Measurement of the ω → π⁺π⁻π⁰ Dalitz plot distribution

Using the production reactions pd→He3ω and pp→ppω, the Dalitz plot distribution for the ω→π⁺π⁻π⁰ decay is studied with the WASA detector at COSY, based on a combined data sample of (4.408±0.042)×10⁴ events. The Dalitz plot density is parametrised by a product of the P-wave phase space and a polynomial expansion in the normalised polar Dalitz plot variables Z and ϕ. For the first time, a deviation from pure P-wave phase space is observed with a significance of 4.1σ. The deviation is parametrised by a linear term 1+2αZ, with α determined to be +0.147±0.036, consistent with the expectations of ρ-meson-type final-state interactions of the P-wave pion pairs.