The eta transition form factor from space- and time-like experimental data

Abstract

The $η$ transition form factor is analysed for the first time in both space- and time-like regions at low and intermediate energies in a model-independent approach through the use of rational approximants. The $η\rightarrow e^+e^-γ$ experimental data provided by the A2 Collaboration in the very low energy region of the dilelectron invariant mass distribution allows for the extraction of the most precise up-to-date slope and curvature parameters of the form factors as well as their values at zero and infinity. The impact of these new results on the mixing parameters of the $η$-$η^\prime$ system, together with the role played by renormalisation dependent effects, and on the determination of the $VPγ$ couplings from $V\to Pγ$ and $P\to Vγ$ radiative decays are also discussed.