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)