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

Abstract

Due to large $\pi \overrightarrow{N}\Delta$ 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 Boltzmann-Uehling-Uhlenbeck 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 $1A\mathrm{GeV}.$