Precise Determination ofρ−ωInterference Parameters from Photoproduction of Vector Mesons Off Nucleon and Nuclei

We have studied $\rho -\omega$ interference in di-pion photoproduction in the energy range 5-7 GeV with a total of 630 000 pion pairs from hydrogen, carbon, and lead. The mass spectra show clear interference in the vicinity of the $\omega$ mass. A branching ratio $\frac{{\Gamma }_{\omega \to 2\pi }}{{\Gamma }_{\omega \to \mathrm{all}}}=(1.22\mathrm{}\pm 0.30)\%$ and a relative phase to the $\rho \to \pi \pi$ amplitude of 96°±15° is found, where the errors include model-dependent uncertainties and are twice the size of the statistical errors alone.