Observation of Δ⁺→ pπ⁰ decay in heavy-ion collisions

Proton-pi (0) coincidences have been measured at the beam energy of 180A MeV in the reaction Ar+Ca studied by TAPS at SIS/GSI. In the proton-pi (0) invariant mass spectrum we observe a significant excess of strength above the background obtained by event mixing. We attribute this signal to the strength distribution M-Delta+ of the Delta (+) baryonic resonance. No correlation is observed in the case of deuteron-pi (0) coincidences. Assuming isotropic emission of pi (0) and Delta (+) from a midrapidity thermal source and isospin symmetry, we determined the global N-Delta/N-pi ratio of 0.79 +/- 0.30(stat) +/-0.2(syst). This value indicates that most pions produced at subthreshold energy in heavy-ion reaction are mediated by the Delta -resonance.