QCD sum rules at finite density in the large-N_c limit: The coupling of   the rho-nucleon system to the D_{13}(1520)
  QCD sum rules are studied for the vector-isovector current at finite baryon density in the limit of large number of colors N_c. For the condensate side it is shown that in this limit the four-quark condensate factorizes also for the finite density case. At the hadronic side the medium dependence is expressed in terms of the current-nucleon forward scattering amplitude. Generalizing vector meson dominance we allow for a direct coupling of the current to the nucleon as well as a coupling via the rho meson. We discuss the N_c dependence of (a) modifications of the pion cloud of the rho meson, (b) mixing with other mesons (in particular a_1 and omega) and (c) resonance-hole excitations R N^{-1}. We show that only the last effect survives in the large-N_c limit. Saturating the sum rules with a simple hadronic ansatz which allows for the excitation of the D_{13}(1520) we determine the coupling of the latter to the rho-nucleon and the photon-nucleon system. These couplings are hard to determine from vacuum physics alone. 
Stefan Leupold and Marcus Post (Giessen U., Germany)