arXiv subjects
Publications and source records attributed to G. Chanfray.
The temperature evolution of the quark condensate is studied using three different methods. In the spirit of a many-body approach we make an expansion in the scalar density up to second order. Our result is consistent chiral perturbation theory to two-loop order.
Using a non-perturbative method based on the selfconsistent Quasi-particle Random-Phase Approximation (QRPA) we describe the properties of the pion in the linear $σ$-model. It is found that the pion is massless in the chiral limit, both at zero- and finite temperature, in accordance with Goldstone's theorem.
We study the evolution with density of the quark condensate in the nuclear medium with interacting nucleons and including the short range correlations. We work with two chiral models, the linear sigma model and the non-linear one. For the last one we use two versions, one which does not satisfy PCAC, and another one which does. We show that the quark condensate, as other observables, is independent on the variant selected. The application to physical pions excludes the linear sigma model as a credible one. In the non-linear models our conclusions are: first there is no systematic reaction imposed by chiral symmetry against symmetry restoration, second, if one keeps only the s-wave pion-nucleon interaction, the quark condensate evolves essentially linearly with density, as if the nucleons were non interacting. The main correction arises from the p-wave pion-nucleon interaction. Last, in the s-wave optical potential, chiral symmetry tolerates but does not impose two body terms. On the other hand the effect of correlations linked to the isospin symmetric amplitude is negligible.
Rho meson propagation in hot hadronic matter is studied in a model with coupling to $ππ$ states. Medium modifications are induced by a change of the pion dispersion relation through collisions with nucleons and $Δ's$ in the fireball. Maintaining gauge invariance dilepton production is calculated and compared to the recent data of the CERES collaboration in central S+Au collisions at 200 GeV/u. The observed enhancement of the rate below the rho meson mass can be largely accounted for.
The s-wave correlated $π$-$π$ strength distribution in cold nuclear matter is investigated. The in-medium single pions are renormalized by means of the usual p-wave couplings to nucleons and $Δ$'s. Two groups of models for the $π$-$π$ interaction are studied. The first one uses phenomenological models which reproduce the phase shifts but do not respect the soft-pion theorems. With increasing density a strong accumulation of strength in the subhreshold region is observed. Slightly above saturation density there even occurs an instability with respect to s-wave $π$-$π$ pair condensation. These effects are strongly moderated or disappear altogether if constraints from chiral symmetry are built into the $π$-$π$ interaction.
The occurrence of poles in the $2π$ propagator in a hot pion medium is studied. The domain of non--trivial solutions to the corresponding bosonic gap equation is investigated as a function of the chemical potential and temperature of the gas.