The Relativistic Heavy Ion Collider (RHIC) at the Brookhaven National Laboratory in the US has started colliding beams of copper ions. RHIC, which is actually two concentric rings 4 km in circumference, was built to create collisions between heavy ions, in particular gold. The use of intermediate-size copper nuclei – resulting in energy densities that are not as high as in earlier gold-ion runs, but more than was produced by colliding gold ions with much lighter deuterons – is important to understanding the new phenomena that have been observed in the heavy-ion collisions.
The energy of the gold-gold collisions was predicted to be sufficient to “melt” protons and neutrons to produce a hot “soup” of free quarks and gluons – the quark-gluon plasma. To date, the gold-gold collisions at RHIC have produced some very intriguing data that indicate the presence of a new form of matter – hotter and denser than anything ever produced in a laboratory. However, while some observations fit with what was expected of quark-gluon plasma, others do not.
So there has been considerable debate over whether the hot, dense matter being created at RHIC is indeed the postulated quark-gluon plasma, or perhaps something even more interesting. Data already in hand show that the quarks in the new form of matter appear to interact quite strongly with one another and with the surrounding gluons, rather than floating freely in the “soup” as the theory of quark-gluon plasma had predicted. Many physicists are beginning to use the term “strongly interacting quark-gluon plasma” to express this understandi
ng.
The deuteron-gold collisions do not exhibit the same behaviour, leading to the suggestion that what is seen in the gold-gold collisions is not an intrinsic property of the gold ions themselves, but is indeed created in the collisions. The copper experiments will provide another control that will help in understanding how the new phenomena observed are turned on and off, and when.
The copper-copper run is expected to last for about 10 weeks, but depends on funding for fiscal year 2005.
