Nuclear Physics Seminar

The photoproduction of pseudo-scalars mesons from complex nuclei at extreme forward angles is evaluated for incident photon energies from 3 to 12 GeV. The total photoproduction amplitude at small angles can be factorized in terms of the Coulomb and the nuclear coherent amplitudes, the quantum interference between them, plus an additional incoherent mechanism; which is very sensitive to the short range correlations caused by the nuclear matter. The elementary photoproduction operators for p0 and h are calculated in terms of t-channel helicity amplitudes using a Regge model that includes r, w and B meson exchange plus Reggeon cuts. The results reproduce quite satisfactorily the available proton data both for p0 and h. The Final State Interactions between the produced mesons and the nuclei both for the coherent and incoherent contributions are taken into account using the multi-collisional intranuclear cascade model (MCMC). The results of the angular distributions of h mesons from Be and Cu around 9 GeV are then used to fit an old dataset from Cornell, providing a consistent interpretation for the experimental yields. The final value for the h-->gg decay width so obtained is 0.490 ± 0.047 keV, which is more than 50% higher than the one obtained using the Cornell parameterization (0.324 ± 0.046 keV). Such re-analysis provides a nice explanation for the discrepant value obtained at Cornell, when compared with the PDG world average of 0.510 ± 0.026 keV. An inadequate treatment of the inelastic background (incoherent part), which was assumed to be isotropic and energy independent by the time of the Cornell analysis, is the most likely cause for such inconsistent result. The Monte Carlo method proposed here may also help for the calculation of inelastic backgrounds in recent experiments of meson production, such as the PrimEx Collaboration at the Jefferson Laboratory, as well as in future experiments with the 12 GeV upgrade of the electron beam.