The Physics of RHIC II
By Jamie Nagle
Nuclear physics in the United States has become a very broad scientific enterprise with many fascinating, cutting edge research aspects trying to answer some of the most important questions in science. Approximately every five years the Department of Energy and National Science Foundation charges the Nuclear Science Advisory Committee (NSAC) to "conduct a new study of the opportunities and priorities for United States nuclear physics research and recommend a long range plan that will provide a framework for coordinated advancement of the Nation's nuclear science research programs over the next decade." Over the past year there have been various Town Meetings for different sub-fields including one for QCD physics in Rutgers, New Jersey in January 2007, and then a meeting of all sub-fields in Galveston, Texas in May 2007 to come to some consensus on these future priorities. One of the most exciting scientific opportunities is at the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory.
At the Galveston meeting a set of presentations tried to answer the questions: What have we learned in the first six years at the Relativistic Heavy Ion Collider? What is the fundamental science at RHIC (heavy ion and spin) in the next decade? What is required to make this science a reality?
The field of relativistic heavy ions aims to first create nuclear matter at energy densities only seen in the very early universe and at the core of stars, and then study its properties in detail. We have succeeded in creating such matter in the laboratory at RHIC, and find that at these very large energy densities the matter equilibrates very rapidity (< 1 fm/c), flows as a nearly perfect liquid (small viscosity), has large color fields, collective excitations, and final hadron distributions that reflect the underlying quark structure. These discoveries together were named the American Institute of Physics (AIP) story of the year in 2005.
These discoveries have peaked the scientific community's interest, and there is strong desire to understand how these properties come about. For example, there are predictions of a lower bound for viscosity of any material, interestingly first calculated using string theory. The future RHIC program can measure the flow of heavy quarks (charm and beauty) with precision to determine if this nuclear matter is the most perfect liquid in nature. Theoretical developments on viscous hydrodynamics are also needed to complete this picture. Additionally, this liquid may respond (like ripples in water) to high energy partons and high statistics measurements will be able to confirm or refute this. If we run RHIC at lower energies, recent calculations predict a phase diagram critical point that may be observed with upgrades detectors. These are just some of the excited measurements of a future RHIC program.
Additionally there is a world wide effort to understand the dynamics of quarks and gluons inside the proton - in particular how they account for the spin of the proton. The RHIC spin program has started to constrain the gluon contribution and the future program includes mapping out this contribution as a function of the momentum fraction of the proton (x), sea-quark contributions, and an exciting program in transverse spin physics.
In order to make this science a reality over the next decade, important detector upgrades to the PHENIX and STAR detectors, a ten-fold increase in Relativistic Heavy Ion Collider collision rate (termed RHIC II), and a focused advancement in theoretical efforts are needed. There are fascinating scientific questions that can be directly answered with RHIC II and an excited young community eager to pursue the science.
In a world of limited budgets and many scientifically compelling ideas, difficult choices are required. Coming out of the Galveston meeting, the RHIC II science program emerged as a recommended priority, but continued hard work is needed to make it happen. There is much more still to be discovered and a deeper understanding to be gained.