An article in an August 2000 issue of the New York Times listed understanding confinement of quarks inside of protons and neutrons as one of the ten fundamental questions in physics to ponder for the 'next millennium or two'. Nearly all of the visible matter in the universe is forever trapped inside the nuclear cores of the atoms that we observe. While scientists believe that the theory of Quantum Chromodynamics, (QCD), can explain confinement, an exact understanding of how QCD works has been extremely elusive. We know that QCD works under the extreme conditions found in high energy particle collisions, but our knowledge of what it is doing under normal conditions found in the every day world is quite limited. Using advances in high speed computing and experimental facilities that could soon be available at laboratories in the United States, scientists hope to go a long way in answering this question within the next decade.
QCD is a theory that describes how quarks and gluons interact with each other. It was originally formulated based on experiments in the 60's and 70's, leading to many Nobel Prizes for experimentalists and theorists alike. QCD describes quarks as carrying three types of charge called color charge: red, green and blue. These names have nothing to do with the colors we are familiar with; it is only a naming scheme. However, like the colors we are familiar with, equal combinations of each color yields a white or color-neutral object. Each of these color charges can be positive or negative (quark or anti-quark). QCD requires that the combinations of quarks that can exist in nature are colorless. The simplest combinations satisfying this are called baryons (three quarks each of different color) and mesons (quark and anti-quark , color and anti-color).
In addition to the quarks and anti quarks, QCD has force-carrying particles called gluons which are exchanged between quarks and antiquarks to produce the force that holds matter together. What is unique about QCD is that the gluons also carry color; they are not white or neutral objects. It is exactly these colored gluons that allow for the complex nature of QCD, but have also made a complete understanding extremely elusive. We can solve for what quarks are doing when they are very close together, but when we try to look at something the size of a proton, all of our tools fail.
Because gluons are not neutral, it should be possible for gluons to interact with each other to form matter containing only gluons with no quarks. All that is required is that the combinations are colorless. Combinations of two or three gluons will do the trick and such states are called glueballs. We have possible experimental evidence for the lightest of these states but our theoretical understanding of the data is confusing. Glueballs unfortunately look like normal mesons with similar properties called quantum numbers. And therein lies the problem - they can become quantum-mechanically mixed up with the normal mesons and distinguishing what is what becomes a difficult task. Other new mesons should also be possible and these would involve combinations of quarks and gluons called hybrid mesons. There are specific combinations that lead to hybrid mesons with quantum numbers that are not allowed for simple quark-anti-quark combinations, called exotic hybrid mesons. The beauty of these exotic hybrids arises from the fact that they cannot become mixed up with other mesons. Recent experimental results have hinted at the existence of a single such state, but the rich spectrum that would directly yield information on the confining force has so far been elusive. Taken as group, glueballs and hybrid mesons are known as gluonic matter or gluonic excitations.
While the theory of QCD has been known for quite some time, we have only recently reached a point where we can start to understand confinement. Advances in a technique called lattice gauge QCD and significant improvements in computation power have combined to make the solution to QCD within reach. Lattice QCD solves QCD exactly in a discretized space-time world, but to do so, it requires massive computational power. Multi-teraflop computers are needed to allow theorists to do these calculations and to make detailed predictions for the spectrum of exotic hybrid mesons. Such computers are just now being put together. However, the final arbiter in determining which model of confinement is correct is experiment. So in parallel, experimenters are using recent developments in technology to carry out experiments that will map out the spectrum of this new type of matter with a focus on those hybrids that are distinctly different from normal mesons. They are the smoking gun unambiguous evidence of gluonic excitations.
Several factors have conspired to make these experiments now possible. It has been realized that a very good way to produce these exotic particles is with beams of high-energy polarized photons. However until recently, no high quality, high intensity source of these photons has been available. The startup of Jefferson Lab and the beams that would be possible with an Upgraded 12 GeV CEBAF have completely changed this landscape. Also, the computer networks and data handling capabilities that are only now becoming available will allow the experimenters to collect and analyze the Petabytes of data necessary to unravel the full spectrum of these exotic particles.
The combination of all of these should make it possible for us to understand QCD and answer the question of confinement over the next decade. We will try to put QCD on the same footing as what we know about atomic physics. We also expect with the very rich nature of QCD, that there will be both exciting discoveries and break throughs as we find the answer to why quarks are forever confined.
