Matter induced $\rho$-$\delta$ mixing : a source of dileptons
  We study the possibility of $\rho-\delta$ mixing via N-N excitations in dense nuclear matter. This mixing induces a peak in the dilepton spectra at an invariant mass equal to that of the $\delta$. We calculate the cross section for dilepton production through the mixing process and we compare its size with that of $\pi-\pi$ annihilation. In-medium masses and mixing angles are also calculated. 
O. Teodorescu, A. K. Dutt-Mazumder, C. Gale