Study of dalitz-plot distributions of the decays eta ---> pi+ pi- pi0 and eta ---> pi+ pi- gamma

Based on 80 884 events, we obtained the matrix element squared for the decay η→π+π−π0:|M(x,y)|2=1−(1.08±0.014)y+(0.03±0.03)y2+(0.05±0.03)x2 where x and y are the usual Dalitz-plot coordinates. The γ-ray energy spectrum was measured for 18 150 η→π+π−γ events. ρ dominance of the π+π− final state is strongly favored over a simple gauge-invariant matrix element.