Probing Hadronic Polarizations with Dilepton Anisotropies

Abstract

We investigate the production of $e^+e^-$ pairs from $p + Be$ and nucleus-nucleus collisions from 2 GeV/A to 200 GeV/A within a covariant transport approach and focus on the dilepton angular anisotropies as a function of the dilepton invariant mass. Whereas the low mass regime yields information about the Dalitz decays of the $\Delta, \eta$ and $\omega$, above M $\approx $0.5 GeV the angular anisotropies provide additional information about the $\pi^+\pi^- \to \rho^0 \to e^+e^-$ channel in the medium. The anisotropy coefficient is found to change its sign for M $>$ 0.5 GeV in case of nucleus-nucleus reactions when increasing the bombarding energy from 2 GeV/A to 200 GeV/A which sheds some light on the $\pi \pi \to \rho$ dynamics.