The atomic nucleus is a quantum bound state of nucleons
(protons and neutrons) each comprising three confined i.e.
permanently bound valance quarks. To the best of our knowledge,
quarks always come in twos (mesons consist of a quark and an
antiquark pair) and threes.
The dynamics of nearly mass-less quarks inside
a nucleon is fully relativistic. Thus
the inertial mass of protons, neutrons,
and hence of practically all the matter around us, is believed
to originate in the confinement of
quarks inside the small volume of the nucleon, of radius
1 in 100,000 of the size of an atom. The mass of the nucleon
and hence of the nucleus, and therefore of about 98% of all
matter known to us, is understood to be the relativistic quark
quantum-zero-point energy. The
indirectly arising interactions, due to the properties of
the confining strongly interacting vacuum state,
determine the scale of hadronic size, and thus mass, and
thus all the properties of the Universe we see around us.
By recreating the
conditions that existed about 10--30 microseconds after the Big Bang,
experiments may now have created a new
vacuum state, in which matter is nearly mass-less, and
quarks are free.
In the Big-Bang epoch it was the high temperature that
revealed this simple deconfined structure of matter known as the
quark--gluon plasma (QGP). In QGP, gluons the stuff that
binds, and the quarks are no longer confined
in hadrons, the collective name for mesons and baryons.
Laboratory experiments `melt' the space between particles setting
confined quarks and gluons free to roam where temperature is high.
Tantalizing
evidence for the quark-gluon plasma, also known as quark matter,
has emerged in recent years from the international particle physics
laboratory, CERN in Geneva.
Attentive readers will note that we have recreated the physical
picture of the once extinct ether. At the beginning of the century,
the ether was discarded as a superfluous concept, and the theory
of relativity had made it all but unobservable. Times have changed:
both quantum mechanics and strong interactions have been discovered
and combined in quantum chromodynamics (QCD), the theory of the
strong force. This new approach has made us turn full circle in our
perceptions about the relativistically invariant
ether, the quantum vacuum state. Today the vacuum is
seen as an active element of the description of physical laws.
To many theorists the process "vacuum
melting" is one of the most interesting aspects of the high-energy
nucleus-nucleus collision program. Thus there is a substantial
theoretical effort to obtain a better
understanding of the properties of hadronic matter at finite
temperature. This is done by numerical simulations of QCD on a
space lattice.
The dense fireball reaches temperatures as high
as 3 10^{12}K before exploding in a micro bang not different
from the Big Bang. There is little precious time to observe
new physics we seek. We must detect reliably the formation of an
unknown phase, existing only as short
as 0.5 10^{-22}sec.
It is more practical to select a small subset of
these particle to study. The criterion for selection is typically that: a) the
particle of interest undergoes a spontaneous and a specific, self-analyzing
decay, that is easy to see against the background of in some cases
many thousands other particles; and b) we are primarily
interested in matter created from the energy of the collision.
These two criteria leave us with three types ofobservables today:
antimatter, strangeness and strange matter, and charmonium, the
bound state of heavy charmed quarks. Numerous experiments were
developed in the past decade to probe the production of these
classic observables, and indeed experiments have seen the type of
behavior that was predicted to appear should deconfinement occur as
hoped for already at collision energies reachable at CERN today.
However, it is too early to conclude if the hoped for discovery has
been made, since we still lack a systematic exploration
as function of collision energy and other parameters of the
key observables.
To facilitate systematic experimental
effort there are two major facilities under construction in USA
and Europe. Within a year, at Brookhaven, the more powerful
Relativistic Heavy Ion Collider (RHIC) will
be completed, practically totally devoted to this new physics.
The hope and expectation is that the 10 times higher energy can be
reached there. This will make the signatures of deconfinement more spectacular,
allowing direct measurement of properties such as latent heat of the
vacuum state transformation. More than 1000 experimental high energy and
nuclear physicists from all ove the entire World, are today
busy preparing for the first beam expected in April 1999, and they
receive ardent support from theorists, with sizable effort sited at
the Department of Physics, University of Arizona.
July 3, 1998