Pion photoproduction and the "anomalous" transition $^{14}$N$_{g.s.}$ --> $^{14}$C$_{g.s.}$

The authors study the reaction /sup 14/N( gamma , pi /sup +/)/sup 14/C /sub g.s./ near threshold. The vanishingly small Gamow-Teller matrix element allows one to probe corrections to the normally dominant sigma . epsilon (Kroll-Ruderman) term. They find that, while the cross section for this process is strongly sensitive to nuclear structure and pion distortion inputs, the pseudoscalar terms reduce the cross section substantially from the Kroll-Ruderman value, irrespective of these uncertainties. Significant improvement is found in the comparison with experiment when the empirical nuclear wave functions obtained from other weak and electromagnetic processes, are used, rather than the shell-model wave functions. (26 refs).