Multi-nucleon pion absorption in the 4He(π+,ppp)n reaction

Three-proton emission cross sections for the 4He(π+,ppp)n reaction were measured at an incident pion kinetic energy of Tpi+=165 MeV over a wide angular range in a kinematically complete experiment. Angular correlations, missing momentum distributions and energy spectra are compared with three- and four-body phase space Monte Carlo calculations. The results provide strong evidence that most of the three-proton coincidences result from three-nucleon absorption. From phase-space integration the total three-nucleon absorption cross section is estimated to be σ3N=4.8 ± 1.0 mb. The cross section involving four nucleons is small and is estimated to be σ4N < 2 mb. On the scale of the total absorption cross section in 4He, multi-nucleon pion absorption seems to represent only a small fraction. (Author) (40 refs., 3 tabs., 15 figs.)