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.

The pi N --> KY and KY --> KY reactions are studied using a dynamical coupled-channel model of meson-baryon interactions at energies where the baryon resonances are strongly excited. The channels included are: pi N, K \Lambda, and K\Sigma. The resonances considered are: N^* [S_{11}(1650), P_{11}(1710), P_{13}(1720),D_{13}(1700)]; \Delta^* [S_{31}(1900), P_{31}(1910), P_{33}(1920)]; \Lambda ^* [S_{01}(1670), P_{01}(1810)] \Sigma^* [P_{11}(1660), D_{13}(1670)]; and K^*(892). The basic non-resonant \pi N --> KY and KY --> KY transition potentials are derived from effective Lagrangians using a unitary transformation method. The dynamical coupled-channel equations are simplified by parametrizing the pi N -->pi N amplitudes in terms of empirical pi N partial-wave amplitudes and a phenomenological off-shell function. Two models have been constructed. Model A is built by fixing all coupling constants and resonance parameters using SU(3) symmetry, the Particle Data Group values, and results from a constituent quark model. Model B is obtained by allowing most of the parameters to vary around the values of model A in fitting the data. Good fits to the available data for pi^- p to K^0 \Lambda, K^0 \Sigma^0 have been achieved. The investigated kinematics region in the center-of-mass frame goes from threshold to 2.5 GeV. The constructed models can be imbedded into associated dynamical coupled-channel studies of kaon photo- and electro-production reactions.Comment: 35 pages, 11 Figure

Radiative pion photoproduction in the ∆(1232) resonance region is studied with the aim to access the ∆ + (1232) magnetic dipole moment. We present a unitary model of the γp → γπN (πN = π 0 p, π + n) reactions, where the πN rescattering is included in an on-shell approximation. In this model, the low energy theorem which couples the γp → γπN process in the limit of a soft final photon to the γp → πN process is exactly satisfied. We study the sensitivity of the γp → γπ 0 p process at higher values of the final photon energy to the ∆ + (1232) magnetic dipole moment. We compare our results with existing data and give predictions for forthcoming measurements of angular and energy distributions. It is found that the photon asymmetry and a helicity cross section are particularly sensitive to the ∆ + magnetic dipole moment.

The γp → γηp reaction in the S 11 (1535) resonance region is investigated as a method to access the S 11 (1535) magnetic dipole moment. To study the feasibility, we perform calculations of the γp → γηp process within an effective Lagrangian approach containing both the S 11 resonant mechanism and a background of non-resonant contributions. Predictions are made for the forthcoming experiments. In particular, we focus on the sensitivity of cross sections and photon asymmetries to the S 11 (1535) magnetic dipole moment.

ObjectivesWhether birth by caesarean section (CS) increases the occurrence of neurodevelopmental disorders, asthma or obesity in childhood is controversial. We tried to demonstrate the association between children born by CS and the occurrence of the above three diseases at the age of 5.5 years.MethodsThe database of the Taiwan Birth Cohort Study which was designed to assess the developmental trajectories of 24 200 children born in 2005 was used in this study. Associations between children born by CS and these three diseases were evaluated before and after controlling for gestational age (GA) at birth, children’s characteristics and disease-related predisposing factors.ResultsChildren born by CS had significant increases in neurodevelopmental disorders (20%), asthma (14%) and obesity (18%) compared with children born by vaginal delivery. The association between neurodevelopmental disorders and CS was attenuated after controlling for GA at birth (OR 1.15; 95% CI 0.98 to 1.34). Occurrence of neurodevelopmental disorders steadily declined with increasing GA up to ≤40–42 weeks. CS and childhood asthma were not significantly associated after controlling for parental history of asthma and GA at birth. Obesity in childhood remained significantly associated with CS (OR 1.13; 95% CI 1.04 to 1.24) after controlling for GA and disease-related factors.ConclusionsOur results implied that the association between CS birth and children’s neurodevelopmental disorders was significantly influenced by GA. CS birth was weakly associated with childhood asthma since parental asthma and preterm births are stronger predisposing factors. The association between CS birth and childhood obesity was robust after controlling for disease-related factors.