Off-shell pions in Boltzmann-Uehling-Uhlenbeck transport theory

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.