[en] Studies of meson spectra via strong decays provide insight regarding QCD at the confinement scale. These studies have led to phenomenological models for QCD such as the constituent quark model. However, QCD allows for a much richer spectrum of meson states which include extra states such as exotics, hybrids, multi-quarks, and glueballs. First discussion of the status of exotic meson searches is given followed by a discussion of plans at Jefferson Lab to double the energy of the machine to 12 GeV, which will allow us to access photoproduction of mesons in search for gluonic excited states

[en] Studies of meson spectra via strong decays provide insight regarding QCD at the confinement scale. These studies have led to phenomenological models for QCD such as the constituent quark model. However, QCD allows for a much richer spectrum of meson states which include extra states such as exotics, hybrids, multi-quarks, and glueballs. First discussion of the status of exotic meson searches is given followed by a discussion of plans at Jefferson Lab to double the energy of the machine to 12 GeV, which will allow us to access photoproduction of mesons in search for gluonic excited states

[en] Motivated by recent experimental results for gluonic hybrid meson candidates and from recent theoretical Lattice QCD and Flux-tube model calculations, photoproduction should provide an ideal hunting ground for gluonic matter. Jefferson Lab offers an excellent opportunity to undertake the study of meson spectroscopy at intermediate energies. Current studies are underway at CLAS which are showing the feasibility of using CLAS as a meson spectrometer

[en] Results are reported on the reaction gammap --> ppp-bar . A high statistic data set was obtained at the Thomas Jefferson National Accelerator Facility utilizing the CLAS detector and a tagged photon beam of 4.8 to 5.2 GeV incident on a liquid hydrogen target. The focus of this study was to search for possible intermediate resonances which decay to proton-antiproton. Both final state protons were detected in the CLAS apparatus whereas the antiproton was identified via missing mass. General features of the data are presented along with results on narrow and broad resonance studies

[en] An amplitude analysis of an exclusive sample of 5765 events from the reaction π^- p → η(prime)π^- p at 18 GeV/c is described. The η(prime)π^- production is dominated by natural parity exchange and by three partial waves: those with J^PC = 1^-+, 2⁺⁺, and 4⁺⁺. A mass-dependent analysis of the partial-wave amplitudes indicates the production of the a₂(1320) meson as well as the a₄(2040) meson, observed for the first time decaying to η(prime)π^-. The dominant, exotic (non-q(bar q)) 1^-+ partial wave is shown to be resonant with a mass of 1.597 ± 0.010_-0.010^+0.045 GeV/c² and a width of 0.340 ± 0.040 ± 0.050 GeV/c². This exotic state, the π₁(1600), is produced with a t dependence which is different from that of the a₂(1320) meson, indicating differences between the production mechanisms for the two states

[en] The reaction π^- p → ωπ^- p, ω → π⁺ π^- π⁰ has been studied at 18 GeV/c. The ωπ^- mass spectrum is found to be dominated by the b₁(1235). Partial Wave Analysis shows that b₁ production is dominated by natural parity exchange. The S-wave and D-wave amplitudes for b₁(1235) → ωπ have been determined, and it is found that the amplitude ratio, |D/S| = 0.269 ± (0.009)_stat ± (0.01)_sys and the phase difference, φ(D-S) = 10.54^o ± (2.4^o)_stat ± (3.9)_sys

[en] We examine the results of two measurements by the CLAS collaboration, one of which claimed evidence for a Θ⁺ pentaquark, whilst the other found no such evidence. The unique feature of these two experiments was that they were performed with the same experimental setup. Using a Bayesian analysis we find that the results of the two experiments are in fact compatible with each other, but that the first measurement did not contain sufficient information to determine unambiguously the existence of a Θ⁺. Further, we suggest a means by which the existence of a new candidate particle can be tested in a rigorous manner

[en] The reaction γp → K⁺K^-p was studied at Jefferson Lab with photon energies from 1.8 to 3.8 GeV using a tagged photon beam. The goal was to search for a Θ⁺⁺ pentaquark, a narrow doubly charged baryon state having strangeness S = +1 and isospin I = 1, in the pK⁺ invariant mass spectrum. No statistically significant evidence of a Θ⁺⁺ was found. Upper limits on the total and differential production cross section for the reaction γp → K^-Θ⁺⁺ were obtained in the mass range from 1.5 to 2.0 GeV/c², with an upper limit of about 0.15 nb, 95% C.L. for a narrow resonance with a mass M_#Theta#_#sup ++# = 1.54 GeV/c². This result places a very stringent upper limit on the Θ⁺⁺ width

[en] The cross section and decay angular distributions for the coherent φ meson photoproduction on the deuteron have been measured for the first time up to a squared four-momentum transfer t =(p_γ-p_φ)² = -2 GeV²/c², using the CLAS detector at the Thomas Jefferson National Accelerator Facility. The cross sections are compared with predictions from a re-scattering model. In a framework of vector meson dominance, the data are consistent with the total φ-N cross section σ_φN at about 10 mb. If vector meson dominance is violated, a larger σ_φN is possible by introducing larger t-slope for the φN → φN process than that for the γN → φN process. The decay angular distributions of the φ are consistent with helicity conservation

[en] For the first time, the reaction gamma d -> Lambda n K+ has been analyzed in order to search for the exotic pentaquark baryon Theta+(1540). The data were taken at Jefferson Lab, using the Hall-B tagged-photon beam of energy between 0.8 and 3.6 GeV and the CEBAF Large Acceptance Spectrometer (CLAS). No statistically significant structures were observed in the nK+ invariant mass distribution. The upper limit on the gamma d -> Lambda Theta+ integrated cross section has been calculated and found to be between 5 and 25 nb, depending on the production model assumed. The upper limit on the differential cross section is also reported