Multinucleon pion absorption on4He into thepppnfinal state

Abstract

Results from a 4π solid angle measurement of the reaction π+4He→pppn at incident pion energies of Tπ+= 70, 118, 162, 239, and 330 MeV 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 particular, and an energy dependent isospin ratio of the cross sections of σppn/σppp=3.6±1.3, 2.6±0.5, 1.8±0.3, 1.4±0.2, and 1.8±0.6 was determined from 70 to 330 MeV. The differential cross sections 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 pion energies, while those from final state interactions are stronger at lower energies. However, both mechanisms combined were found to account for not more than one-third of the total pppn multinucleon yield. The remaining strength is reasonably well reproduced by phase space models, but shows a dependence on the incident pion's orbital angular momentum. The isospin structure of the (ppp) and (ppn) final states is not understood, nor are some structures in their distributions. The four-nucleon yield (pppn) was found to be weak (1–8 % of the total absorption cross section) and shows no evidence for a “double-Δ” excitation.Received 18 June 1997DOI:https://doi.org/10.1103/PhysRevC.56.1872©1997 American Physical Society