Fig. 2.

Determination of the ${\eta }^{\prime }$ TFF by means of Eq. (2.3) in comparison to data from BESIII [108] (statistical and systematic errors added in quadrature) scaled with $F_{{\eta }^{\prime }\gamma \gamma }$ from Table 1 and the VMD model of Eq. (D.23) with $\varphi$ resonance contribution according to Eq. (D.22); for the kinematic range accessible in ${\eta }^{\prime }$ decays (left) and a larger time-like region including the $\varphi$ resonance with inset magnifying the low-s region (right). See also Ref. [109] for an earlier version of this figure