Dilepton production in microscopic transport theory with in-medium $\rho$-meson spectral function

Abstract

We use the microscopic GiBUU transport model to calculate dilepton ($e^{+}{e}^{-}$) production in heavy-ion collisions at SIS18 energies focusing on the effect of collisional broadening of the $\rho$-meson. The collisional width of the $\rho$-meson at finite temperature and baryon density in nuclear matter is calculated on the basis of the collision integral of the GiBUU model. A systematic comparison with HADES data on dilepton production in heavy-ion collisions is performed. The collisional broadening of the $\rho$ improves the agreement between theory and experiment for the dilepton invariant-mass distributions near the $\rho$ pole mass and for the excess radiation in Au+Au at $1.23A$ GeV. We furthermore show that some remaining underprediction of the experimental dilepton spectra in C+C at $1A$ GeV and Au+Au at $1.23A$ GeV at intermediate invariant masses $0.2-0.4$ GeV can be accounted for by adjusting the $pn$ bremsstrahlung cross section in a way to agree with the inclusive dilepton spectrum from $dp$ collisions at $1.25A$ GeV.

Comments: Relevant result figures replaced, correcting small bug in pion propagation, Conclusions unchanged