We investigate the modification of meson spectral densities in dense nuclear matter at zero temperature. These effects are studied in a fully relativistic mean field model which goes beyond the linear density approximation and also includes baryon resonances. In particular, the role of $N^{*}\mathrm{}\left(1520\right)\mathrm{}$ and $N^{*}\mathrm{}\left(1720\right)\mathrm{}$ on the $\rho$ meson spectral density is highlighted. Even though the nucleon-nucleon loop and the nucleon-resonance loop contribute with the opposite sign, an overall reduction of $\rho$ meson mass is still observed at high density. Importantly, it is shown that the resonances cause substantial broadening of the $\rho$ meson spectral density in matter and also induce nontrivial momentum dependence. We study the dispersion relations and collective oscillations induced by the $\rho$ meson propagation in nuclear matter together with the influence of the mixing of $\rho$ with the $a_0$ meson. The relevant expression for the plasma frequency is also recovered analytically in the appropriate limit. The spectral density of the $a_0$ meson is also shown.

• Received 14 December 2001

DOI:https://doi.org/10.1103/PhysRevC.66.015209