**Photoproduction of pseudo scalars mesons from Complex
nuclei up to 12 GeV**

Dr. Tolio Rodrigues

Institute of Physics

University of Sau Paulo

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
° and *n* are calculated in terms of *t*-channel helicity
amplitudes using a Regge model that includes *p*,
and *B* meson exchange plus Reggeon cuts. The results reproduce
quite satisfactorily the available proton data both for *n*° and
*n*. 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 for the angular distributions of n 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
*n*-->*yy* decay width so obtained si 0.490 + 0.047 ke V, 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.