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Herbert Nachbagauer

Publications and source records attributed to Herbert Nachbagauer.

13 recordsLinked to original sources
arXiv1998

Time evolution of observables out of thermal equilibrium

We propose a new approximation-technique to deal with the exact macroscopic integro-differential evolution equations of statistical systems which self-consistently accounts for dissipative effects. Concentrating on one and two point equal-time correlators, we develop the self-consistent method and apply it to a scalar field theory with quartic self-interaction. We derive the effective equations of motion and the corresponding macroscopic effective Hamiltonian. Non-locality in time appears in a natural way necessary to account for entropy generating processes.

hep-ph↗
arXiv1998

Dissipative Time Evolution of Observables in Non-equilibrium Statistical Quantum Systems

We discuss differential-- versus integral--equation based methods describing out--of thermal equilibrium systems and emphasize the importance of a well defined reduction to statistical observables. Applying the projection operator approach, we investigate on the time evolution of expectation values of linear and quadratic polynomials in position and momentum for a statistical anharmonic oscillator with quartic potential. Based on the exact integro-differential equations of motion, we study the first and naive second order approximation which breaks down at secular time-scales. A method is proposed to improve the expansion by a non--perturbative resummation of all quadratic operator correlators consistent with energy conservation for all times. Motion cannot be described by an effective Hamiltonian local in time reflecting non-unitarity of the dissipative entropy generating evolution. We numerically integrate the consistently improved equations of motion for large times. We relate entropy to the uncertainty product, both being expressible in terms of the observables under consideration.

hep-th↗
arXiv1997

Wigner Functionals and their Dynamics in Quantum-Field-Theory

We reformulate time evolution of systems in mixed states in terms of the classical observables of correlators using the Weyl correspondence rule. The resulting equation of motion for the Wigner functional of the density matrix is found to be of the Liouville type. To illustrate the methods developed, we explicitly consider a scalar theory with quartic self-interaction and derive the short time behaviour with the non-interacting thermal density matrix as initial condition. In the scalar case, the complete correlator hierarchy is studied and restrictions are derived for spatially homogeneous initial conditions and systems with unbroken symmetry.

hep-th↗
arXiv1997

The Quantum Liouville Equation for the Effective Action

Starting from the von Neumann equation, we construct the quantum evolution equation for the effective action for systems in mixed states. This allows us to find the hierarchy of equations which describe the time evolution of equal time correlators. The method is applied explicitly to a scalar theory with quartic self-interaction.

hep-th↗
arXiv1995

The dynamics of cosmological perturbations in thermal $λϕ^4$ theory

Using a recent thermal-field-theory approach to cosmological perturbations, the exact solutions that were found for collisionless ultrarelativistic matter are generalized to include the effects from weak self-interactions in a $λϕ^4$ model through order $λ^{3/2}$. This includes the effects of a resummation of thermal masses and associated nonlocal gravitational vertices, thus going far beyond classical kinetic theory. Explicit solutions for all the scalar, vector, and tensor modes are obtained for a radiation-dominated Einstein-de Sitter model containing a weakly interacting scalar plasma with or without the admixture of an independent component of perfect radiation fluid.

gr-qc↗
arXiv1995

The dynamics of a thermal non-equilibrium anharmonic oscillator

We propose an non-standard method to calculate non-equilibrium physical observables. Considering the generic example of an anharmonic quantum oscillator, we take advantage of the fact that the commutator algebra of second order polynomials in creation/annihilation operators closes. We solve the von~Neumann equation for the density-operator exactly in the mean field approximation and study the time evolution of the particle number and the expectation value < X^2 > .

hep-ph↗
arXiv1995

Spherically symmetric quark-gluon plasma field configurations

We study field configurations in a hot quark-gluon plasma with spherical symmetry. We show that the electric fields point into radial direction and solve the effective non-abelian equations of motions. The corresponding charge density has a localized contribution which has a gauge invariant interpretation as a pointlike color charge. We discuss configurations oscillating periodically in time. Furthermore, we calculate the electric field induced by a constant local charge that is removed from the plasma for $t>0$ as a model of a decaying heavy quark.

hep-ph↗
arXiv1995

Finite temperature formalism for nonabelian gauge theories in the physical phase space

We establish a new framework of finite temperature field theory for Yang-Mills theories in the physical phase space eliminating all unphysical degrees of freedoms. Relating our method to the imaginary time formalism of James and Landshoff in temporal axial gauge, we calculate the two-loop pressure and provide a systematic and unique method to construct the additional vertices encountered in their approach.

hep-ph↗
arXiv1994

On the consistent solution of the gap--equation for spontaneously broken $λΦ^4$-theory

We present a self--consistent solution of the finite temperature gap--equation for $λΦ^4$ theory beyond the Hartree-Fock approximation using a composite operator effective action. We find that in a spontaneously broken theory not only the so--called daisy and superdaisy graphs contribute to the resummed mass, but also resummed non--local diagrams are of the same order, thus altering the effective mass for small values of the latter.

hep-ph↗
arXiv1994

Finite Temperature Effective Potential for Spontaneously Broken $λΦ^4$ Theory

We present a self-consistent calculation of the finite temperature effective potential for $λΦ^4$ theory in four dimensions using a composite operator effective action. We find that in a spontaneously broken theory not only the so-called daisy and superdaisy graphs contribute to the next-to-leading order thermal mass, but also resummed non-local diagrams are of the same order, thus altering the effective potential at small effective mass.

hep-ph↗