Hadron properties in the medium The case of virtual mesons

The modifications of hadron properties in the nuclear medium are usually described within the framework of specific quark models. We show that the properties of virtual mesons which are exchanged between nucleons in a nucleus are also modified from a more conventional point of view. Using the relativistic (σ, ω) model to describe the nuclear medium, we identify the elementary contributions to both the nucleon and meson masses. This enables us to separate contributions coming from the direct coupling to nucleons in the Fermi sea and contributions from vacuum polarization. This is essential in order to avoid double counting when one modifies by hand both meson and nucleon masses in the medium. We apply our formalism to calculate the modification of the spin-orbit potential in the medium and the contribution from nucleon resonances. These examples exhibit vertex corrections. Finally, we consider the modifications of the electromagnetic current in terms of Meson Exchange Currents.