[en] This experiment was one of the first photoproduction experiments performed at Jefferson Lab using the CLAS and the Photon Tagger. The event reconstruction and the photon flux determination procedures have been developed and were proven to work well as we can see from the cross section measurement of the γp → π⁺n reaction. The preliminary results at CLAS for this reaction agree very well with previous world data. The analysis procedure has been developed to analyze the double-pion photoproduction. The differential cross sections for the γp → Pπ⁺π⁰n reaction have been measured with incident photon energies between 1 and 2 GeV. The Chew-Low extrapolation technique was used to extract the associated γπ → ππ cross sections from the differential cross sections. The extrapolation procedure of extracting the pole cross section has been explored. F^3π was obtained from the γπ → ππ cross sections. The results show a momentum dependence of the F^3π amplitude in which they agree with Holstein's calculation. These measurements test fundamental predictions of low energies QCD. Future work on this analysis will help reduce the uncertainty in F^3π, and extend the measurements to the lower and higher s regions

[en] The reaction γp → π⁺ K^- K⁺n was studied at Jefferson Lab using a tagged photon beam with an energy range of 3-5.47 GeV. A narrow baryon state with strangeness S = +1 and mass M = 1555 ± 10 MeV/c² was observed in the nK⁺ invariant mass spectrum. The peak's width is consistent with the CLAS resolution (FWHM = 26 MeV/c²), and its statistical significance is 7.8 ± 1.0 σ. A baryon with positive strangeness has exotic structure and cannot be described in the framework of the naive constituent quark model. The mass of the observed state is consistent with the mass predicted by a chiral soliton model for the Θ⁺ baryon. In addition, the pK⁺ invariant mass distribution was analyzed in the reaction γ p → K^- K⁺p with high statistics in search of doubly-charged exotic baryon states. No resonance structures were found in this spectrum

[en] We present a measurement of the induced proton polarization P_n in π⁰ electroproduction on the proton around the Δ resonance. The measurement was made at a central invariant mass and a squared four-momentum transfer of W = 1231 MeV and Q² = 0.126 GeV²/c², respectively. We measured a large induced polarization, P_n = -0.397 ± 0.055 ± 0.009. The data suggest that the scalar background is larger than expected from a recent effective Hamiltonian model

[en] New cross sections for the reaction ep → e(prime)ηp are reported for total center of mass energy W=1.5--2.3 GeV and invariant squared momentum transfer Q²=0.13--3.3 GeV². This large kinematic range allows extraction of new information about response functions, photocouplings, and ηN coupling strengths of baryon resonances. A sharp structure is seen at W ∼ 1.7 GeV. The shape of the differential cross section is indicative of the presence of a P-wave resonance that persists to high Q². Improved values are derived for the photon coupling amplitude for the S₁₁(1535) resonance. The new data greatly expands the Q² range covered and an interpretation of all data with a consistent parameterization is provided