Search

The helicity amplitudes of the N(1520) and N(1535) resonances in the γ ∗ p → N ∗ electromagnetic transition are studied in the constituent quark model using the impulse approximation, with the proton and resonances assumed to be in three-quark configurations. The comparison of theoretical results and experimental data on the helicity amplitudes A 1 / 2 , A 3 / 2 , and S 1 / 2 indicates that the N(1520) and N(1535) resonances are primarily composed of three-quark L = 1 states but may contain additional components. However, it is improbable that contributions from meson clouds will be dominant at low Q 2 . [ABSTRACT FROM AUTHOR]

A review of our works providing a theoretical description of incoherent pion photoproduction on the deuteron is presented. The existing γ d → π N N data are analysed, especially those obtained more recently by the CLAS Collaboration at JLab, the A2 Collaboration at MAMI at Mainz, and the PION@MAX-lab Collaboration at Lund. A procedure, which accounts for the final-state interactions (FSI), is applied to extract γ n → π N differential cross sections from the deuteron data. The role of FSI is discussed. We also comment on the results of other works in connection with some discrepancies seen in comparing the model with experiment. The electromagnetic properties of baryon N ∗ resonances, improved using the extracted γ n → π N differential cross sections are presented. A model description of the cross section data from charged-pion photoproduction reactions γ d → π ± N N near threshold is also given. The γ d → π - p p data are used to extract the E 0 + multipole and total cross section of the reaction γ n → π - p near threshold. [ABSTRACT FROM AUTHOR]

In a previous study, we compared experimental data of the total and angular differential cross sections of π 0 photo-production off the proton to theoretical models implemented in version 9.6.p02 of the GEANT4 Monte Carlo simulation toolkit (Amar et al. in Eur Phys J A 55:62, 2019). This paper describes a similar work but for the exclusive and differential cross sections of the π + photo-production off the proton. The Chiral Invariant Phase Space (CHIPS) and Bertini Cascade (BERT) models were compared to the experimental data from the CEBAF Large Acceptance Spectrometer (CLAS) of Jefferson Lab and the phenomenological Scattering Analysis Interactive Dial-in (SAID) model. Relativistic Breit–Wigner fits were used to compare the description of the nucleon resonance region up to 3 GeV from SAID, CLAS and CHIPS. As observed in the π 0 case, the CHIPS model provides a reasonable description of the invariant mass distribution while the limited tabulated data set of the BERT model is not adequate. Furthermore, the γ p → n π + differential cross sections d σ / d Ω show significant differences between CHIPS and both CLAS and SAID data pointing to some needed improvements in its physics. [ABSTRACT FROM AUTHOR]