134 research outputs found
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
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