"Since two fermions cannot turn into three fermions, the experimental observation of three-jet events in e+e- annihilation, first accomplished by the TASSO collaboration in June 1979 and confirmed by the other collaborations at PETRA two months later, implies the discovery of a new particle. Similar to the quarks, this new particle hadronizes into a jet, and therefore cannot be a color singlet. These three-jet events are most naturally explained by a hard noncollinear bremsstrahlung 
. [...] Thus the 1979 discovery of the second gauge particle, the gluon, occurred more than fifty years after that of the photon. This particle is also the first [...] gauge particle with self-interactions. Four years later, in 1983, the second and the third non-Abelian gauge particles, the W and the Z, were discovered at the CERN proton-antiproton collider by the UA1 and UA2 collaborations."
S.L. Wu
