[en] At medium energies where the strong coupling constant is very large and perturbative methods can no longer be applied, the nature of confinement remains one of the key challenges in our present understanding of QCD. A necessary step is undoubtedly to identify the relevant degrees of freedom in this energy regime and the effective forces between them. Thus, a precise knowledge of the spectrum of baryon resonances and their properties is needed. A large number of states has been found in different analyses. However, only a few of them are well established and confirmed by different experiments. It has been shown that particular resonances have large effects on polarization observables. At Jefferson Laboratory, a major program has been proposed to perform a large set of photoproduction experiments using linearly- and circularly-polarized tagged photon beams as well as longitudinally and transversely-polarized targets. In double-pion production, a major obstacle in the determination of the couplings of baryon resonances are the considerable contributions from non-resonant mechanisms, m this case, polarization observables, which are sensitive to very small resonance contributions, will be helpful in the evaluation of N* properties

[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] 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

[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] The reaction ²H(e,e(prime) p)n has been studied with full kinematic coverage for photon virtuality 1.75 < 5.5 ∼ GeV². Comparisons of experimental data with theory indicate that for very low values of neutron recoil momentum (p_n < 100 MeV/c) the neutron is primarily a spectator and the reaction can be described by the plane-wave impulse approximation. For 100 < 750 MeV/c proton-neutron rescattering dominates the cross section, while Δ production followed by the NΔ → NN transition is the primary contribution at higher momenta

[en] High-quality cross sections for the reaction γd → π⁰d have been measured using the CLAS at Jefferson Lab over a wide energy range near and above the η-meson photoproduction threshold. At backward c.m. angles for the outgoing pions, we observe a resonance-like structure near E_γ = 700 MeV. Our model analysis shows that it can be explained by η excitation in the intermediate state. The effect is the result of the contribution of the N(1535)S₁₁ resonance to the amplitudes of the subprocesses occurring between the two nucleons and of a two-step process in which the excitation of an intermediate η meson dominates