Dispersive analysis for $\eta \to \gamma {\gamma }^{\ast }$

C. Hanhart, A. Kupsc, U. -G. Meißner +2

A dispersion integral is derived that connects data on $\eta \to {\pi }^{+}{\pi }^{-}\gamma$ to the $\eta \to \gamma {\gamma }^{\ast }$ transition form factor. A detailed analysis of the uncertainties is provided. We find for t…