The number and type of measurements needed to ascertain the amplitudes for pseudoscalar meson photoproduction are analyzed in this paper.It is found that 8 carefully selected measurements can determine the four transversity amplitudes without discrete ambiguities. That number of measurements is one less than previously believed. We approach this problem in two distinct ways: (1) solving for the amplitude magnitudes and phases directly; and (2) using a bilinear helicity product formulation to map an algebra of measurements over to the well-known algebra of the 4 × 4 Gamma matrices. It is shown that the latter method leads to an alternate proof that 8 carefully chosen experiments suffice for determining the transversity amplitudes completely. In addition, Fierz transformations of the Gamma matrices are used to develop useful linear and nonlinear relationships between the spin observables. These relationships not only help in finding complete sets of experiments, but also yield important constraints between the 16 observables for this reaction. Typeset using REVT E XRecently, Keaton and Workman (KW) [2] argued that selecting a complete set of observables is more complicated than the above BDS rule. However, KW were not able to provide sufficient conditions for resolving all ambiguities. Their work inspired us to investigate the problem of determining which experiments can provide a "complete set," e.g. those

Eta photo-and electroproduction on the nucleon is studied using an isobar model. The model contains Born terms, and contributions from vector meson exchanges and nucleon resonances. Our results are compared with recent eta photoproduction data for differential and total cross sections, beam asymmetry, and target asymmetry, as well as electroproduction data. Besides the dominant S 11 (1535) resonance, we show that the second S 11 resonance, S 11 (1650), is also necessary to be included in order to extract S 11 (1535) resonance parameters properly. In addition, the beam asymmetry data allow us to extract very small (< 0.1%) N * → ηN decay branching ratios of the D 13 (1520) and F 15 (1680) resonances because of the overwhelming swave dominance. This model (ETA-MAID) is implemented as a part of the MAID program 1 .

A reggeized model for η and η ′ photoproduction on the nucleon is presented. In this model, t-channel vector meson exchanges are described in terms of Regge trajectories to comply with the correct high energy behavior. We compare this reggeized model with an isobar model (η-MAID), where the t-channel exchanges are described by ρ and ω poles. Both models contain the same resonance contributions, and describe current γp → ηp data up to E lab γ = 2 GeV quite well, but only the reggeized model can be successfully extended to higher energies. For the γp → η ′ p reaction, the reggeized model is found to be able to give a satisfactory description. For the differential cross section data from SAPHIR, we find that the observed linear forward rise in cos θ near E lab γ = 1.6 GeV can be well described by the interference of an S 11 resonance and the Regge trajectory exchanges without any need for an additional P -wave resonance.

A coupled-channel (CC) approach has been developed to investigate kaon photoproduction on the nucleon. In addition to direct K + Λ production, our CC approach accounts for strangeness production including K + Λ final state interactions with both π 0 p and π + n intermediate states. Calculations for the γp → K + Λ reaction have been performed, and compared with the recent data from SAPHIR, with emphasis on the CC effects. We show that the CC effects are significant at the level of inducing 20% changes on total cross sections; thereby, demonstrating the need to include πN channels to correctly describe the γp → K + Λ reaction.