“…Several pieces of information are available : the partial widths [9] are known with an accuracy of the order 10% cross-checked by several means, the η spectrum as function of the photon momentum has been measured long ago [10,11] and provides useful information. Finally, measurements of the η ′ spectrum as function of the dipion invariant-mass have been performed twelve times, with various levels of precision, and the corresponding data are published as papers from several Collaborations [13,14,15,16,17,18] or are available as PhD theses [19,20,21,22]. The latest measurements have been performed recently by CERN Collaborations [23,24].…”