MULTINUCLEON PION ABSORPTION ON HE-4 INTO THE PPPN FINAL-STATE

Results from a 4 pi solid angle measurement of the reaction pi(+4)He-- >pppn at incident pion energies of Tpi+= 70, 118, 162, 239, and 330 Me V are presented. Integrated cross sections are given for the reactions where three nucleons participate, leading to energetic (ppp) or (ppn) final states, and for states where four nucleons are involved (pppn). The two three-nucleon absorption modes were investigated in particula r, and an energy dependent isospin ratio of the cross sections of sigm a(ppn)/sigma(ppp)=3.6+/-1.3, 2.6+/-0.5, 1.8+/-0.3, 1.4+/-9.2, and 1.8/-0.6 was determined from 70 to 330 MeV. The differential cross sectio ns were described by a complete set of eight independent variables and compared to simple cascade and phase space models. From this analysis the contributions from initial state interactions to the multinucleon absorption cross sections were found to be more important at higher p ion energies, while those from final state interactions are stronger a t lower energies. However, both mechanisms combined were found to acco unt for not more than one-third of the total pppn multinucleon yield. The remaining strength is reasonably well reproduced by phase space mo dels, but shows a dependence on the incident pion's orbital angular mo mentum. The isospin structure of the (ppp) and (ppn) final states is n ot understood, nor are some structures in their distributions. The fou r-nucleon yield (pppn) was found to be weak (1-8 % of the total absorp tion cross section) and shows no evidence for a ''double-Delta'' excit ation.