Dispersive analysis for η→ γγ.

A dispersion integral is derived that connects data on η→ π π γ to the η→ γγ transition form factor. A detailed analysis of the uncertainties is provided. We find for the slope of the η transition form factor at the origin $b_{\eta}= (2.05 \ \mbox{}^{+0.22}_{-0.10} )~\mathrm{GeV}^{-2}$. Using an additional, plausible assumption, one finds for the corresponding slope of the η′ transition form factor, $b_{\eta'} = (1.53\ \mbox{}^{+0.15}_{-0.08} )~\mathrm{GeV}^{-2}$. Both values are consistent with all recent data, but differ from some previous theoretical analyses. [ABSTRACT FROM AUTHOR]