Harald Fritzsch, one of the pioneers of quantum chromodynamics, recalls some of the background to the development of the theory 40 years ago.
About 60 years ago, many new particles were discovered, in particular the four Δ resonances, the six hyperons and the four K mesons. The Δ resonances, with a mass of about 1230 MeV, were observed in pion–nucleon collisions at what was then the Radiation Laboratory in Berkeley. The hyperons and K mesons were discovered in cosmic-ray experiments.
Murray Gell-Mann and Yuval Ne’eman succeeded in describing the new particles in a symmetry scheme based on the group SU(3), the group of unitary 3 × 3 matrices with determinant 1 (Gell-Mann 1962, Ne’eman 1961). SU(3)-symmetry is an extension of isospin symmetry, which was introduced in 1932 by Werner Heisenberg and is described by the group SU(2).
The observed hadrons are members of specific representations of SU(3). The baryons are octets and decuplets, the mesons are octets and singlets. The baryon octet contains the two nucleons, the three Σ hyperons, the Λ hyperon and the two Ξ hyperons (see figure 1). The members of the meson octet are the three pions, the η meson, the two K mesons and the two K mesons.
In 1961, nine baryon resonances were known, including the four Δ resonances. These resonances could not be members of an octet. Gell-Mann and Ne’eman suggested that they should be described by an SU(3)-decuplet but one particle was missing. They predicted that this particle, the Ω–, should soon be discovered with a mass of around 1680 MeV. It was observed in 1964 at the Brookhaven National Laboratory by Nicholas Samios and his group. Thus the baryon resonances were members of an SU(3) decuplet.
It was not clear at the time why the members of the simplest SU(3) representation, the triplet representation, were not observed in experiments. These particles would have non-integral electric charges: 2/3 or –1/3.