He3(π−,n)H2andHe4(π−,n)H3at 50-295 MeV

The cross section of ${\pi }^{-}+{}_{}{}^3{}_{}{}^{}\mathrm{He}\to d+n$ and ${\pi }^{-}+{}_{}{}^4{}_{}{}^{}\mathrm{He}\to t+n$ has been measured at energies between $T=50\mathrm{} \mathrm{and} 295$ MeV and angles between ${\theta }_n\approx 20° \mathrm{and} 150°$ covering the momentum transfer range $q\approx 0.4-1.0$ GeV/c. A phenomenological analysis of the data is attempted to identify the characteristic $T$, $\theta$, and $q$ dependences of the ($\pi$,$n$) reaction dynamics. Most conspicuous is the angular dependence of the ${}_{}{}^4{}_{}{}^{}\mathrm{He}$(${\pi }^{-}$,$n$)${}_{}{}^3{}_{}{}^{}\mathrm{He}$ which mirrors characteristic features of $\pi$ + He elastic scattering. The $T$ dependence of ($\pi$,$n$) in ${}_{}{}^{3,4}{}_{}{}^{}\mathrm{He}$ (for $\theta <70\mathrm{}°$) is similar to $\pi +N$ or $\pi$ + He elastic scattering. The clear dependence of $T$ and $\theta$ is indicative of $\pi +N$ (off-shell) scattering in the basic pion-absorption process. The nuclear form factor $q$ dependence is most pronounced for forward angles, and the largely exponential falloff with $q$ is similar to that of analogous nuclear reactions. Comparisons with theoretical calculations show that crucial ingredients are missing in currently used models and general directions of needed refinements are indicated.