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

Proton-π0 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-π0 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 NΔ of the Δ baryonic resonance. No correlation is observed in the case of deuteron-π0 coincidences. Assuming isotropic emission of π0 and Δ+ from a midrapidity thermal source and isospin symmetry, we determined the global NΔ/Nπ 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 Δ-resonance. (orig.)