Meson Photo-Production in GEANT4 for $E_\gamma = 0.225$-3.0 GeV using the $\gamma + p \rightarrow p + \pi^{0}$ reaction

Abstract.

The study of nuclear resonances in the intermediate energy regime requires accurate modeling of the experimental physics background. The cross sections in the GEANT4 toolkit do not adequately treat meson photo-production. The purpose of this study is to compare experimental data and theoretical models implemented in version 9.6.p02 of the GEANT4 Monte Carlo simulation toolkit for the exclusive and differential cross sections of $\pi^0$ photo-production. Two models were studied: the Chiral Invariant Phase Space (CHIPS) and Bertini Cascade (BERT). The cross section data from the 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 $\Delta(1232)$, $N(1520)$, $N(1650)$ and $N(1875)$ resonances from SAID, CLAS and CHIPS, giving agreements to within 3% for the mass of each resonance. The limited tabulated data set of BERT does not allow for an accurate description of the energy distribution, even following its improvement in version 10p02. The angular distributions of the cross sections show significant differences between CHIPS and both CLAS and SAID data, the latter being the most accurate model to describe the $\gamma p\rightarrow p\pi^0$ exclusive cross section. The physics in CHIPS appears to be correct but it needs further development for the individual resonances contributions that could be useful since it is also the only model able to treat electromagnetically induced meson production for $A > 1$.