Pion absorption in flight on 4He

We have measured double and triple coincidences following π+ and π- absorption on 4He at various pion energies. Differential and integrated cross sections are determined for all possible absorption channels. We study and compare the two-nucleon absorption on nucleon pairs with isospin I=0 and I=1. In the dNN final-state two absorption mechanisms are identified, one with only one spectator nucleon and one with phase-space behavior. The existence of a three-nucleon absorption component with a spectator nucleon is established and a four nucleon phase-space-like contribution is also identified and found to be small. The total absorption cross section is directly determined by adding the integrated cross sections of all measued reaction channels. (orig.)