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The s-wave meson-baryon interaction is studied using the lowest-order chiral Lagrangian in a unitary coupled-channels Bethe-Salpeter equation. In the strangeness S = −1 sector the low-energy K−p dynamics leads to the dynamical generation of the Λ(1405) as a K̄N state, along with a good description of the K−p scattering observables. At higher energies, the Λ(1670) is also found to be generated dy...

Recent measurements of nucleon resonance transition form factors with CLAS at Jefferson Lab are discussed. The new data confirm the assertion of the symmetric constituent quark model of the Roper as the first radial excitation of the nucleon. The data on high Q nπ production better constrain the branching ratios βNπ and βNη for the S11(1535). For the first time, the longitudinal transition ampl...

The coherent photoproduction of η-mesons on spin-zero nuclei is studied in a relativistic, non-local model, which we have previously applied to the coherent photoproduction of pions. We find that different off-shell extrapolations of the elementary production operator lead to large effects in the cross section. We also show that the almost complete suppression of the N (1535) seen in earlier st...

Recent measurements of nucleon resonance transition form factors with CLAS at Jefferson Lab are discussed. The new data resolve a long-standing puzzle of the nature of the Roper resonance, and confirm the assertion of the symmetric constituent quark model of the Roper as the first radial excitation of the nucleon. The data on high Q nπ production confirm the slow fall off of the S11(1535) trans...

Each Gr-functor of the type (φ, f) of a Gr-category of the type (Π, C) has the obstruction be an element k ∈ H(Π, C). When this obstruction vanishes, there exists a bijection between congruence classes of Gr-functors of the type (φ, f) and the cohomology group H(Π,C). Then the relation of Gr-category theory and the group extension problem can be established and used to prove that each Gr-catego...