  Due to large pi N --> Delta cross section a pion acquires a large width in
nuclear matter. Therefore, a Delta-resonance decaying in the nuclear medium can
produce far off-shell pions, which will gradually lose their virtuality while
propagating to the vacuum. To describe such off-shell dynamics, we implemented
a pion spectral function for finite nuclear matter density -- calculated on the
basis of the Delta-hole model -- in the BUU transport approach. The pion
off-shell dynamics leads to an enhancement of the low-p_t pion yields in the
collisions of Au+Au at 1 A GeV.
