The concept of strange quark matter, single multiquark bags (in the sense of the MIT bag model) containing roughly equal numbers of u, d, and s quarks, dates back to the work of Jaffe [1] and Chin and Kerman [2]. In 1984 Witten [3] proposed that strange quark matter might be absolutely stable and might indeed be the true ground state of baryonic matter. In essence, the use of three types of quarks in ``building'' a single large bag avoids some of the energy penalty stemming from the Pauli principle when only two types are available. In addition, the negative charge of the strange quark causes the resulting droplets of quark matter to be nearly neutral, thus avoiding instabilities due to Coulomb repulsion energy.
Strange quark matter has many fascinating properties, and its existence would have major impacts on physics, astrophysics, cosmology, and possibly on technology as well [4]. A recent international workshop [5] reviewed the physics of strange quark matter and its role in astrophysics and cosmology. A remarkable conclusion from a recent letter of intent for a CERN strangelet search [6] was that ``this hypothesis [strange quark matter] has not been scathed by seven years of intense and combined effort of cosmologists, astrophysicists, and hunters of exotica.''
In essence, even though the consequences of the existence of strange quark matter are major, e.g. neutron stars are really strange quark matter stars (or perhaps have cores thereof), there is sufficient ambiguity in the parameters, and sufficiently limited data that the basic question remains open. Further, experiments with the requisite sensitivity (such as E864) have only recently been initiated.
Farhi and Jaffe [7] worked out the physics of strange quark matter in the context of the MIT bag model. The term strangelets refers to strange quark matter systems of low baryon number, A, which are stable against strong decay but not necessarily against weak decay. It is these which might be accessible in AGS high energy heavy ion experiments.
As is discussed in the section on physics goals, a sensitivity of
, the goal of E864, does allow significant
constraints on the parameters of strange quark matter.
However, there is a more general motivation for a high energy heavy ion search
with high sensitivity and broad reach. These collision systems represent a
new ``entrance channel'' which has not been previously explored, and which
make new regimes of quantum numbers experimentally accessible. Strange quark
matter is the best known of such possible states, but there are others as
well. The SU(3) chiral soliton (Skyrme) model predicts light (A=2,3,4)
clusters with multiple strangeness, which could be stable with respect to
strong and even weak interactions. These predictions are speculative but
sufficiently dramatic to merit an experimental test. The chiral solitons are
not strangelets; the solitons are bound states of individual hyperons, while
the quarks in a strangelet form a single state, and are not bound as a
collection of individual baryons. Moreover, solitons may form bound
composites with modest baryon numbers like 
, while strangelets are
expected to be unstable when 
.
The fundamental point here is that high energy heavy ion collisions produce a system with high density of strangeness and baryon number in both configuration space and momentum space. States with multiple strangeness and/or baryon number can be produced which would have negligible production rates in other collision processes.
In addition, the collision system for high energy heavy ions is dynamically different from that encountered in ``elementary particle'' collisions. It has been speculated that if QCD is ``slightly'' broken or if, contrary to current thinking, QCD does not lead to absolute confinement of color, states of bare color might be much more readily produced in high energy heavy ion collisions than in simpler collision systems. These aspects are described more fully in the section on physics goals.
In summary, these collision systems provide a new dynamical regime whose study may result in discoveries of new elementary particle and nuclear systems.