They came. They saw. They liked. In early December, a subcommittee to Jefferson Laboratory's Program Advisory Committee reviewed the latest iteration of plans for Hall D.

While final, formal approval from the Department of Energy is pending, the committee's report characterized the hall's photon-based physics as "...well-suited for definitive searches for exotic states [of mesons] that are required according to our current understanding of QCD." The report also stated that "JLab is uniquely suited to carry out this program of searching for exotic states."

"Most of the conceptual design has been completed," says Elton Smith, a JLab staff physicist who is serving as the Laboratory's liaison with the Hall D group. "Now we have to go forward and complete the technical design. That will require prototyping of equipment and, in some cases, deciding between alternative solutions. In each instance, we'll be choosing what we believe to be the optimal installation."

When and if built, Hall D would be the fourth experimental hall at the Lab. It would be located on the east end of the North Linac. Hall D would operate at 12 billion electron volts (GeV) and would require the addition of 10 new cryomodules. The physics proposal comes from the Hall D collaboration, composed of 80 nuclear and particle physicists from 25 institutions. The collaboration is led by Alex Dzierba, a spokesperson from Indiana University, and Curtis Meyer, acting co-spokesperson from Carnegie Mellon University.

The Hall D addition is a vital part of a desired, Lab-wide beam-energy upgrade to 12 GeV by 2006. (For JLab to achieve 12 GeV, planners will need to fully exploit the accelerator's total capacity for 50 eight-cavity cryomodules in both linacs, plus several more in the injector region. Currently, 41 1/4 cryomodules are used to deliver beams up to 5 1/2 GeV.)



Looking for the New Physics

Researchers believe new vistas in nuclear physics research will open as a result of the upgrades. The purpose of Hall D experiments would be to detect the production and decays of a class of particles known as mesons. These mesons can be created prolifically in photon-induced reactions. Scientists wish to study mesons systematically in order to find unusual varieties and to better understand how the two quarks, which make up these mesons interact via the strong force-carrying particles known as gluons.

"It's always interesting to look at new physics and what kinds of experiments can be done," Smith says. "The focus in Hall D will be on the discovery of hybrid mesons. If we find them, then we'll learn even more about how the nucleus of an atom is put together."

Photon beams are electrically neutral and so cannot be steered and focused by magnets, as can electrons. However, photons can be collimated and sent out in a straight line in a regular stream. In order to produce photons and track mesonic interactions a state-of-the-art detector will be constructed. Components used to detect the reaction products include a superconducting solenoidal magnet, drift chambers, calorimeters and a lead-glass array. The solenoid magnet and lead-glass array have already been constructed at other national laboratories and are being reserved for use in Hall D. They will be moved to JLab and upgraded as part of the hall construction project.

Researchers are also counting on the continuing, rapid evolution of computer power. In order to analyze the enormous volume of experimental data flowing from Hall D, researchers will need computers at least 10 times faster than today's models. It is a safe assumption that these will be forthcoming given increasing microchip sophistication and progression of computer clock speeds.

If the necessary funds are budgeted and allocated, construction on Hall D should begin no later than 2005. "Hall D is not yet funded," Smith points out. "But I expect that Hall D is something the Lab will pursue vigorously as an integral part of the energy upgrade."



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