Meson Particle-Hole Dynamics
  Particle-hole modes with quantum numbers of pions and negative kaons can propagate in nuclear matter. We discuss possible manifestations of these modes in experiments on heavy-ion collisions and on neutrino-nucleus scattering. Calculations of reaction rates in medium can be harmed by double counting, which arises because the very same quantum numbers can be carried in medium as by single particle excitations as by multi-particle ones. We argue that the optical theorem written in terms of the non-equilibrium Green's functions provides a convenient formalism (closed diagram technique) void of double counting. 
E.E. Kolomeitsev and D.N. Voskresensky