 92-0
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