The Nuclear Physics Long Range Plan: The Stakes for BNL
by Tom Ludlam
The Department of Energy and National Science Foundation are now in the final stages of a year-long effort to prepare a Long Range Plan for Nuclear Science that will set the agenda for the next ten years of U.S. research in nuclear physics.
These Long Range Plan studies, which have been carried out roughly every five years since the 1970s, are a big factor in shaping the scientific agenda that defines the field of nuclear physics. The Long Range Plan of 1983 initiated a new direction for the field, stating “We identify a relativistic heavy ion collider as the highest priority for the next major facility to be constructed, with the potential of addressing a new scientific frontier of fundamental importance.” Having realized that potential, BNL is a major player in the current exercise, with much at stake.
In the years since RHIC turned on in 2000 the new collider and its four detectors, BRAHMS, PHENIX, PHOBOS, and STAR, have been at the center of some of the most exciting scientific discoveries worldwide, and BNL has become a flagship laboratory for U.S. nuclear physics. In a period of six years RHIC has provided heavy ion as well as proton and deuteron beams over a wide range of collision energies. It has become the world’s first and only collider of spin-polarized proton beams, and has exceeded its design performance goals for both heavy ion and proton-proton collisions. Never before has a high energy collider delivered so much so quickly. With a pace of scientific output rivaling that of any of the world’s major accelerator facilities, the RHIC scientists, along with the rest of the nuclear physics community, are taking a long look into the future.
The future promise emerges in large measure from the discoveries in high energy collisions of gold nuclei at RHIC, leading to the conclusion that the state of matter called the Quark-Gluon Plasma can indeed be produced under laboratory conditions. Created at temperatures greater than a trillion degrees, and densities higher than anything previously observed in our universe, this matter surprised the scientific world by exhibiting the properties of an extraordinary liquid—the most nearly perfect liquid ever seen.
Along the way, RHIC’s unique capability for accelerating and colliding high energy beams of spin-polarized protons is now producing important new results aimed at resolving one of the great puzzles of modern physics – Why is it that the quarks which are bound together to form a proton, each quark having its own spin, have been found to contribute only a small fraction of the actual spin of the proton? The RHIC spin experiments will soon be able to establish whether the missing spin is carried by the gluons – the quanta of energy that bind the quarks together.
These results have a common thread that leads to a rich vein of fundamental scientific questions regarding elemental the force that binds the quarks into nuclear matter and is ultimately responsible for the origin and structure of the visible mass of our universe. This “strong force” is described by the theory of quantum chromodynamics, or QCD. The Quark-Gluon Plasma, whose surprising properties we have just begun study, is thought to be one of possibly many phases of matter made up of quarks and gluons at extreme temperatures and densities. Similarly, QCD, at some fundamental level that we do not yet understand, determines how the simple properties of the proton emerge from the complicated substructure of quarks, anti-quarks and gluons revealed in measurements such as the RHIC spin experiments.
The discoveries at RHIC offer a unique new window to explore the mysteries of QCD, addressing questions of scientific importance that reach well beyond heavy ions and nuclear physics. To exploit this new opportunity, an international community of scientists has developed plans for new facilities at Brookhaven, building on the present RHIC complex of accelerators and detectors with:
- A sequence of upgrades to the PHENIX and STAR detectors.
- RHIC II: A major accelerator upgrade to increase the luminosity (collision rate) for heavy ion beams at RHIC.
- eRHIC: The addition of a high-energy spin-polarized electron beam to collide with the heavy ion and polarized proton beams of RHIC, as well as a new large detector.
RHIC II and the upgrades to the existing detectors will enable more intense beams of ions and protons for a deeper study of the basic properties of Quark-Gluon Plasma and an extended program to study the proton spin. eRHIC will provide high energy beams of polarized electrons to collide with the polarized proton beams of RHIC, allowing for precision imaging of the quarks and gluons to determine the full spin and spatial structure of the proton. Electrons colliding with heavy ions in eRHIC will also allow definitive experiments on yet another fundamental state of matter – a super-dense assembly of gluons that may have been glimpsed as a precursor to the formation of Quark-Gluon Plasma in the RHIC experiments, and which BNL theorist Larry McLerran and his colleagues have dubbed the Color Glass Condensate.
Taken together, this series of upgrades from the present RHIC, to RHIC II, and ultimately eRHIC is designed to provide a comprehensive “QCD Laboratory” to study the nature of quark-gluon matter, its relationship to the properties of the early universe and the transition to “ordinary” matter of protons, neutrons, and nuclei; the detailed properties of the “glue” that binds matter in these various forms; and the full understanding of how the seemingly complex QCD structures combine to form the observed properties of the proton.
The R&D, design, and construction of the new components required to realize RHIC II and eRHIC will be equally challenging, and will require investments in construction totaling hundreds of millions of dollars over the coming decade. A graphic summary of this long-term outlook is shown in the accompanying figure.
This vision for Brookhaven is an ambitious one, with huge scientific potential. Not surprisingly, other nuclear physics laboratories around the country have plans of their own. The purpose of the Long Range Plan exercise that is now going on across the entire nuclear physics community is to lay out all of these plans, debate the relative scientific merits, weigh them against the resources available over the next ten years (assuming various budget scenarios), and come up with an overall plan with a set of priorities that will guide the big-picture decisions to be made by the funding agencies during this decadal period. In short, the RHIC community needs to make a compelling case to achieve a high priority in this science-driven, budget-constrained planning process.
The process has been unfolding since last January, when the American Physical Society’s Division of Nuclear Physics hosted a set of “Town Meeting” workshops to gather input from all segments of nuclear physics. It was at these meetings that each of the laboratories and research areas in the field presented its vision and its case for inclusion in the final plan. The agendas, presentations, and summary “white papers” for these meetings can be found here.
The output from these meetings is now the input for a final Working Group meeting of the DOE/NSF Nuclear Science Advisory Committee (NSAC) to hammer out the final plan. This meeting will take place during the week of April 30 – May 4 in Galveston, Texas. This Working Group consists of ~50 members and is broadly representative of the various scientific and institutional interests across the nuclear physics community. The charge given to NSAC by the NSF and DOE Office of Science, as well as previous Long Range Plan reports can be found here.