On Wednesday, May 28, the Relativistic Heavy Ion Collider (RHIC) and Alternating Gradient Synchrotron (AGS) Users' Meeting featured a special, all-day symposium entitled "RHIC & Its Impact on Nuclear Science."
The talks started with Gordon Baym of the University of Illinois, Urbana-Champaign, who gave a historical perspective of the RHIC heavy-ion program from its inception at a 1974 workshop held at Bear Mountain. There, T.D. Lee asked whether the vacuum was a medium whose properties could be changed - and suggested investigating this question by "distributing high energy or high nucleon density over a relatively large volume." Baym's talk included the first ideas about how to do this -- for example, by colliding heavy ions -- and early predictions about what kinds of matter such collisions might produce, including the possibility of quark matter as an "ultimate state." Well before RHIC was built, scientists were talking about potential connections with astrophysical concepts such as neutron stars, supernovae, cosmic rays, and mini black holes, and even condensed matter physics, Baym said.
One field with surprising connections to RHIC physics - string theory - was then explored in depth by Makoto Natsuume of the KEK Laboratory in Japan. The connections, he emphasized, run in both directions, with elements of string theory being useful for understanding the quark matter created at RHIC, particularly its low viscosity, and RHIC being useful for testing aspects of string theory. His appeal to heavy-ion physicists: "Ask not what string theory will do for you, but what together we can do for the future of physics."
Gerry Bunce of BNL's Physics Department then gave a historical overview of the spin physics program at RHIC, emphasizing the importance of technological and theoretical advances, as well as financial investments from collaborators at the RIKEN laboratory in Japan, and the donation from Renaissance Technologies that helped fund the 2006 polarized proton run. The moral of his "fable" was that investments in technologies and ideas that predated the specific mission of investigating the proton "spin mystery" - and having adequate running time to implement, learn, and make improvements - have all been essential to the program's success.
Jacques Soffer of Temple University then elaborated on the physics results coming out of the spin program and how they relate to the problem of proton spin. He discussed the theoretical QCD underpinning for RHIC spin measurements being used to extract information on the contribution of gluons to the proton's spin, and to those planned to detect W bosons to gain access to the polarization of antiquarks in the so-called "sea" inside the proton.
The afternoon session began with Thomas Roser of Brookhaven's Collider Accelerator Department, who gave an overview of RHIC's accelerator achievements, including dramatic improvements in luminosity for both the heavy-ion and spin programs. He then described planned upgrades to further improve luminosity as well as the addition of an electron beam ion source (EBIS) and an electron ion collider (eRHIC), which would allow collisions with new species of heavy ions and electrons, respectively, as well as ways of using lower-energy electron beams to further improve overall luminosity.
Theorist Berndt Mueller of Duke University then gave his perspective of how RHIC is moving from the discovery phase to "precision" science studies of the properties of quark-gluon matter, including many features that theorists are predicting with newly refined techniques and that experimentalists can explore. He described the experimental and theoretical surprises already found at RHIC as "a gold mine." Extracting the gold, he said, will require sustained collaboration among theorists and experimentalists on precision data acquisition and interpretation.
Jamie Nagle of the University of Colorado continued this theme by discussing some key heavy-ion findings and specific matter properties yet to be explored. In view of the Large Hadron Collider (LHC) coming online, Nagle suggested there would be exciting competition. Similar competition among the four experimental collaborations at RHIC resulted in high-quality physics, he said, suggesting that competition with LHC would do the same. With improved luminosity reducing error bars on critical measurements at RHIC and new results coming out of the LHC, the next decade promises to be a "golden age of heavy ions," Nagle said, with an excited young community eager to pursue the science.
Bernd Surrow of the Massachusetts Institute of Technology gave a similarly enthusiastic talk about the future of the polarized proton program at RHIC. He emphasized the considerable constraints RHIC data have already placed on the gluon contribution to proton spin, and discussed the plan to improve those constraints and add new ones for antiquark polarization, with more extensive data anticipated over the coming several years. In addition, Surrow discussed what is being learned and planned for measurements of transverse spin sensitivities at RHIC.
Steve Vigdor, Associate Laboratory Director for Nuclear & Particle Physics, wrapped up the symposium with an outlook on the future of RHIC. He described the three stages of the long-range plan for nuclear physics at the Lab, which are: 1) a luminosity upgrade enabling RHIC-II science - made possible with stochastic cooling - to be completed as early as 2012; 2) between 2016 and 2021, an upgrade project such as the first stage of an electron ion collider, or a large improvement in proton collision luminosity fueled by an innovative beam cooling technique, or AGS precision experiments; and 3) implementation of a full-capability electron ion collider (eRHIC), which would start producing collisions in the 2020s.
Vigdor also stressed the value of Brookhaven's involvement in what he calls the "physics of the universe," experiments like ATLAS at the LHC in Switzerland, neutrino oscillation experiments at the Daya Bay reactor in China and subsequently utilizing a beam produced at Fermilab and directed toward a huge detector in the Homestake Mine in South Dakota, and the Large Synoptic Survey Telescope in Chile.
"It's important for the future of RHIC to maintain a healthy program in particle physics," he said.
See related story: "RHIC, AGS Users' Meeting Reflects on Past, Looks Toward Future of Nuclear Physics".