Analysis of rho - omega interference in the pion form - factor

The formalism underlying the analysis of e + e − → π + π − in the ρ − ω interference region is carefully revisited. We show that the standard neglect of the pure I = 0 omega, ω I , " direct " coupling to ππ is not valid, and extract those combinations of the direct coupling and ρ-ω mixing allowed by experiment. The latter is shown to be only very weakly constrained by experiment , and we conclude that data from the e + e − → π + π − interference region cannot be used to fix the value of ρ−ω mixing in a model-independent way unless the errors on the experimental phase can be significantly reduced. Certain other modifications of the usual formalism necessitated by the unavoidable momentum-dependence of ρ−ω mixing are also discussed.