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The Physics Background for non-scientists.

Physicists have overwhelming evidence that nuclear matter is made of point-like quarks and gluons, but no one has ever seen a free quark or gluon. The quarks and gluons seem to be confined inside hadrons such as protons and neutrons. Inside these entities the quarks and gluons behave as free particles. It is part of the theory called Quantum Chromo Dynamics (QCD). It is a consequence of the properties of empty space, i.e. of the vacuum. By compressing nuclei so that individual protons and neutrons overlap or dump an enormous amount of energy into a very small volume, it should be possible to create a much greater volume in which there are free quarks and gluons. This state is called a Quark-Gluon Plasma (QGP). If physicists can create it, the study of QGP should cast light on QCD and the problem of confinement.

If physicists are able to create a QGP it will expand and decay into hadrons (strongly interacting particles such as protons, neutrons, and pions) and will recreate the scenario that occurred at about a micro-second after the Big Bang. At that time the energy which is inside our present universe occupied a volume of about the size of our solar system. It consisted entirely of the QGP at a temperature of about 1,000,000,000,000 degrees. It was rapidly expanding and cooling. Suddenly the temperature fell below a critical value and a phase transition occurred. This process is analogous to droplets forming in cooled steam. For the first time in the history of the universe, nuclear matter (protons, neutrons, etc.) was created. Recreating and studying the process, even on a very small scale, may teach physicists something about the evolution of the universe.

As a by-product, the study of the QGP may also give scientists information about the properties of the vacuum. The present understanding of the vacuum is that it is anything but empty. It is full of fluctuations; particles are appearing and disappearing. The vacuum can be characterized by the type of fluctuations that occur. Theorists have speculated that following the decay of a QGP, occasionally a different state of the vacuum might be produced.

How will the Quark Gluon Plasma be created and studied?

The cartoon below illustrates how it might be possible to create the QGP. Scientists believe that if nuclei are accelerated to velocities which are very close to the speed of light, and then made to collide head-on, they will either be compressed sufficiently or dump enough energy in a small volume to produce the QGP.

Facilities have been built in the US and Europe to accelerate nuclei to higher and higher velocities and to study these collisions. To date, there is no clear evidence of the creation of a QGP.

With the completion of the construction of Relativistic Heavy Ion Collider (RHIC) in 1999 a major advance will occur. The collider will accelerate gold in two separate rings which will circulate in opposite directions around a pair of superconducting magnet rings in a tunnel 3.8 kilometers in circumference. The gold nuclei will reach speeds which are equal to 99.995% of the speed of light, and in their collisions energy densities will be produced equivalent to the mass of the earth in volume 10 x 10 x 10 meters.

The gold nuclei will be made to collide at four locations around the rings. Here sophisticated detectors will be placed to examine the consequences of the collisions. Since the precise outcome of these collisions is not known, each detector is optimized to be sensitive to different possible outcomes. One of the four detectors is called Phobos.

An Overview of Phobos

The Phobos concept is based on the premise that interesting collisions will be rare but that when they do occur the new physics will be readily identified. Thus the Phobos detector is designed to be able to examine and analyze a very large number of unselected gold-gold collisions. For each collision the detector gives a global picture of the consequences of the collision and detailed information about a small subset of the nuclear fragments ejected from the high energy density region.

The PHOBOS Detector

PHOBOS consists of many silicon detectors surrounding the interaction region. With these detectors physicists will be able to count the total number of produced particles and study the angular distributions of all the products. With this array they will be on the look out for unusual events, fluctuations in the number of particles and angular distribution. Physicists know from other branches of physics that a characteristic of phase transitions are fluctuations in physical observables. In order to obtain more detailed information about these events the PHOBOS detector will also have two high quality magnetic spectrometers which will study, in detail, 1% of the produced particles.

The PHOBOS detector will be able to measure quantities such as the temperature, size, and density of the fireball produced in the collision. It will also study the ratios of the various particles produced. With this information it should be possible to both detect and study a phase transition that might occur between QGP and ordinary nuclear matter. With PHOBOS, the group hopes to discover the QGP and learn more about confinement and the early universe.