We calculate the pion spectrum and its electromagnetic annihilation vertex in nuclear matter taking into account the strong πNΔ interaction. For nuclear densities the pion kinetic energy is approximately compensated by the interaction, but also the free-pion annihilation matrix element is almost canceled by the contribution from the interaction with the medium. Consequently, the enhancement of the dilepton production rate due to the softening of the pion spectrum is strongly reduced.

• Received 16 November 1989

DOI:https://doi.org/10.1103/PhysRevLett.64.1502