1. See e.g., F. Wilczek, in QCD – 20 Years Later, ed. by P.M. Zerwas, H.A. Kastrup (World Scientific Publ. Co., Singapore, 1993)
2. M. Gell-Mann, Phys. Lett. 8, 214 (1964)
3. G. Zweig, Int. J. Mod. Phys. A 25, 3863 (2010) and earlier references given there
4. J.C. Pati, A. Salam, Phys. Rev. D10, 275 (1974)
5. W. Buchmüller, in Nucleon-Nucleon and Nucleon-Antinucleon Interactions, ed. by H. Mitter, W. Plessas (Springer, Wien/New York, 1985)