Measurement of the $\omega \to {\pi }^{+}{\pi }^{-}{\pi }^0$ Dalitz plot distribution

Using the production reactions $pd\to {}^{3}He\omega$ and $pp\to pp\omega$,theDalitzplotdistributionforthe$\omega \to \pi^+ \pi^- \pi^0$ decay is studied with the WASA detector at COSY, based on a combined data sample of $(4.408\pm 0.042)\times {10}^4$ 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 $\varphi$. For the first time, a deviation from pure $P$-wave phase space is observed with a significance of $4.1\sigma$. The deviation is parametrised by a linear term $1+2\alpha Z$, with $\alpha$ determined to be $+0.147\pm 0.036$, consistent with the expectations of $\rho$-meson-type final-state interactions of the $P$-wave pion pairs.