In electron-positron annihilation
(Frascati seminar at March 7, 1960 by
Bruno Touschek
)
one observes threshold enhancements:
Ann.Phys.323,1215(2008);
Eur.Phys.Lett.81,61002(2008); 84,51002(2008);
Phys.Rev.D80,074001(2009)