High precision measurements of the differential cross sections for π0 photoproduction at forward angles for two nuclei, 12C and 208Pb, have been performed for incident photon energies of 4.9-5.5 GeV to extract the π0→γγ decay width. The experiment was done at Jefferson Lab using the Hall B photon tagger and a high-resolution multichannel calorimeter. The π0→γγ decay width was extracted by fitting the measured cross sections using recently updated theoretical models for the process. The resulting value for the decay width is Γ(π0→γγ)=7.82±0.14(stat)±0.17(syst)  eV. With the 2.8% total uncertainty, this result is a factor of 2.5 more precise than the current Particle Data Group average of this fundamental quantity, and it is consistent with current theoretical predictions.

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The exclusive reactions p ! K 0 K n and p ! K 0 K 0 p have been studied in the photon energy range 1.6-3.8 GeV, searching for evidence of the exotic baryon 1540 in the decays ! nK and ! pK 0. Data were collected with the CLAS detector at the Thomas Jefferson National Accelerator Facility. The integrated luminosity was about 70 pb ÿ1. The reactions have been isolated by detecting the K and proton directly, the neutral kaon via its decay to K S ! ÿ and the neutron or neutral kaon via the missing mass technique. The mass and width of known hyperons such as , ÿ and 1116 were used as a check of the mass determination accuracy and experimental resolution. Approximately 100 000 1520's and 150 000 's were observed in the K 0 K n and K 0 K 0 p final state, respectively. No evidence for the pentaquark was found in the nK or pK S invariant mass spectra. Upper limits were set on the production cross section of the reaction p ! K 0 as functions of center-of-mass angle, nK and pK S masses. Combining the results of the two reactions, the 95% C.L. upper limit on the total cross section for a resonance peaked at 1540 MeV was found to be 0.7 nb. Within most of the available theoretical models, this corresponds to an upper limit on the width, ÿ , ranging between 0.01 and 7 MeV.

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The exclusive reaction gammap-->K0K+n was studied in the photon energy range between 1.6 and 3.8 GeV searching for evidence of the exotic baryon Theta+ (1540)-->nK+. The decay to nK+requires the assignment of strangeness S=+1 to any observed resonance. Data were collected with the CLAS detector at the Thomas Jefferson National Accelerator Facility corresponding to an integrated luminosity of 70 pb-1. No evidence for the Theta+ pentaquark was found. Upper limits were set on the production cross section as function of center-of-mass angle and nK+ mass. The 95% C.L. upper limit on the total cross section for a narrow resonance at 1540 MeV was found to be 0.8 nb.