Systematics of the (π+,p) reaction with light nuclei

Recent (${\pi }^{+},p$) angular distribution measurements for ground state transitions in several light nuclei exhibit energy independence in both shape and magnitude over a wide energy range. They also exhibit a simple exponential decrease with momentum transfer, $\exp (-\frac{q}{\lambda })$. The parameter $\lambda$ has a smooth variation with target mass $A$. The magnitudes of the cross sections are proportional to the spectroscopic strength of the transition except for ${}_{}{}^{11}{}_{}{}^{}\mathrm{B}({\pi }^{+},p)$, which is a factor of 3 high and ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}({\pi }^{+},p)$, which is a factor of 10 low. A single neutron pickup distorted-wave calculation appears to be incapable of reproducing the experimental data and will require a major modification. A two nucleon model is briefly examined and shows potential for success.