QUARK-GLUON PLASMA AND NUCELAR COLLISIONS

Johann RAFELSKI


Professor of Physics, University of Arizona

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.

Quark Deconfinement

As we squeeze the normal nuclear matter in the collision at relativistic energy, the individual nucleons dissolve, and their constituents form a new phase in a transformation that is at first sight, reminiscent of the conversion of atomic- into plasma-phase of normal matter: in the conventional matter the hot atomic gas (here hot hadronic gas) turns into ions and electrons (here quarks and gluons). However, there is a profound difference: the melting of the vacuum is believed to be a sudden change of structure, akin to the transformation from ice to water, rather than the gradual change that accompanies the conversion of atomic into ion plasma matter. From the fundamental perspective, the study of this `vacuum melting' and the exploration of other physical properties of the quantum vacuum is the primary objective of the nucleus-nucleus high energy collision experimental program.

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.

Quark Matter

It is clear that quark matter in the unusual molten vacuum is no ordinary matter. Roughly speaking, we need to compress ordinary matter by a factor of about 10^{15} to produce quark matter. It takes pressures in excess of 10^{30} atmospheres to achieve these densities. With the energies available in collisions between nuclei at CERN, and perhaps also at the Brookhaven National Laboratory on Long Island near New York, we can now create these extreme conditions. Before the experimental program has begun 12 years ago, some have believed that small nuclear projectiles will pass through a big target nucleus without much interaction. The reality has proven that strongly interacting matter behaves as called: a large nucleus turns out to be a very hard, non-transparent target to impinging lighter projectile, and experiences the required acceleration of a few times 10^{31}g as it slows down.

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.

Experimental Methods

The direct electromagnetic probes, that is directly produced photons and electron-positron, muon-antimuon pairs are witnesses to the earliest moments of the reaction, but their production rates are in general very small, and thus the experimental yield is dominated by the secondary decay processes of the much more often produced hadronic (strongly interacting) particles. Hadronic multiplicity is growing rapidly with both energy and size of the projectile-target nuclei. With up to 10 mesons in the final state per each participating baryon seen at CERN, the observation and understanding of these particles poses a formidable challenge.

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.

A decade from now

Our hopes already reach to the yet more powerful CERN's facility ALICE at the Large Hadron Collider presently under construction. The energetic collisions we are hoping to study at LHC in 6 years will rival the most energetic cosmic ray collision events ever observed. We will deepen our understanding of the origin of mass, allow to study of the modern day ether, and the conversion of energy into matter.

Publications after1991

July 3, 1998