The search for higher symmetries

The eight-baryon puzzle

Let us begin with a very simple observation: there are eight baryons. By this I mean there are eight positive-parity particles with spin one-half and nucleon number one (the nucleons, the ∧, the ∑s, and the Ξs), and that the masses of these particles are close together, all lying within 20% of their common mean mass. There are no other particles with the same parity, spin, and nucleon number which lie at all close in mass to these eight. Now, eight is a disquietingly large number of particles; although there is no fundamental reason why we should not have eight independent particles in a field theory, with eight arbitrary masses and eight sets of arbitrary coupling constants, still, it would be very pleasant if there were some way to reduce the number of entities. Two methods immediately suggest themselves:

1. (i) Perhaps some of the baryons are fundamental and the others are composite meson–baryon states. This was the central idea of the original Sakata model, where p, n, and ∧ were taken as fundamental. Then the approximate mass degeneracy would arise because the masses of the mesons are small compared to the masses of the fundamental baryons. This idea has not met with much success.

2. (ii) Perhaps the strong interactions are approximately symmetric under a group larger than the ordinary isospin–hypercharge group.

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