Test particle description of transport processes for states with a continuous mass spectrum

Aiming at a description of transport processes where the dynamically generated width of the states is potentially large a transport equation beyond the quasiparticle approximation is derived in first order gradient expansion. An effective particle number is identified which is exactly conserved by the coarse grained transport equation. Using a test particle ansatz for this conserved quantity allows to rewrite the transport equation into equations of motion for test particles. The two-body collision terms are formulated in terms of the test particles which gain non-trivial renormalization factors due to the coarse graining process.Comment: 10 pages, LaTeX, no figures, Talk given at the workshop ``Kadanoff-Baym Equations --- Progress and Perspectives for Many-Body Physics'', Rostock, Germany, 20.-24. September 199

Determination of vector meson properties by matching resonance saturation to a constituent quark model

The properties of mesonic resonances can be calculated in terms of the low-energy coefficients of chiral perturbation theory (chiPT) by extending unitarized chiPT to higher energies. On the other hand these low-energy coefficients can be calculated in two different models, namely (i) by assuming resonance saturation and (ii) within a constituent quark model. By matching the expressions of the two models combined with the results of unitarized chiPT and the Weinberg sum rules the properties of vector and axial-vector mesons can be calculated in the combined large-N_c and chiral limit.Comment: completely rewritten, 3 pages, 1 figure, contribution for QNP 2002: Quarks and Nuclear Physics, June 9-14, 2002, Juelich, German

Towards an effective field theory for vector mesons

The assumption that vector mesons dominate the interactions of hadrons with electromagnetism (vector-meson dominance - VMD) provides an important phenomenological concept. On the other hand, a clear microscopic derivation is still missing and there are cases where VMD drastically fails, e.g. for the omega transition form factor. In principle, effective field theories with their systematic expansion and power counting could provide a tool to assess the validity of VMD and more generally to describe the interactions of vector mesons at low energies. Though the systematic development is still in an infant stage we present here a Lagrangian for light pseudoscalar and vector mesons which is inspired by ideas from effective field theories. The Lagrangian is used to calculate electromagnetic meson form factors. It turns out that one can reproduce both the successes of VMD concerning the pion form factors and the deviations from VMD concerning the omega transition form factor.Comment: Talk presented at the 50th International Winter Meeting on Nuclear Physics, 23-27 January 2012, Bormio (Italy

Interpretation and resolution of pinch singularities in non-equilibrium quantum field theory

Ill-defined pinch singularities arising in a perturbative expansion in out of equilibrium quantum field theory have a natural analogue to standard scattering theory. We explicitly demonstrate that the occurrence of such terms is directly related to Fermi's golden rule known from elementary scattering theory and is thus of no mystery. We further argue that within the process of thermalization of a plasma one has to resum such contributions to all orders as the process itself is of non-perturbative nature. In this way the resummed propagators obtain a finite width. Within the Markov approximation of kinetic theory the actual phase space distribution at a given time of the evolution enters explicitly.Comment: REVTeX, 11 pages, 3 eps-figures, minor changes, some remarks adde