Stable baryonic matter in the Universe currently appears to be composed of up and down quarks. However, a system of 3A up, down, and strange quarks would have a lower energy per baryon than normal nuclear matter. The energy savings comes from the decreased Fermi levels of a system with 3 different flavors instead of only two. The quarks in this case would not form individual baryons, but would have wave functions ranging over the entire size of the system. Color must still be confined, so it is still possible to talk about baryon number when discussing such a system. This would represent a new ground state of matter.
In 1984 it was suggested that such strange quark matter, or `strange matter,' might be both stable and bound at zero temperature and pressure. Normal nuclear matter does not decay to this true QCD ground state due to the high order weak process necessary to produce the strange quarks in abundance. The radically different wave functions between nuclear and strange matter would also greatly inhibit such decay. However, certain specific processes can be envisioned that would produce strange matter.
It is natural to divide the spectrum of strange matter into 3 categories by size. Each requires a different model. Bulk strange matter is sufficiently large that no surface effects need to be considered. The concepts needed to explain bulk strange matter are useful in more detailed models of `strangelets,' nuggets of strange matter with a baryon number . Very small strangelets would resemble superheavy isotopes of known elements. A model of very small strangelets requires a different approach, as these systems are too small to simply apply a bulk model with surface corrections.
Naturally occuring strangelets may have formed in the early stages of the Universe. Strange matter may also be formed in the present under high pressure in the dense interiors of neutron stars. In order to detect naturally occuring strange matter, or to produce it in the lab, it is necessary to determine its stability. An understanding of the interactions between strange and normal matter is also important.
Several searches have been conducted for natural sources of strangelets, both astronomical and terrestrial. Accelerator experiments have attempted to produce strangelets artificially, and more are planned as new facilities become available. Occasionally, speculations on the properties of and uses for strangelets have resembled science fiction.
Finally, there are many details which must be thoroughly investigated in order to make meaningful specific predictions from the models I will discuss. (For example the choice of renormalization scale.) Rather than attempt this, I will be interested in revealing the general properties of strange matter predicted by these models.