E04-005: Search for New Forms of Hadronic Matter in Photoproduction
In the past 40 years, a picture has emerged in which quarks are the fundamental entities from which protons and neutrons -- and hence, the nucleus of the atom -- is constructed. The theory of Quantum Chromodynamics (QCD) has also emerged as the description of the interaction between quarks that forms this nuclear matter. QCD introduces the mediator of this interaction, the gluon, which itself can be an active participant in the interaction, and thus can be a constituent of nuclear matter. Particles in which the gluon is a constituent may exhibit properties that distinguish these particles from nuclear matter constructed solely from quarks. Searches for such "hybrid states" have yielded tantalizing results at Brookhaven National Laboratory and the Crystal Barrel at CERN.
There have been suggestions that the photon beam generated from high-energy electrons may provide a fertile environment to produce these states at Jefferson Lab. In fact, the principle motivation for the construction of the Hall D detector is to search for hybrids. However, Hall B's CEBAF Large Acceptance Spectrometer (CLAS), using the highest energy electrons currently available, near 6 GeV, will attempt to identify these particles from the observed final states to which they decay.
In addition, the high data rate available with CLAS will allow several other experiments to run simultaneously. Based on suggestive results from an earlier high-energy run, a search for pentaquark baryons, constructed of four quarks and one anti-quark, will be run concurrently. The same data will afford a plan to measure cascade baryon spectroscopy. Cascade resonances, due to their unique combination of quarks, make them an ideal laboratory for understanding the interaction of quarks within protons and neutrons, which also gives insight into the nature QCD in the predominant low-energy regime, where the theory is least understood.