Theory of Pion Absorption Applied to the ReactionLi6(π−,2n)He4

The absorption of pions from a low-energy $S$ state, by a nucleus with the emission of two fast nucleons, is treated in an impulse approximation in which the pion interacts only with the pair of ejected nucleons. We consider the particular reaction ${\mathrm{Li}}^6({\pi }^{-},2n){\mathrm{He}}^4$. The two-nucleon absorption process is given by a theory, previously applied to the pion absorption by a deuteron, which is developed from low-energy pion-nucleon scattering theory. The relative motion of the absorbing nucleon pair is treated in detail, including effects of short-range repulsion and tensor potentials. Distributions of the emitted nucleons in momentum and angle are calculated in this theory, and are compared with other results for some theories and with experimental data.