An intriguing possibility that atomic nuclei could be only an isomeric meta-stable state of a more general system of baryon number A was first discussed by Bodmer (1971). He assumed that for a sufficiently large A there exists a

"collapsed nucleus" Ca of much smaller radius and much higher density than the "normal nucleus" Na. At a given A, the radius of Ca was supposed to be ~ 1/3 of the normal nucleus, so that the density of the collapsed nuclei was about thirty times higher than of the normal ones. The hypothetical Ca nucleus was much tighter bounded than Na and would correspond to the true ground state of the matter. Bodmer considered three models of Ca . Two of them were related to an abnormal state of nuclear and hyperonic matter with an enormous binding energy; such states are similar to the "abnormal matter" of Lee & Wick (1974), discussed in § 8.9. In the third model, Ca was built of the uds matter and had small positive electric charge. Using qualitative arguments, Bodmer showed that the existence of Ca is not in conflict with experimental data, if metastable Na are sufficiently long-lived isomers. For example, in one specific case he obtained the lifetime of Na significantly longer than 1024 yr. The paper by Bodmer was of qualitative character; the MIT Bag Model of hadrons was formulated three years later.

Quantitative studies started only after the formulation of the MIT Bag Model (Chodos et al., 1974; DeGrand et al., 1975). Eight years after the Bod-mer paper, the possibility of the existence of long-lived (as compared to the strong-interaction timescale ~ 10_22 s) "quark nuclei" was studied within the Bag Model by Chin & Kerman (1979). They stressed the necessity of s quarks for the stability of their "multiquark droplets". These uds droplets of A > 10 were shown to be stable with respect to strong interactions and decayed via weak leptonic processes. With the lifetime longer than 10_4 s, the multiquark droplets of Chin & Kerman were actually metastable and converted eventually into nuclei, in contrast to the strict strange matter hypothesis which would made them absolutely stable. Similar ideas on the existence of droplets composed of u, d, and s quarks were put forward independently and nearly simultaneously by Terazawa, who proposed to call them "super-hypernuclei" (Terazawa 1979, as quoted by Terazawa 1989a,b). However, his paper, published in symposium proceedings, was unnoticed at that time and had no impact on the later development of the idea of strange matter in the mid 1980s.

The strange-matter investigations got a real start after the classical paper by Witten (1984) who proposed a cosmological scenario, in which "quark nuggets" with equal numbers of u, d and s quarks could be formed. He suggested that these nuggets could appear during the hadronization epoch in the early Universe, ~ 10_5 s after the Big Bang. During a first-order phase transition, associated with the hadronization, some fraction of matter could condense as quark nuggets which, if stable, could form a component of the dark matter in the today Universe. Using the simplest version of the MIT Bag Model with massless non-interacting quarks (§ 8.5), Witten showed that for reasonable values of the bag constant the uds matter could be absolutely stable (the true ground state at zero pressure).

Shortly after the Witten's paper, Farhi & Jaffe (1984) published the paper entitled "Strange matter". They explored in detail properties of the uds matter, including the effects of finite s quark mass and lowest-order QCD interactions (see § 8.6). Using their version of the MIT Bag Model they demonstrated the existence of a wide region in the Bag-Model parameter space, in which uds quarks could form the ground state of the matter.

Since then, the strange matter hypothesis has been addressed in numerous papers, from theoretical and experimental points of view. In particular, strange matter has been searched for in cosmic rays and in relativistic heavy-ion collisions. The physics of strange matter is reviewed by Glendenning (2000), Weber (1999), Madsen (1999), and Weber (2005). A detailed review of earlier work, done before 1991, is given by Madsen & Haensel (1991).

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