OBSERVATION OF DELTA(-]P-PI(0) DECAY IN NUCLEAR-MATTER())

Proton-pi(0) coincidences have been measured at the beam energy of 180 A MeV in the reaction Ar+Ca studied by TAPS at SIS/GSI. In the proton- pi(0) invariant mass spectrum we evidence a significant excess of coun ts above the background obtained by event mixing. We assign this signa l to the strength distribution of the Delta baryonic resonance. From t he measured yields we deduce a pi/Delta ratio of 0.9+0.3 (preliminary value), pointing that most (if not all) pions produced in subthreshold energy heavy-ion reaction are mediated by the Delta-resonance.