Abstract
We present a dispersive representation of the η' transition form factor that allows one to account, in a consistent way, for the effects of ρ - ω mixing in both the isoscalar and the isovector contributions. Using this formalism, we analyze recent data on η' → π+π-γ to constrain the isovector part of the form factor, individually and in combination with data for the pion vector form factor, which suggests a tension in the ρ - ω mixing parameter. As a first application, we use our results, in combination with the most recent input for the isoscalar part of the form factor, to predict the corresponding spectrum of η' → ℓ+ℓ-γ , in particular we find the slope parameter bη' = 1.455(24) GeV-2 . With forthcoming data on the latter process, our results establish the necessary framework to improve the evaluation of the η' -pole contribution to the anomalous magnetic moment of the muon using experimental input from both η' decay channels.