## Dispersive analysis for $η\to γγ^*$    [PDF]

C. Hanhart, A. Kupsc, U. -G. Meißner, F. Stollenwerk, A. Wirzba
A dispersion integral is derived that connects data on $\eta\to \pi^+\pi^-\gamma$ to the $\eta\to \gamma\gamma^*$ transition form factor. A detailed analysis of the uncertainties is provided. We find for the slope of the $\eta$ transition form factor at the origin $b_\eta = (2.05 {}^{+0.22}_{-0.10}) {GeV}^{-2}$. Using an additional, plausible assumption, one finds for the corresponding slope of the $\eta'$ transition form factor, $b_{\eta'} = (1.58 {}^{+0.18}_{-0.13}) {GeV}^{-2}$. Both values are consistent with all recent data, but differ from some previous theoretical analyses.
View original: http://arxiv.org/abs/1307.5654