Physics of strange matter. Chapter 27

All known nuclei are made of the two nucleons, the proton and the neutron. Besides those two lightest baryons there exist still a couple of other stable (but weakly decaying) baryons, the hyperons. These were found for the first time in cosmic ray experiments and were entitled as strange because of their unusual long lifetimes of about 10-10 sec. For their notation in the hadron zoo of elementary particles one introduced the new quantum number strangeness (or hypercharge). In the quark picture baryons are interpreted as being made out of three constituents, the quarks. Correspondingly the proton (uud) and neutron (udd) are made out of the two lightest quarks, the up and down quark. For the description of the hyperons a third flavour, the strange quark, was demanded. The lightest hyperon, the Λ-particle (uds), and the three Σ -particles (uus,uds,dds) contain one strange quark, the two more heavy Ξ-particles (uss,dss) contains two strange quark and the Ω-particle (sss) is solely made put of three strange quarks. Up to now strangeness remains an experimentally as theoretically rather largely unexplored degree of freedom in strongly interacting baryonic matter